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Recent Advances in Date Palm Water Management

By
Ali Montazar | UCCE Irrigation and Water Management Advisor, San Diego, Riverside and Imperial Counties and Robert Krueger| Horticulturist and Research Leader, USDA-ARS National Clonal Germplasm Repository for Citrus and Dates
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Recent Advances in Date Palm Water Management

Dates (Phoenix dactylifera L.), one of the world’s oldest cultivated fruits, originated in the Middle East, with its distribution extending to the U.S. in the last century. The geographical distribution of commercial date production is limited to arid and semi-arid regions where there is not abundant water supply. The low desert of California with nearly 11,000 acres of date palms is the major date production area within the U.S. followed by Arizona (Montazar et al. 2020). Since the date industry is currently economically successful and robust in California, date production is expected to increase as many new groves have been planted in recent years and more are currently being planted.

The date palm is drought tolerant; however, more accurate irrigation scheduling and water management during its flowering and fruiting season is critical for healthy date palms and high-quality fruit production. Date palm growers have started to adopt the use of microirrigation, but in many instances, irrigation management is based upon data developed decades ago in flood-irrigated orchards.

Figure 1. A drip irrigated date palm (top) and an integrated drip-flood irrigated date palm (bottom) in the Coachella Valley.

An irrigation study was conducted to acquire relevant information on crop water consumption and develop more accurate crop coefficients values for date palms in California. Extensive data collection was carried out in six mature date palm orchards in the Coachella and Imperial valleys over a three-year period. The experimental orchards represent various soil types and conditions, irrigation management practices, canopy features and the most common date cultivars in the region (‘Medjool’ and ‘Deglet Noor’). The orchards have relatively heterogeneous soil; however, the dominant soil texture varies from sandy loam to silty loam and silty-clay loam. A combination of surface renewal and eddy covariance equipment (flux tower, Figure 2) was utilized to measure actual crop evapotranspiration (ETa) at each site. This article provides some effective irrigation and water management tips in California date palms based on the findings of this study.

Recent Advances in Date Palm Water Management
Figure 2. Flux tower set up at one of the experimental sites located in Thermal, Calif. An aerial view of the tower from a distance (top) and a ground view of the tower (bottom).

Both micro/drip and flood irrigation are common practices in the low desert region, and some growers, who have installed microirrigation systems in their orchards irrigate their date palms through an integrated micro-flood irrigation system (Figure 1). The results of our recent date palm irrigation management survey demonstrated 31% of grower responders use only flood irrigation, 19% use only microirrigation and 50% follow an integrated micro-flood irrigation management approach. The survey also illustrated drip irrigation systems dominate microirrigation systems, with nearly 88% of grower responders reporting using drip irrigation and 12% using microsprinkler irrigation.

Consumptive Water Use in Date Palms
The results of this study demonstrated considerable variability in date palm consumptive water use (Figures 3 and 4). The cumulative date palm water use over a 12-month period across the six sites ranged from 51.7 in. (site 5) to 59.1 in. (site 3) with a mean daily ETa of 0.28 in d-1 in June-July and 0.04 in d-1 in December at the site with the highest crop water consumption.

Figure 3. Cumulative date palm actual evapotranspiration or consumptive water use (ETa) at the experimental sites over a 12-month study period (May 2019 to April 2020).

The results revealed clearly that water consumption of date palms varies significantly depending upon site-specific conditions. Various factors may influence date palm crop water use including irrigation management practices, salinity and/or soil differences, groundwater table and ground shading or canopy cover (and tree height), this last providing a good estimation of canopy size/volume and the amount of light that it can intercept. For instance, the cumulative consumptive water use over the 12-month period was 58.8 in. in a non-salt-affected sandy loam soil date palm under flood irrigation (site 4) with an average density of 50 trees per acre and an average canopy cover and tree height of 81% and 36.1 ft., respectively. In comparison, the cumulative annual consumptive water use was determined to be 51.7 in. in a silty clay loam saline-sodic date palm (site 5 under an integrated microsprinkler/flood irrigation system) with an average canopy cover of 55%, density of 60 trees per acre and tree height of 24 ft.

Figure 4. Daily actual evapotranspiration at sites 3 and 5 over the 12-month study period (May 2019 to April 2020).

Crop Coefficient Values for Date Palms
The results indicate that there is substantial difference in crop coefficient (Ka) values of date palms, both spatially and temporally (Figure 5). For instance, at site 4, the average monthly crop coefficient value varied between 0.64 in December and 0.88 in June. Date trees at this site experienced mild to moderate water stress during July, but soil moisture was maintained at a desired level during the remainder of the study period.

Figure 5. Calculated monthly actual crop coefficient (Ka) values at the experimental date palms over the 12-month study period. The observed daily actual evapotranspiration (ETa) and Spatial CIMIS reference evapotranspiration (ETo) in each of the experimental date palms were used to compute the monthly Ka values.

The soil types and conditions at sites 2, 3 and 4 were similar, and the canopy features were relatively similar as well. Both sites 3 and 4 had a density of 50 plants ac-1, whereas the density was 52 plants ac-1 at site 2. All three sites had the ‘Deglet Noor’ date cultivar. Slight differences were found among the monthly crop coefficient values of these orchards that were likely related to irrigation management differences. An integrated irrigation system consisting of drip and flood irrigation was used at sites 2 and 3. Both sites are considered fully irrigated orchards over the study period with the water applied of 7.7 ac-ft/ac at site 2 and 7.9 ac-ft/ac at site 3, with relatively high soil water availability the entire season. The average Ka values for the 12-month period were 0.81, 0.82 and 0.81 at sites 2, 3 and 4, respectively. Within the year, the monthly range of values was from 0.63 (December) to 0.90 (June) at site 3. These values likely represent the ‘potential’ Kc values for the date palms since the applied irrigation water in these orchards were 50% to 60% higher than the measured ETa and both soil moisture and canopy temperature data suggested that no water deficit occurred over the 12-month period.

Figure 6. Whole-soil-profile representations of mean ECe (electrical conductivity of the saturation extract) distribution of observed values at six experimental date palm sites (left) and the relationship of seasonal actual evapotranspiration and mean annual actual crop coefficient (Ka) versus electrical conductivity of the saturation extract (ECe) (right). The average ECe of the entire soil profile (4 ft. depth) from the whole soil samples at each date palm site were used to represent the average ECe of crop root zone in each of the experimental sites.

The monthly Ka value varied from 0.62 in November to 0.75 in June at site 5. This orchard is regularly irrigated by micro-sprinkler and occasionally flood irrigated to leach out heavy salt accumulation in the entire soil profile. Across the six sites, sites 3 and 5 had the highest and lowest Ka values averaged over the 12-month period, respectively. An average 12-month Ka value of 0.70 was obtained at site 5, which is nearly 17 % lower than the average 12-month Ka value of site 3. These sites have the same date cultivar (Deglet Noor); however, site 5 had a higher planting density and smaller trees. The reduction of tree growth at site 5 is likely associated with the physiologic adjustment of trees to the long exposure to high salinity-sodicity environments.

Soil Salinity and Date Trees’ Growth
Soil salinity varied considerably amongst the experimental sites (Figure 6, see page 12). The mean ECe (soil electrical conductivity) at the experimental sites showed that while the entire soil profile is saline at the site close to the Salton Sea (site 5, average ECe of the top 4 ft. of the soil > 12 dS m-1), relatively low values of ECe (average ECe of the top 4 ft. of the soil < 5 dS m-1) were observed within the crop root zone at the other experimental sites. The soil particle size distribution at site 5 has higher clay content below the topsoil than the other sites, which along with a high content of soluble salts and high soluble sodium resulted in both water penetration and subsurface drainage problems.

To quantify the impacts of salinity on date palm crop water use, the relations of cumulative ETa and mean annual actual crop coefficient as a function of ECe were derived. Inverse relationships were found between the seasonal actual ET and ECe; and between the mean annual Ka and ECe (Figure 6b). The average ECe of the entire soil profile (4 ft. depth) from the soil samples at each date palm site were used in this analysis to represent an average ECe for the corresponding orchards. As was mentioned, other drivers are involved in ETa and Ka values including canopy features and cultivars, soil types and irrigation management practices. These parameters do not directly contribute to the observed linear relationships; however, they indirectly influence the results. For instance, the reduced vegetative growth at site 5 may have resulted from salinity and drainage issues and the soil properties. The soil is categorized as “silty loam” with silt content greater than 50% at the top 1.2 m, and therefore, it has a very low infiltration rate. Average weight of date fruits was 31% lower at site 5 in compared with site 4 (both are ‘Deglet Noor’ cultivar).
Earlier studies indicated all aspects of date palm vegetative growth may negatively respond to salinity including the rate of production of new leaves and the size of the leaf canopy Zapata and Martinez-Cob 2002; Tripler et al. 2007). The percentage of light interception was 30% lower in the date palm irrigated by saline water (15.0 dS m-1) in a recent study (Al-Muaini et al. 2019).

Recommendations
Date palms need variable amounts of irrigation water depending on time of year, canopy cover percentage and tree height, soil types and conditions, and irrigation management. To sustain date production in the desert region, growers need to integrate microirrigation and flood irrigation together. It helps to fill the soil profile for this deep-rooted tree crop specifically at the time that drip irrigation does not have the capacity to accomplish this. During mid-June to early July, one might need to apply more than 100 gallons per day per tree. Depending on the capacity of the microirrigation system, it would be beneficial to have one flood irrigation during this period. Another flood irrigation during early season (March) is also recommended. We need to keep in mind that effective irrigation management in the desert environment is different than other regions. In the desert environment, irrigation needs to maintain crop water needs and soil salinity at the same time even for an adapted and stress-tolerant crop such as date. The two or three flood irrigation events (depending on the soil types and conditions) integrated with drip irrigation can maintain crop water needs and salinity and optimize the economic benefits of date production in the low desert region.

Growers may use the crop coefficient values developed by this study along with CIMIS ETo data to calculate crop water needs in different times of year. It is highly recommended that growers monitor soil moisture at least at the depth of 1 to 2 ft. Soil moisture at the depth of 1 to 2 ft. is a good indicator of soil water availability to date palms under microirrigation (a threshold of 25 to 30 centibar could be considered for the sandy loam soils in the Coachella Valley.)

References
Montazar, A., Krueger, R., Corwin, D., Pourreza, A., Little, C., Rios, S., Snyder, R.L. 2020. Determination of Actual Evapotranspiration and Crop Coefficients of California Date Palms Using the Residual of Energy Balance Approach. Water, 12 (8), 2253.
Zapata, N., Martínez-Cob, A. 2002. Evaluation of the surface renewal method to estimate wheat evapotranspiration. Agric. Water Manage., 55(2), 141–157.
Tripler, E., Ben-Gal, A., Shani, U. 2007. Consequence of salinity and excess boron on growth, evapotranspiration and ion uptake in date palm (Phoenix dactylifera, L., cv. Medjool). Plant and Soil, 297:147–155.
Al-Muaini, A., Green, S., Dakheel, A., Abdullah, A., Abou-Dahr, W., Dixon, S., Kemp, P., Clothier, B. 2019. Irrigation management with saline groundwater of a date palm cultivar in the hyper-arid United Arab Emirates. Agric. Water Manage., 211 (1): 123-131.

It’s All in the Tank Mix: New Ways to Blend Soil Health Products for Higher Profits

By
Cassidy Million | Ph.D., Director of Ag Science, Heliae Agriculture
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Figure 1. Utilizing products or a blend of multiple products that provide soil health-like properties for specific soil and crop responses is a newly emerging idea.

Tank mixing, the process of combining multiple crop protection chemicals, adjuvants or fertility products into the same tank, is a well-known practice. We all understand the benefits of reduced passes across the field, increasing efficiency of your applications and saving precious time. However, it’s time to shift our mindsets and tanks into applying old techniques to new concepts. The concept I am referring to is soil health. You may be thinking, ‘This is not a new concept’; however, utilizing products or a blend of multiple products that provide soil health-like properties for specific soil and crop responses is a newly emerging idea (Figure 1).

Figure 1. Utilizing products or a blend of multiple products that provide soil health-like properties for specific soil and crop responses is a newly emerging idea.

For this article, I will refer to products currently classified as biostimulants, biologicals and carbon-based products as “soil health” products. Products tailored for impacting specific soil health properties are more readily available than ever, and, frankly, this flooding of the marketplace creates much confusion among end users on performance expectations. From my own experience, I have encountered frustrated growers with numerous concerns such as, ‘These products don’t work for me’, ‘There is too much variability,’ or ‘I am unsure of what products I need.’ These are valid concerns; however, there may be some very simple solutions to addressing these problems (how we mix them in the tank or apply them to the field). When incorporating these soil health products, addressing a few basic points before getting started can help ensure the results we hope to achieve in the field. These include:
How can I understand the mode of action or the primary function of the product?
What is the soil health problem or challenge/s in your field?

Which soil health or blend of soil health products are available for my desired results?
How do I ensure my soil health blend is properly stored and tank mixed to achieve optimum success in the field?

Soil Health Products
Simply put, soil health products are inputs that provide soil health-like properties. To help break this down, let’s separate these products into two categories: living biologicals and non-living soil health products. Living biologicals are just what you think they are: The products that contain living fungi and/or bacteria that can be applied to the soil. Non-living soil health products encompass things such as enzymes, humic substances, microalgae and other microbial foods, macroalgae like seaweeds and kelps, and biochars to name a few. Keep in mind that while we can break them into two simple categories, it doesn’t mean these products are the same within each category. On the contrary, each of these products has a different mode of action and provides a different outcome when applied to the soil. To help demonstrate this, Table 1, shows a condensed list of products with their corresponding mode of action. When we think of these products, we usually think of them as a standalone application or a single addition into our ag chemical tank mixes; this is where things are about to get shook up (pun intended).

Table 1

As the presence of soil health products grows in the marketplace with a predicted 10% increase over the next eight years, there have been increasingly more research studies evaluating the synergies or complementary effects of combining products. The main takeaway from this research reveals that mixing these different products together can give growers the benefit of multiple different modes of action that optimize results. These findings demonstrate the right combination of products achieves better or higher results than acting alone. However, this information should be taken with caution; just as adding modes of action with ag chemicals improves herbicide control rates, the wrong mix of chemicals can also create antagonism and cause a disaster. The same can be seen when tank mixing soil health products. This antagonism may not express itself as you may see in pesticide tank mix but could be just as devasting to your crop or act in a way to impede the mode of action of the products you mixed. Understanding a soil health product’s mode of action and how it interacts with other products are critical components to achieving tank mix or “blend” success.

Start with a Solution in Mind
Before you can get started blending soil health products, you must decide what you want to utilize in your operation, which can be a daunting step in this process due to the confusion in the marketplace. Thus, the first question to ask is what are you trying to achieve? You need to start with an end goal in mind. I would suggest you evaluate what is the yield limiting factor in your field. For example, is soil compaction leading to poor soil drainage and water holding capacity issues? Is your soil sample showing adequate nutrient levels, but nutrients aren’t seeming to move into the plants? Are nutrients not staying in your soils and leaching away? You may find that you have more than one issue. Once you can identify the main issue/s you are having in your field you can begin to select the correct mode of action/s to address the problems. Table 2 will walk you through a few common scenarios that are encountered in many fields that would benefit from soil health products and examples of products that would be a solution to those problems.

Table 2

Building Your Soil Health Blend for Success
Now that we understand mode of action solution-based approaches, we can now begin to build a soil health blend. The next step are things to consider such as incompatibilities or antagonisms between products or in the field. Just like pesticide tank mixes physical, biological and chemical incompatibilities can occur with soil health products. Table 3, is a soil health blend checklist of questions you should ask while evaluating and building your soil health blend. The questions outlined in Table 3, not only encompass the mixing of the products but also bring to light other considerations such as shelf-life, viability and storage and handling requirements that may be different from other inputs we are more commonly used to.

Table 3

Soil health blends are the future of unlocking higher yields, more effective nutrient use, and improved crop quality. Understanding product mode of action and seeking sound advice from your retailer, CCA, or trusted advisor is key. To continue to improve our outputs we must be focused on our inputs. I challenge all of you to start thinking on how soil health blends can be utilized as a tool for success. If we think in terms of nutrient inputs, we would never consider only applying one standalone nutrient to a crop, and the same holds true for adding soil health inputs. Diversifying our inputs based on product mode of action, environmental needs and adopting the principal of blending soil health products will increase our chances of success in the field.

Table 4

Resources
Tank mixing guide: https://ag.purdue.edu/department/extension/ppp/ppp-publications/ppp-122.pdf-
Research on combining biostumulants: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6243119/

Challenges and Prospects for Biopesticide Discovery

By
Philippe Rolshausen | Ph.D., Professor of Cooperative Extension, Department of Botany and Plant Sciences, UC Riverside
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Figure 1. Microbial shift following decline of citrus trees caused by Huanglongbing (Ginnan et al. 2020).

The world population is projected to reach about 10 billion individuals by 2050. As a result, the agricultural sector needs to produce more food despite several looming challenges, including the shrinking of arable land, erratic and extreme weather patterns and pest and disease outbreaks. Higher agricultural outputs per surface basis is one key element to solve the equation of a growing demand, which inevitably results in an increase of agrochemical inputs (pesticides, fertilizers). The global agrochemical market size is valued at $235.2 billion in 2023 and is expected to reach $280 billion by 2028, growing at a compound annual growth rate (CAGR) of 3.7% during the forecast period. The major driving force behind this uptrend comes from the organic sector. Hence, the global agricultural biologicals market (biopesticides, biostimulants and biofertilizers) is projected t

grow at a CAGR of 13.8% to reach $27.9 billion by 2029 from $14.6 billion in 2023.
There is a growing consumer demand for organically grown and safe agricultural products. There is also a collective awareness of the potential risks associated with pesticide use, including impact on human health (e.g., increase of cancers linked to pesticide use), contamination of environmental resources (e.g., leaching of chemicals in groundwater) and non-target side effects on wildlife (e.g., bee population collapse). This has led to a tightening of policies by governments and regulatory agencies for conventional chemical use and chemical residue limits and the phasing out of harsh synthetic agrochemicals. Growers are at the forefront of the sustainable ag movement and have adapted their farming practices to meet both consumer and agency demands. Replacing synthetic chemicals with organic products or biologicals in their agrochemical arsenal can be challenging because of the limited scientific information on their mode of action and range of efficacy. In this article, we will mostly focus the discussion on sustainable strategies to control diseases with biopesticides, the discovery pipeline within the framework of my research and the prospects for the future.

In Integrated Pest Management
Fostering sustainable farming practices that promote and enhance agroecosystem health and increase biodiversity are instrumental to our long-term success. Integrated pest and disease management programs (IPM or IPDM) have been central to this philosophy. The framework of IPM programs is to reduce synthetic agrochemical inputs and rely first on alternative strategies, such as adapted management practices, use of resistant/tolerant plant varieties and deployment of organic agrochemicals and biologicals. IPM programs were at times challenging to implement to manage certain crop diseases because of the lack of relevant alternatives to synthetic agrochemicals. Organic agrochemicals suffer from a short window of activity and under high disease pressure they must be applied often. Also, those products are not free of environmental risks. For example, the copper-based Bordeaux mixtures have been used for over a century in viticulture and are known to impoverish vineyard soil biota and affect aromas in wine.

Biologicals also often suffered from a lack of credibility because of their inconsistency to control diseases. From a grower standpoint, there is too much economical risk to rely on uncertainty. However, recent scientific discoveries and technological advances have helped strengthen IPM programs in many ways. First, they have and continue to improve the formulations of agrochemicals (conventional, organic and biological) and, most importantly, their deliveries to target pests and pathogens. Scientific reports indicated that only 0.1% to 1% of sprayed material reaches its target depending on the system. Thus, increasing delivery efficiency will reduce leaching of chemicals in the environment. Second, the advent of ‘omics’ technologies and affordability of sequencing costs have allowed industry to identify biological candidates in a quick and reliable manner. This has revolutionized the field of microbiology (among others) that traditionally used ancestral culturing techniques to single out biological agents from environmental samples, which equated to finding a needle in a haystack. In today’s era, we can generate large-sequence datasets that capture not only entire microbial communities associated with plants or environments but also shed light on their biological functions. To be able to see in a high-throughput manner ‘who does what’ has streamlined the discovery of biological products.

Finding Beneficial Microbes for Diseased Systems
My lab has been using omics tools to identify beneficial organisms inhabiting citrus and grapevine that could be leveraged to manage Huanglongbing (HLB) and Pierce’s disease (PD), respectively. Both diseases are caused by bacterial pathogens transmitted by insect vectors following feeding on the plant host. In citrus HLB, the bacterium (Candidatus Liberibacter asiaticus) lives in the phloem tissues that transport sugars downwards, from leaves to fruits and roots. In grapevine, the bacterium (Xylella fastidiosa) lives in the xylem tissues that transport water and nutrients upwards from roots to leaves. In both pathosystems, the buildup of bacteria in the hosts’ vasculature leads to disruption of channel transport to a point that becomes detrimental to the plant. Citrus and grapevine decline can happen in as quickly as two years, but in some cases, the hosts can sustain the infection for several years. Our hypothesis is the microbial communities living with the host provide protection against the pathogens with the goal of leveraging their benefits for crop protection.

Figure 1. Microbial shift following decline of citrus trees caused by Huanglongbing (Ginnan et al. 2020).

A survey of citrus orchards in Florida and sequencing of the microbial communities associated with roots of trees with a range of disease symptoms revealed interesting information on the disease etiology (Figure 1). Our results showed there was an initial decline of keystone taxa (native organisms that play a role in the stability of an ecosystem) and symbionts such as mycorrhizae fungi for trees that contracted the disease (Phase I). This was followed by a microbe-mediated response to infection, with enrichment of several beneficial organisms that have the capabilities to stimulate the tree immune response or provide direct antibiosis to pathogens (Phase II). In the late phase of the infection (Phase III), we measured an increase of soilborne parasites and pathogens (Fusarium, Phytophthora) and saprophytic fungi that decompose decayed roots. Those findings indicate either HLB made trees more vulnerable to root pathogens or vice versa, but in either case, this synergistic effect caused trees to decline at a faster pace. This research led to the pursuit of two strategies that are now being evaluated to combat HLB. The first strategy is to re-introduce into the system beneficial microbes or microbial natural products isolated from Phase II and verify these can support tree immunity. The second strategy is to identify cultural practices like soil amendments that support the keystone and symbiotic communities.

Similar to citrus, we surveyed vineyards and profiled the microbiome of grapevine with a range of Pierce’s disease symptoms (Figure 2). Here, we found interesting data when looking at the vine lignified shoot tissues (canes). In comparison to the root system, there are very few microbes living in the plant vasculature and even more so in annual tissues like canes that are pruned off every year. This is a perfect environment for the pathogenic bacterium to thrive due to the limited microbial competition. Our results showed that, just like in citrus roots, there was a microbial-mediated response to infection and that two bacteria that also inhabit the vascular system correlated negatively with the pathogen Xylella fastidiosa. In other words, when those two beneficial bacteria were present and abundant, the pathogenic bacterium was low, and vines were healthy. Our group further re-isolated those beneficial bacteria and confirmed in greenhouse bioassays that when they were inoculated to grapevine, they provided protection against Pierce’s disease. These biological control agents are currently being evaluated at UC Davis in field trials. UC patented those technologies, and we are partnering with the private sector to develop injectable or sprayable products that could be commercialized for Pierce’s disease management.

Figure 2. Microbial mediated response to grapevine Pierce’s disease (PD) infection. A) Mild PD infection; B) intermediate PD infection; and C) severe PD infection. Bottom panel shows the bacterial response to infection in intermediate PD infection (Deyett and Rolshausen 2019).

There is a push from agricultural commodities to replace synthetic agrochemicals with environmentally friendly solutions. There are a lot of incentives for agrochemical companies to develop bio-based products, including an easier path to EPA registration. The lower residual toxicity levels of biopesticides make them the perfect choice to meet stringent environmental standards and address food safety and quality. However, biologicals still suffer from higher production costs because they are not produced and distributed in high volumes like their synthetic chemical counterparts. Increasing scalability and improving consistency in efficacy are two major hurdles for biopesticides to gain larger market share in the future.

References
Ginnan et al. 2020. Phytobiomes. https://doi.org/10.1094/PBIOMES-04-20-0027-R
Deyett and Rolshausen 2019. Frontiers in Plant Science. https://doi.org/10.3389/fpls.2019.01246

Tips for Managing Vine Nutrition in Vineyards

By
Tian Tian | UCCE Viticulture Farm Advisor, Kern County
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Understanding vine nutrient status and determining their nutrient needs are as important as fertilization itself (all photos by T. Tian.)

Proper nutrition management allows vines to grow healthy canopies and produce fruit with desirable quality. Fertilization is used to correct nutrient deficiency and improve vine productivity. Even in vines without foliar symptoms, growers may fertilize as a routine practice to compensate nutrient loss at harvest and prevent nutrient deficiency. As a result, sometimes managing vine nutrition simply means applying fertilizers. I would argue that understanding vine nutrient status and determining their nutrient needs are as important as fertilization itself.

Grapevines have lower fertilization requirements than many agricultural crops.

Overfertilization does not offer many benefits from economic or vine productivity points of view. Instead, it could compromise vine balance, decrease fruit quality and negatively affect the environment. Let’s use nitrogen (N) as an example here. Excessive N additions lead to jungle-like canopy with limited light penetration and air circulation, increase disease pressure and negatively affect fruit quality. Excessive amounts of nitrate in the soil also increase the risk of groundwater and surface water contamination.

Supplying vines with ample but not excessive nutrients is easier said than done. In this article, I will start the story with summarizing previous work on whole vine nutrient budget and then extend the discussion to determining vine nutrient requirements and fertilization.

In terms of macronutrients, annual growth is a strong sink for nitrogen, phosphorus, potassium, calcium and magnesium between bud break and veraison.

Whole Vine Nutrient Budget
Studies were conducted under field conditions in California and Oregon and in potted systems in South Africa to track nutrient uptake, movement and distribution among vine organs at different phenological stages.

In terms of macronutrients, annual growth is a strong sink for N, phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) between bud break and veraison. Required nutrients of shoots, leaves and clusters can be obtained from two pools: nutrients remobilized from permanent organs and those taken from soil. In mature vines, up to 50% of N and P in new growth can be supplied by the stored reserve in trunks and roots. On the other hand, less than 15% of K, Ca and Mg are remobilized from the reserve, since only a small percentage of those nutrients can be recycled during leaf fall. Clearly, nutrients obtained from the soil still account for a large portion of required nutrients, even in the mature vines. Young vines have less nutrient reserve than older vines, and thus rely more on nutrients supplied by root systems. Nitrogen uptake peaks between bud break and bloom, while uptake of other macronutrients usually reach the max between bloom and veraison. Nutrient uptake also takes place after harvest when nutrients are available in the soil and weather conditions are favorable.

Even if the tissue nutrient concentration of a vineyard is slightly below the given normal range, it does not mean vines surely experience nutrient deficiency.

The uptake and distribution of micronutrients is less understood as compared to macronutrients. Dr. Paul Schreiner, scientist at USDA-ARS, studies the budget of micronutrient in young and older ‘Pinot noir’ vines in Oregon’s Willamette Valley. In young vines, boron, zinc, manganese and copper were taken between bud break and harvest, with the peak of uptake occurring between bloom and veraison. The uptake and allocation of micronutrients appears less consistent in mature vines.

Please note that research findings reflect vine nutrient budget under specific conditions and should be interpreted with caution. Factors like soil nutrient availability, irrigation practices, rootstock and scion combination and weather conditions have large impacts on nutrient uptake and allocation. Clearly, vine nutrient demand varies between vineyards. So, how should one determine whether fertilization is needed at a specific site?

Determine Nutrient Requirement of Vines
Nutrient analyses of leaf blades and leaf petioles at bloom and veraison are indicators of vine nutrient status. Many testing labs provide the comparison of leaf nutrient concentration to the normal range for each nutrient.

We recommend growers not relying solely on numbers on lab results to make fertilization decisions. First, variability in vine nutrient requirement is expected between vineyards and across seasons. Even if the tissue nutrient concentration of a vineyard is slightly below the given normal range, it does not mean vines surely experience nutrient deficiency. Second, normal ranges of leaf nutrients are estimated in experiments where vines received a specific nutrient at different rates and had its concentration over a wide range of leaf tissues. Given the difference in experimental setup, the normal range determined can vary between studies. Thus, the normal range for nutrients should be used only as references when it comes to interpreting vine nutrient requirements. Historical nutrient data, vine growth and production goals are important to consider. For example, fast-growing canopies may have lower leaf N concentration, especially in newly expanded leaves, but does not mean that vines require N fertilization. Instead, rapid shoot growth often indicates ample N and water availability in the soil.

Fertilization
In vineyards where fertilization is needed, growers and CCAs often ask me about the amount, formula and application timing of fertilizer. Rather than providing generalized answers for those questions, I would like to share some tips. Please feel free to reach out for questions related to your vineyards.

About 3 lb N, 0.5 lb P and 5 lb K are removed from a vineyard with each ton of harvested fruit. Many would use those numbers to calculate how much fertilizer is needed to compensate the loss during harvest. In reality, supplementing vines only with nutrients removed from the vineyard may not be sufficient. For instance, data from western Oregon showed that young ‘Pinot noir’ vines acquired 12.5 lb N, 3 lb P and 25 lb K per acre via uptake with the crop level at 2.0 ton/acre.

Compost is an affordable, slow-releasing fertilizer. Applying compost in the spring supplements vines with nutrients, boost soil microbial activity, improves soil water penetration by aggregation and enhances water holding capacity. Increasing soil organic matter is good practice for soil health in general.

Soil pH plays key roles on nutrient availability. Most nutrients become more available in soils with neutral pH. Regular soil sampling and testing can help with managing and adjusting soil pH if needed.

If nutrient deficiency is observed late in the season, immediate fertilization may not alleviate the symptoms. It is because the period when nutrient deficiency becomes evident would not be coincident with the period when nutrient uptake peaks. Making applications at the right timing in the following season may be more effective on correcting deficiency.
Even though vines obtain nutrients mainly from the soil by roots, foliar application can be an additional tool to supplement vines with nutrients. In the past, we successfully increased fruit N at harvest in wine grapes by applying urea to foliage between fruit set and veraison. Foliar application of P and Mg was found to reduce leaf symptoms in wine grapes. Some PCAs suggest foliar Ca and Mg sprays between bloom and veraison can increase berry firmness and reduce powdery mildew occurrence in table grapes.

Applying fertilizers in small doses may improve fertilization efficiency in vineyards with shallow root systems. Theoretically, frequent fertilization with lower doses allows roots to better catch nutrients and reduce leaching.

References
Araujo FJ and Williams LE. 1988. Dry matter and nitrogen partitioning and root growth of young field-grown Thompson Seedless grapevines. Vitis 27:21-32.
Conradie WJ. 1980. Seasonal uptake of nutrients by Chenin blanc in sand culture: I. Nitrogen. S Afr J Enol Vitic 1:59-65.
Conradie WJ. 1981. Seasonal uptake of nutrients by Chenin blanc in sand culture: II. Phosphorus, potassium, calcium and magnesium. S Afr J Enol Vitic 2:7-14.
Schreiner RP, Scagel CF and Baham J. 2006. Nutrient uptake and distribution in a mature ‘Pinot noir’ vineyard. HortScience 41:336-345
Schreiner RP. 2016. Nutrient uptake and distribution in young Pinot noir grapevines over two seasons. Am J Enol Vitic 67:436-448

Navel Orangeworm Hullsplit Spray Considerations in 2023

By
Sudan Gyawaly | UCCE IPM Advisor, Northern Sacramento Valley
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If the beginning of Nonpareil hullsplit coincides with egg laying time of the second flight of NOW, the risk of NOW damage increases. (all photos by Vicky Boyd.)

From the standpoint of navel orangeworm (NOW) management in almonds, hullsplit is the most critical fruit developmental stage. Almond fruits open at their suture during this period and become susceptible to NOW infestation. If the beginning of Nonpareil hullsplit coincides with egg laying time of the second flight of NOW, the risk of NOW damage increases. Therefore, understanding NOW activity and hullsplit status is vital for timely insecticide applications. The nuts on the southwest side of the canopy mature earlier, so the initiation of hullsplit in a block should be confirmed by regularly checking on the southwest side of the canopy. An orchard ladder or pruning tower can aid in reaching the fruits from the top of the representative trees to detect early split. Also, knowing the difference between true hullsplit and blank nut split is essential because blank nut hullsplit usually begins one to two weeks before the sound nut hullsplit begins. Nuts in edge trees, especially on the southern edge of an orchard, often start to split several days to a week ahead of trees within the orchard. Edge and blank splits signal the approach of hull split of sound nuts inside the orchard. Hullsplit timing varies among tree water status, varieties, geographic regions and weather conditions; however, prediction models are available to predict hullsplit timing for various locations. Research and field experience tell us that spraying earlier, rather than later, is the most effective strategy for reducing NOW damage.

NOW Traps and Spray Decision
NOW monitoring (egg, male and female) traps do not provide a specific treatment threshold; however, they can be highly beneficial for spray decision-making since they inform the grower about seasonal pest activities. Egg traps are helpful to set the egg-laying biofix for overwintering adults in the spring and track the heat units to determine the completion of that generation and the beginning of the second generation (i.e., 1050 degree-days). The second generation infests the early hullsplit nuts, and the timing for that generation is critically important to minimize the nut damage. The utility of egg traps for guiding spray timing would be less clear for the later generations because of the continuation of egg-laying due to overlapping adults among generations. However, the density of egg laying in egg traps and female moth activity (in Peterson traps or similar tools) are good indicators of potential NOW pressure and, possibly, nut damage.

Experience shows that unless an orchard is in an isolated location with a history of very low NOW damage and low NOW population, most almond orchards in the Northern Sacramento Valley warrant at least one spray at hullsplit. However, missing proper spray timing and not practicing other cultural practices such as winter sanitation may result in higher-than-acceptable levels of nut damage. Therefore, minimizing NOW damage requires practicing cultural techniques, such as winter sanitation and timely harvest, monitoring pest populations and making well-timed insecticide sprays. Once hulsplit begins and eggs are being laid in egg traps, spray to protect early hullsplit, watch traps tracking females (egg and/or Peterson traps) and check early split nuts for worms to make preharvest spray decisions and decide on harvest timing. If NOW pressure is high, it might be worth a talk with the processor to look at timely harvest and kernel (meats) production vs later harvest for inshell. If NOW pressure is high, damage can increase very rapidly, and every day the nuts are in the trees increases the danger of further damage.

Determining Spray Timing and Numbers
The number and timing of hullsplit sprays depend on orchard history, in-season pest monitoring information, use of other cultural practices such as winter sanitation and timely harvest, and overall risk of NOW immigration to the orchard. Typically, the first hullsplit spray for NOW is made when the eggs are being laid and the hullsplit begins. Commonly used, relatively reduced-risk insecticides only kill eggs and larvae. Some are effective against adults; however, insecticides must contact adults to be effective. Spraying during the night or early morning is more effective than spraying during the day. These insecticides can be effective for two to four weeks; a second application may be necessary if you continuously find high numbers of eggs, females and males in their respective traps and you cannot harvest the nuts before the third flight activity. Early hullsplit pollinizers split about two weeks after Nonpareil; the hullsplit typically starts when the NOW third flight begins. For many growers requiring a second hullsplit spray, a second application can help both Nonpareil and pollinizers. Late-season pollinizer varieties can be at risk from the latter portion of the third and fourth flights if not harvested on time.

Additional Considerations
Provide good spray coverage. Good spray coverage is essential for effective NOW control. Many years of study have indicated spray coverage is the function of tree height, sprayer speed and spray volume. In mature almond orchards, it has been shown that high-volume spray (150 to 200 GPA ) with a slower spraying speed (2 mph) provides better coverage and NOW control, though it is an unconventional practice for many growers.

Harvest on time. The goal of timely harvest is to get the almond harvested once 100% hull split occurs for all varieties, and if possible, before the beginning of the third generation for Nonpareil and before the fourth generation for other late varieties. Ensuring that nuts are at the proper maturity stage during the harvest is critical. Too early harvest can lead to mold development and chipping issues during the processing, reducing the nut quality.
Know your insecticides. Insecticides differ in their mode of action, efficacies to NOW and toxicity to beneficials (Table 1). Knowing those facts can help select the best insecticide for a particular situation. Most of the insecticides labeled for NOW target newly hatched larvae and have some impacts on eggs. Some have effects on all stages. Insecticides applied for NOW management at hullsplit must kill the egg or larva before feeding damage occurs. Another difference among NOW insecticide is their potential effects on spider mite predators such as predatory mites and sixspotted thrips. Non-selective insecticides, in general, are more toxic to those beneficial than selective insecticides. Therefore, the tradeoff of using broad-spectrum insecticide should always be considered.

Disclaimer: Products listed in this table do not constitute a recommendation, and many of the active ingredients presented in this article can be purchased under multiple trade names.

Understand seasonal changes in NOW biology and behavior. NOW development rate changes during the season. The first and second generation of NOW needs about 1050 degree-days to complete one generation. However, after that, NOW can complete a generation in only 750 degree-days due to better nutritional quality offered by the fresh nuts. Temperature plays a significant role, too, as higher temperatures during the summer accelerate egg hatching and larvae development.

Do Wider Beds in Fresh Market Onion Production Increase Total Water Productivity?

By
Jairo Diaz | Director, UC ANR Desert Research and Education Center, Daniel Geisseler | UCCE Nutrient Management Specialist and Roberto Soto| Faculty Member, Universidad Autonoma de Baja California
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Do Wider Beds in Fresh Market Onion Production Increase Total Water Productivity?

Imperial County, located in Southern California, has a large agricultural sector with close to 500,000 acres in production. The region’s major agricultural sectors are cattle feedlots, forages and vegetables. Imperial County and surrounding areas produce an estimated two-thirds of the nation’s vegetables in the winter. In the last decade, the use of pressurized irrigation systems (solid set sprinklers and drip irrigation) and wider beds in vegetable production has gained popularity in Imperial County. Imperial County is a top fresh market onion region with over 3,000 acres in annual production. Agriculture in Imperial County relies on just one water source: the Colorado River. As the Colorado River basin (CRB) is reaching more than 20 years of continuous drought, the US Bureau of Reclamation is working with basin users to develop strategies that may reduce water usage between 2 and 4 million acre-feet per year. Most of the projected water cuts are expected from agriculture due to its high share of water use. Agricultural users and water regulators in the CRB are discussing strategies to promote water conservation practices and improve water use efficiency while keeping a large and productive agricultural sector that feeds Americans with high-quality food all year long. The objective of this study was to assess the effect of two bed sizes and two irrigation amounts on onion yield and water productivity.

Do Wider Beds in Fresh Market Onion Production Increase Total Water Productivity?
Figure 1. Onion sizes (L-R prepack, medium, jumbo, colossal and super colossal) per season. Terena in 2019-20 (top), and Hornet in 2020-21 (bottom).

Methods
This study was performed during two growing seasons (2019-20 and 2020-21) at the UC Desert Research and Extension Center (DREC) in Holtville, Calif. The major soil unit in testing areas is a Holtville clay with clay, silt and sand proportions of approximately 42%, 19% and 39%, respectively.

Table 1. Growth stage onion crop coefficient values used in this study (Montazar 2019).

Two irrigation levels were evaluated: 100% and 130% of crop evapotranspiration (ETc). ETc was computed using potential evapotranspiration from the California Irrigation Management Information Systems (CIMIS) station at DREC (Meloland station #87) and adjusted by stage-specific crop coefficients (Table 1) developed by Ali Montazar (2019) for Imperial County onion production. This study used sprinkler irrigation for crop emergence and until the beginning of bulbing to ensure an adequate establishment. This is a normal practice in our region. Irrigation treatments using drip systems were established in January of every season. Water amounts delivered through the drip irrigation system were computed using a daily water balance approach (irrigation needs = ETc – rainfall). Drip irrigation treatments started at bulb initiation (about eight leaves and a bulb diameter that was twice that of the neck.) Onion yields and sized distribution were measured at harvest. A two-sample t-test was performed to compare irrigation levels per bed size with SAS software. Total water productivity (TWP) was computed for each irrigation treatment. The TWP included rain and irrigation amounts. The TWP was computed as the total fresh onion yield divided by the total water use.

Table 2. Onion cultivars, seasonal conditions and irrigation amounts per season.

The drip tape was installed between 2- and 4-inches depth. One drip tape line was installed near the center of the 40-inch bed. Three drip tape lines were buried in the 80-inch bed plots. 4 and 12 lines of onions were planted in the 40-inch and 80-inch bed rows, respectively. Research plots were 50 feet long and comprised four rows on 40-inch beds and three rows on 80-inch beds.

Results and Discussion
Table 2 shows onion cultivars, seasonal conditions and irrigation amounts per season. Terena and Hornet varieties are short-day, yellow hybrid onions (Figure 1). Terena variety has a globe shape and Hornet is a grano-shaped onion.

Plant density used in this study is in the range used by growers in Imperial County (Table 3). Plant density on 80-inch beds was about 50% higher than onions planted on 40-inch beds.

Table 3. Plant density by cultivar and bed size.

Total yields responded to irrigation amounts except in the trial with Hornet (2020-21) on 80-inch beds (Table 4). In the Hornet trial (2020-21) on 80-inch beds, we noticed the irrigation treatment using 130% ETc yielded a higher proportion of pre-pack sizes compared to other treatments. We also noticed Hornet grown on 80-inch beds did not yield super colossal sizes. These results indicate Hornet seeds were planted too close in the 80-inch beds, reducing bulb size in the higher-irrigation treatment.

Table 4. Effect of irrigation rates on fresh market onion size distribution and total yield.

Terena treatments in 2019-20 produced a larger proportion (63% to 90%) of high-value bulbs (jumbo, colossal and super colossal) than Hornet in 2020-21(49% to 78%). High-value bulbs produced by irrigation treatments at 130% ETc were consistently higher than in the 100% ETc treatments. When onions were sorted by sizes and per-unit yields were computed, Terena’s yields were approximately 20% higher than Hornet’s yields.

Irrigation levels did not affect total water productivity (TWP) for Terena (2019-20) grown on 80-inch beds and Hornet (2020-2021) grown on 40-inch beds (Table 5). Total water productivity decreased as irrigation increased in the following trials: Terena (2019-20) on 40-inch beds and Hornet (2020-21) on 80-inch beds. Total yields on 80-inch beds increased between 10% and 31% compared to total yields on 40-inch beds (Table 5). Total water productivity was higher when onions were harvested on 80-inch beds than onions produced on 40-inch bed systems. Total water productivity of onions using 80-inch beds increased between 8% and 32% compared to onions grown on 40-inch beds.

Table 5. Total yields and water productivity by cultivar.

Highlights
Terena variety produced higher yields than Hornet variety.

High-value bulbs (jumbo, colossal and super colossal) produced by irrigation treatments at 130% ETc were consistently higher than with 100% ETc.

Total water productivity did not increase as irrigation levels increased regardless of bed size.

Fresh market onions on 80-inch beds were more water efficient than onions produced using 40-inch beds.

References
Montazar, A. 2019. Preliminary estimation of dehydrator onion crop water needs in the Imperial Valley. Agricultural Briefs 22(7):131-135. University of California Cooperative Extension – Imperial County.
This research included funds from the California Department of Food and Agriculture’s Fertilizer Research and Education Program.
For more information, contact Jairo Diaz at jdiazr@ucanr.edu or 760-791-0521.

Biostimulants and the Role of Cytokinin in Crop Performance

By
Kirk Van Leuven | PCA, CCA
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Citrus leaf senescence caused by magnesium deficiency manifests when cytokinin and chlorophyll are oxidized out of leaves (all photos courtesy K. Van Leuven.)

Biostimulant products come from things existing in nature which are useful as solutions to agricultural problems. They are seen as ways to meet challenges in nutrient use efficiency, managing crop stresses and arriving at the end goal of improving crop yield and quality.
These products have created some tension in the agricultural community, particularly around regulatory requirements because they don’t fit squarely into either of the two traditional crop input categories. They’re not fertilizers and they’re not pest control chemicals. The crop input industry is organized around these historical categories. So where do biostimulants fit in and how do we begin to understand how to work with them?
Admittedly, overreliance on fertilizers and chemicals creates several problems for agriculture. With yield and quality being the goal of everyone involved, biostimulants can supplement vigor and crop resilience and improve yield and quality while reducing the environmental load of potentially harmful materials.

The newer science of molecular biology helps us to understand what goes on in the machinery of a plant as it reacts to its environment. Depending on conditions and the availability of nutrients and other essentials, the plant’s genetic code conducts its commitment to growth, using polarity and gravity as directional forces. Von Liebig understood 200 years ago how nutritional shortages could limit yield. What we understand now is that nutrition is only part of the equation. We can have soil nutrient levels at optimum and still have growth stalled out if other requirements are not met. At the center of it all is the genetic code. The rate of growth depends on signaling by hormones and the activation of genes inside the plant as it senses conditions around it. Gene activity is increased, or suppressed, in response to nutrient status and other factors like hormone concentration and environmental conditions. It has been estimated that over 30% of all the genes in higher plants have something to do with hormones.

Almonds treated with foliar cytokinin on the right are consistently larger and heavier.

Cytokinin in the Plant
One category of biostimulants is products which contain natural hormones like plant extracts and seaweed. The growth hormone cytokinin (CK) is an active component in some of these products. In the plant, the level of available CK can be increased in two different ways: stimulating synthesis of larger amounts of natural CK or adding exogenous applications of CK-containing materials. Applications of CK-containing seaweed products have become mainstream over the last 40 years. This can help with several plant processes at various stages of growth.

Natural systemic CK is produced primarily in the meristematic zones of the roots and apical growth points and is needed for cell division and other processes throughout the growth cycle. It is needed for ongoing formation of all tissue and controls the size and shape of leaves, as well as the how the leaves are arranged on a stem. Whether plants grow from a crown or from a stock or trunk they benefit from higher CK levels. CK determines the number of cells and potential size of a leaf. For crops like leafy vegetables, the size, quality and shelf life of the crop rely on a constant supply of CK during growth. CK is the hormone associated with holding back leaf senescence and is known as the juvenility hormone.

CK also regulates chlorophyll synthesis in the leaves, and increased CK is known to improve photosynthesis. When we think about the influence that cell division also has on the size of the leaves, we realize CK is essential to the overall photosynthetic capacity of the plant. Photosynthesis is also supported longer in the growing season with senescence delayed by higher CK levels. We will also discuss the role that CK has in the transport of sugars to other parts of the plant. CK is important for plant processes and growth throughout the plant’s life cycle.

Leaves and nuts on cytokinin treated almonds are uniformly larger and more photosynthetic with less senescence observed.

Besides the meristematic zones of the roots and apical tips, there is another place in the plant where CK is produced and plays very important roles. That is in the points where any new vascular tissue is being developed. Vascular tissue conducts the movement of water, nutrients and other solutes throughout the plant. CK is not only required for the formation and growth of new vascular tissue, it is also a regulator in the movement of photosynthates and other solutes through the phloem. Phloem tissue has associated companion cells which are responsible for metabolism and the regulation of phloem function including sugar transport. These companion cells are connected to the sieve tube elements and form bridges to exchange signaling for the transport of sugars from source to sinks. Simply stated, the presence of CK in younger tissue is what calls for the movement of energy and nutrition from the source to the sink.

Similarly, in flowering crops, CK is needed for the development of reproductive tissue and can increase bloom and flower retention. During these times in the growth cycle when new tissue is developing, a spike in the pool of available hormones is necessary. When flowering takes place in unfavorable conditions, the strength of the flowers and pollen is limited by the lack of CK. If adequate hormones are not available, the genetic potential of the plant is compromised. Pollen quality improves with higher levels of CK. Fertilization and the cell division which takes place immediately afterward require available CK. It’s well-established that the quality and size of fruits and nuts are improved by higher levels of CK for cell division at fruit set and the early growth stages afterward. A tighter cell density at this stage contributes to improved size and quality after cell enlargement and crop bulking take place.

Corn lower leaf senescence indicates shortages of cytokinin as growth hormone is translocated downward to support growth of roots. Brace roots also indicate a shortage of growth hormone.

In Research
Several crops have to flower during hotter weather. Summer heat can take a toll on crop yield. Molecular research into the effects of high temperatures on crops during pollination was conducted by Stoller Group, now a division of Corteva Agriscience. It was found that genes responsible for production of enzymes which break CK down by oxidation become upregulated in the plant during higher temperatures. Under better conditions, as previously mentioned, the amount of CK in the plant peaks at the time of flowering to assist in the process of fertilization. Lower CK during hot weather diminishes pollen strength and reduces the shedding of pollen, and the resulting weakened embryos have a higher potential to abort. Infertility of flowers can result from heat-induced male sterility.

Stoller found supplying exogenous CK provided significant improvement to pollination during heat events. This strategy has been used extensively to improve pollination and grain or fruit set across a wide variety of crops by Stoller agronomists since this discovery 10 years ago.

CK is a biostimulant with benefits that have been discovered and researched extensively. Because of the many natural and bioidentical products containing CK, it serves as a great example of how biostimulant products can enhance and improve plant performance at different stages of growth. Increased understanding of plant physiology, molecular biology and genetics has led to many validations of biostimulant technologies to improve crop production and quality. CK products are only one example of the biological and biostimulant products which are entering the market and emerging as solutions to the problems of water and nutrient use efficiency, stress management, weather resistance and other improvements to crop yield and quality.

New Instances of Herbicide Resistance in California Small Grain Crops

By
Nick Clark | UCCE Agronomy Advisor, Kings, Tulare and Fresno counties, Anil Shrestha | Ph.D., Professor, California State University, Fresno, Jennifer Valdez-Herrera | Graduate Student Researcher, UC Davis, and Katherine Waselkov | Ph.D., Professor, California State University, Fresno
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Figure 1. Herbicide-resistant common chickweed grows unscathed to the top of the triticale canopy in this field after it was treated with ALS inhibitor herbicide (all photos by N. Clark.)

Small grain cereals (wheat, triticale, rye, oats and barley) are planted each year in California on about 550,000 acres from the northern border near Oregon to the southern border near Mexico. These versatile crops can be harvested as grain, straw, hay, green-chop or silage. Today, much of the small grain cereals grown in California are used for dairy cattle feed. Dairy sales topped $7.6 billion in 2021, 14.8% of the state’s crop cash receipts.

Crop yield and quality reduction from weeds is a major concern of small grain producers. From 2015-19, PCAs recommended herbicides on an average of 941,000 small grain acres/year. Many fields were treated more than once for weed control (de Souza Dias et al. 2021). During this period, the most common herbicide modes of action on applied acres were acetolactate synthase (ALS) inhibitors (37.8%), synthetic auxins (31.8%) and protoporphyrinogen oxidase (PPO) inhibitors (17.7%). Major concerning weeds in California small grain crops include littleseed canarygrass (Phalaris minor), Italian ryegrass (Lolium multiflorum), shepherd’s-purse (Capsella bursa-pastoris), London rocket (Sisymbrium irio), mustards (Brassica spp.), little mallow/cheeseweed (Malva parviflora), coast fiddleneck (Amsinckia menziesii var. intermedia), burning nettle (Urtica urens) and common chickweed (Stelleria media).

In 2021, concerns from PCAs in the southern San Joaquin Valley became urgent as they noticed a pattern of failures to control common chickweed (escapes) several years in a row. Some fields were becoming overgrown with the weed (Figure 1).

Figure 1. Herbicide-resistant common chickweed grows unscathed to the top of the triticale canopy in this field after it was treated with ALS inhibitor herbicide (all photos by N. Clark.)

An Emerging Problem
As PCAs and pesticide product representatives noticed some ALS inhibitor herbicides weren’t working as well as expected to control common chickweed in small grains, they conducted field trials and reached out to UCCE Farm Advisors Nick Clark and Jose Dias and Specialist Brand Hanson. In 2021 when it was too difficult to dismiss the repeated escapes as mistakes in applications or poor environmental conditions, several UCCE Farm Advisors gave more attention to fields with escapes and conducted early evaluations attempting to repeat results PCAs and product reps saw in their trials.

The PCAs were concerned they were seeing effects of herbicide resistance. This is a genetic phenomenon occurring naturally as weeds are challenged to live when exposed to herbicides. Genetic variation in the weed population gives some individuals natural resistance to the damaging effects of the herbicide. The problem arises when the same herbicide is used repeatedly, and those naturally resistant individuals continue to reproduce and become dominant in the weed population.

The diligence of these PCAs is the reason the broader ag community alerted early to this potentially serious problem.

Early Warning Grows into Full-Blown Research
Early testing in a field with high common chickweed pressure and documented escapes from ALS inhibitors showed Clark, Dias and Hanson that herbicide resistance could not be ruled out as a potential cause of the escapes. In that field, Clark and Dias applied tribenuron methyl (Dupont Express Herbicide with Totalsol Soluble Granules) and pyroxsulam (Simplicity CA), two frequently used ALS inhibitors in California, at two and four times the highest allowable label rate. Clark and Dias observed that tribenuron slightly reduced the common chickweed growth and pyroxsulam had virtually no effect when compared to an untreated control (Figure 2). This evidence warranted more intensive studies on common chickweed populations with ALS inhibitor escapes. Clark partnered with California State University, Fresno Professor Anil Shrestha to conduct controlled environment trials. These were designed to learn whether these weeds were resistant to ALS inhibitors.

Figure 2. Early results from a replicated field trial could not rule out the possibility of ALS inhibitor herbicide genetic resistance in common chickweed found in triticale in the southern San Joaquin Valley. “UTC” means “untreated control.” Skinny bars represent one standard deviation. Treatments under similar lowercase letters are not significantly different from each other according to Tukey’s HSD test (a= 0.05).

Seeds were collected by Clark and technicians Ben Halleck (UCCE) and Walter Martinez (Tulare County) from common chickweed in ALS inhibitor escape fields and an organic field where there was no recent ALS inhibitor application. The organic seeds were ALS inhibitor susceptible controls because their population was not pressured to evolve resistance. The weed seeds were planted in pots in a greenhouse and grown under a shade structure at Fresno State until they reached the ideal growth stage for ALS inhibitor control. After several experiment repetitions, Clark, Shrestha and Fresno State students Paola Vidales, Kiera Searcy and Jennifer Vidales confirmed what PCAs worried about. The common chickweed identified as ALS inhibitor escapes was in fact genetically resistant to tribenuron methyl and pyroxsulam (Heap 2023).

The UCCE and Fresno State team observed that the ALS inhibitor resistant common chickweed responded to increasing doses of pyroxsulam but not to tribenuron methyl. When the dose of pyroxsulam was increased up to eight times the max label rate, the treated common chickweed became more stunted and more of the plants died. Because the plants were not controlled with the maximum allowable rate, they are considered resistant. When using tribenuron methyl, the common chickweed plants sprayed with eight times the max allowable label rate appeared the same and survived as much as plants sprayed with half the label rate. Many of the plants survived the herbicide applications and eventually flowered, suggesting they may be reproductive.

The UCCE and Fresno State team held public field days at Fresno State. Product reps, PCAs and students gathered to observe the results of the study (Figure 3). The value of the field day was two-fold: 1) ag professionals and students learned about the confirmation of common chickweed populations in California that are genetically resistant to ALS inhibitors, and 2) the research team gleaned important insights from the ag professionals’ feedback. That feedback has been critical in guiding ongoing research.

Figure 3. Fresno State Masters student Jennifer Valdez-Herrera (right) teaches field day attendees about the findings of ALS herbicide resistance in common chickweed populations found in the southern San Joaquin Valley with real-life demonstrations of these weeds surviving various ALS inhibitor herbicide applications.

Future Directions
The two different responses between the herbicides gave clues to the researchers about the ways the plants were expressing resistance to the herbicides. Fresno State professor Katherine Waselkov, specializing in genetics of herbicide resistance, joined the team to begin exploring the genetic basis of this problem in California. The major question Waselkov is researching is whether the Central Valley populations of chickweed showing resistance to ALS inhibitors have mutations in the ALS gene, which codes for the enzyme targeted by the herbicides. By extracting DNA, conducting PCR and sequencing the entire ALS gene, the lab can detect possible resistance mutations that occur in this enzyme. Changes in several amino acids that directly interact with the herbicides are likely to cause resistance. These changes have been detected in other countries’ chickweed infestations (Marshall et al. 2010; Laforest and Soufiane 2018). Waselkov’s different approach will also screen for other less common mutations that could cause resistance.

Pyroxsulam and tribenuron methyl are not the only ALS inhibitor herbicides used in small grains. Industry professionals pointed out mesosulfuron-methyl (Osprey Herbicide), although used less commonly, is also regularly applied to small grains. Additionally, it was pointed out that small grain fields where common chickweed was resistant to ALS inhibitors were recently rotated out of or into alfalfa, a common rotation partner, where multiple ALS inhibitor herbicides are applied to control several important weeds. For these reasons, the UCCE and Fresno State team continues to expand the controlled environment study to determine the presence of ALS inhibitor resistance in common chickweed with more herbicide products.

Fresno State Masters student Jennifer Valdez-Herrera is earning a degree in plant science through further study into ALS inhibitor resistance in common chickweed in California. Studying under Shrestha, Waselkov and Clark, Valdez-Herrera is addressing all the issues above in addition to studying the impact that uncontrolled common chickweed has on yield and quality of small grains.

Growers and PCAs are apt to point out ALS inhibitors are a very important tool for controlling multiple serious weeds in small grains and alfalfa. Further, there are still several other herbicide modes of action registered for California small grains and alfalfa which have activity against common chickweed. The PCAs are the first line of defense in the field and the first consultants for knowledge about how to remedy weed problems. Our team continues to consult PCAs to guide future research endeavors in this field to make sure the solutions we seek are relevant and applicable to the industry.

References
Dias, J. L. C. dS., Clark, N., Mathesius, K., Light, S., Hanson, B., Lundy, M. E., Shrestha, A., 2021. Poor control of common chickweed with ALS-inhibitor herbicides reported in multiple small grain fields in the southern San Joaquin Valley. Is it a new case of herbicide resistance in California? UC Weed Science Blog. Retrieved 7/28/23, https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=50181.
Heap, I., 2023. The International Herbicide-Resistant Weed Database. Retrieved 7/28/23, http://weedscience.org/Pages/Case.aspx?ResistID=24253.
Laforest, M., and B. Soufiane. 2018. Coevolution of two sulfonylurea-resistant common chickweed (Stellaria media) biotypes with different mutations in the acetolactate synthase gene. Weed Science 66: 439-445.
Marshall, R., R. Hull, and S. R. Moss. 2010. Target site resistance to ALS inhibiting herbicides in Papaver rhoeas and Stellaria media biotypes from the UK. Weed Research 50: 621-630.

Groundwater Banking on California Croplands

By
Eryn Wingate | CCA, Board Member, Western Region Certified Crop Advisers
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On-farm groundwater recharge methods include winter flooding, skipped row flooding, subsurface reverse tile drainage, recharge ponds and unlined canals or irrigation ditches (photo courtesy Almond Board of California.)

California relies heavily on groundwater, with 40% of annual demand supplied by aquifers during non-drought years and upwards of 60% during dry years. Approximately 83% of people in California receive part of their annual supply from groundwater, and many communities are exclusively reliant on well water for both ag and domestic purposes(3). Chronic groundwater overdraft, estimated at 2 million acre-feet per year since the 1960s, leaves California communities and farmland vulnerable to water shortages and rising surface water and pumping costs(1).

Groundwater depletion also causes subsidence, or land sinkage, as dry aquifers collapse. Subsidence damages infrastructure, such as roads, powerlines, pipes and canals. The California Aqueduct’s flow capacity has reduced due to increasing subsidence rates, with some areas surrounding the infrastructure sinking almost 1.25 inches per month(4). Groundwater recharge efforts have begun to reverse subsidence in some regions, but aquifer degradation and sunken land is often irreversible, and recharge efforts fail. The collapsed aquifers are irreparably damaged and can no longer store water.

California experienced the worst drought on record from 2012 to 2015, exacerbating groundwater pumping overdraft, water shortages and subsidence. Recharge during rainy years does not replenish the loss, and between 2010 and 2020, roughly 28% of monitored wells in California declined by 5 to 25 feet(3,6,8). State officials responded by passing the Sustainable Groundwater Management Act (SGMA) in 2014 to bring pumping and recharge into balance by 2040. Local groundwater sustainability agencies, tasked with developing recharge implementation plans, will likely include Agricultural Managed Aquifer Recharge (AgMAR) to meet requirements and balance the groundwater pumping budget(7).

On-farm groundwater recharge methods include winter flooding, skipped row flooding, subsurface reverse tile drainage, recharge ponds and unlined canals or irrigation ditches. Traditional recharge methods like drainage basins require land dedicated solely to water percolation, limiting groundwater banking with high infrastructure construction costs and spatial constraints. Winter surface flooding on irrigated agricultural land can be implemented widely at low cost and with few infrastructure or management changes at the field level. With 5 million acres of California farmland suitable for AgMAR, groundwater banking can scale up dramatically if even a small percentage of acreage can dually purpose for recharge during rainy years(3,6).

University research and early commercial implementation on orchards, vineyards and other crops indicate AgMAR can effectively recharge aquifers and benefit production in multiple ways(3,5,6,7). AgMAR helps growers and landowners secure irrigation water availability for dry years while preventing further aquifer degradation and subsidence. When properly managed, recharge efforts can also improve soil quality by leaching excess salinity below the root zone. Well water quality also improves when clean stormwater dilutes nitrate and total dissolved solids (TDS) accumulated in the basin after years of ag chemical leaching and pumping overdraft. Better soil and irrigation water quality improve crop health and fertilizer use efficiency, leading to lower production costs or increased yield and crop quality.

Implementation
Growers and landowners implementing AgMAR must consider soil suitability for recharge, crop tolerance, water application timing and field management practices to protect both crop health and groundwater quality. Research and field studies conducted over the last 10 years can guide site selection and implementation, but land managers must evaluate their own soils and crops to adjust AgMAR protocols to fit the unique conditions at each ranch.

Site Selection
The Soil Agricultural Groundwater Banking Index (SAGBI), developed by Toby ‘O Geen and collaborators at UC Davis, provides a scoring system to determine farmland’s suitability for AgMAR6. SAGBI assigns scores in five categories, including deep percolation, root zone residence time, topography, chemical limitations and soil surface condition. The weighted average of scores in all categories is used to classify ag land as Excellent, Good, Moderately Good, Moderately Poor, Poor or Very Poor for agricultural groundwater banking. The best ground for recharge is on flat land with sandy or sandy loam soils. Ideally, the soil should have fast water penetration and infiltration, and little to no chemical limitations such as high salinity, nitrates or pesticide residue that could contaminate groundwater if leached into the basin(6,8).

Over 17.5 million acres of farmland in California have been scored using data from the USDA-NRCS Soil Survey Database, and 5 million acres were rated as Excellent, Good or Moderately Good for AgMAR. Most of the land suitable for recharge is found in the eastern Central Valley as well as some locations in Santa Maria, Salinas and Napa(6).

Land managers can look up their ground’s SAGBI score on UC Davis’ web-based mapping app at casoilresource.lawr.ucdavis.edu/sagbi/. Recharge suitability indicated by SAGBI scores should be verified on the field level by soil testing and site evaluation prior to AgMAR implementation.

Recharge methods from left to right: surface application to orchards, subsurface application to orchards and basins/water conveyance structures (illustrations by Jennifer Natali, courtesy Almond Board of California.)

Crop Suitability
Crop suitability for AgMAR depends on tolerance to soil saturation, crop value and financial risk, and likelihood of nitrate leaching due to typical fertilization patterns. Groundwater banking on agricultural lands is safest during winter dormancy or when fields are fallow. Surface applications typically flood the field with 6 to 8 inches of water that drains in a week or less depending on soil permeability(2,3,5). Oxygen levels in the soil decline due to standing water, and if saturation persists for too long, the crop’s roots may be harmed. Stressed and damaged root systems cannot absorb water and nutrients effectively, resulting in yield decline later in the season. Damaged roots are also more vulnerable to soilborne diseases such as phytophthora and fusarium, so fields with known disease pressure may not be good candidates for recharge(5,7). Growers can avoid root damage and even improve soil health by choosing appropriate fields and planning surface application to match soil drainage rates and crop tolerance thresholds.

Crops suitable for AgMAR include alfalfa, wine grapes, tomatoes, almonds and other tree nuts. Annual cropping fields can also be used for recharge during the fallow period between plantings. Alfalfa presents a good candidate for recharge because its relatively low value poses less financial risk compared with specialty crops if recharge damages production. AgMAR field experiments with alfalfa found no decline in root health or yield after winter flooding on well-draining soils, demonstrating growers can safely carry out recharge programs if the crop is in its dormant stage. Since alfalfa is sensitive to soil saturation during the growing season, researchers suggest rotating recharge sites to older crops scheduled for replanting the following year, especially if flooding events are expected in late winter or early spring(3,7).

Areas of soil suitability for groundwater banking in California taken from UC Davis web-based application. The application uses the Soil Agricultural Groundwater Banking Index to score specific farmland suitability.

Trees and vines are excellent candidates for groundwater banking, if flooded during winter dormancy, well before budbreak. Crop sensitivity to water logging varies with rootstock, but growers are advised to limit standing water duration to two days to avoid root damage. Wine grapes are fairly tolerant to soil saturation, and since they typically receive less nitrogen fertilizer than other crops, residual nitrate levels during the winter are low. Tomatoes, almonds and other tree nuts generally receive higher nitrogen application rates, posing greater risk to groundwater quality if excess nitrate leaches during flooding events. Low nitrate levels and saturation tolerance position vineyards as the safest crop candidate for recharge, but groundwater banking will be more effective if implemented on the state’s vast almond acreage as well(5,8).

UC Davis research funded in part by the California Almond Board demonstrated AgMAR efficacy on two commercial almond orchards in the Central Valley from 2015 to 20175. Researchers applied a total of 24 inches of water per year, split into multiple flooding events during winter dormancy for two consecutive years. One of the orchards, located near Dehli, Calif., has highly permeable, sandy soil. The second orchard, located in Modesto, has moderately permeable, sandy loam soil. Flood water at both locations percolated below the root zone in less than a week, and researchers found no negative impacts on tree water status, root health or yield at either site. Winter flooding proved an efficient recharge method, with over 90% of applied water percolating below the root zone on the sandy soil and over 80% percolation on the sandy loam. Similar studies on pistachios, wine grapes and tomatoes also resulted in effective groundwater banking while maintaining root health and productivity(2,3,5).

California almond growers have begun implementing AgMAR on their own orchards with promising results. Mark McKean, a prominent grower in the Fresno area, began testing flood irrigation and recharge in 2010, and by 2020 he had experimented with groundwater banking on 350 acres. In 2016, McKean banked 2 ac-ft/ac and in 2019 1.4 ac-ft/ac on a three-year-old almond orchard. McKean found better percolation when flood water was applied for short durations rather than one long set, and he reduces his fall nitrogen application rate on fields slated for winter recharge to prevent excess nitrate leaching below the root zone(7).

When properly managed, recharge efforts can also improve soil quality by leaching excess salinity below the root zone (photo courtesy Almond Board of California.)

Other growers have installed subsurface groundwater banking systems to pump recharge water below the root zone and bypass the risk posed by surface flooding. Diverted stormwater or other source water is piped down to a reverse tile drain system at least 8 feet below the soil surface. Several subsurface systems have been installed in the Central Valley since they were introduced in 2017, and some water districts offer incentives programs to help cover the costs. Subsurface recharge systems are expensive to install, but they facilitate high recharge volumes without impacting roots and crop health(7).

Water Quality Considerations
Groundwater banking on ag lands has the potential to significantly improve water resource security in California, but AgMAR poses a risk to groundwater quality if excess salts, nitrates and other residual contaminants are leached down to aquifers. Preliminary soil testing can assess field suitability for recharge before each rainy season, and sites with excess nitrate, salinity or pesticide residue may be passed up in favor of sites with optimum chemical characteristics. Fields with higher residual contamination may still be good candidates for recharge if enough clean stormwater is available to dilute nitrate and salinity down to safe drinking water quality standards. Growers can also prepare for winter recharge by reducing the proportion of annual nitrogen fertilizer applied in the fall.
Residual contaminants in the vadose zone, the unsaturated area between the soil and the groundwater table, may also pose a risk to groundwater quality. Salts, nitrate and ag chemicals accumulated in the vadose zone after years of commercial agriculture may be mobilized by high-volume recharge events and leach down to the groundwater basin. Hannah Waterhouse and colleagues at UC Davis analyzed soil core data down to 30 feet on 12 fields in the Kings groundwater basin to quantify potential risk of nitrate and salt contamination to aquifers8. The study compared the effects of soil permeability, crop type and fertilizer management on nitrate and salt accumulation in topsoil and below. Fields with lower water infiltration rates had higher nitrate and salinity levels compared with more permeable ground. Soils with slower water infiltration rates stored on average 732 lbs N/ac while lighter, well-draining soils stored 542 lbs N/ac within the 30-foot profile(8). Information gleaned from this study and other research can help determine the source and volume of water required for recharge at each site to ensure that leached contaminants are sufficiently diluted to protect well water quality.

Crop type and grower management also strongly affected nitrate and salinity levels. High nitrogen application rates on tomatoes and almonds were reflected in the soil profile, while wine grapes with lower N applications and deep root systems almost always contained the lowest nitrate levels. Elevated residual nitrate found on one outlier vineyard was explained by the grower’s fertilizer management. While other wine grapes received split N applications, the field with unusually high residual nitrate received the entire year’s N supply in one shot at the beginning of the season, demonstrating management’s strong impact on nitrate leaching8. Regardless of crop type, growers implementing AgMAR can protect underlying groundwater by testing the soil’s N level in fall and adjusting fertilizer management to prevent nitrate leaching.

Groundwater quality monitoring and collaboration between growers, researchers and water agencies will help to safely implement AgMAR and improve recommendations to meet differing requirements at each ranch. Further research is required to understand how the vadose zone’s characteristics will impact groundwater quality in response to AgMAR, but initial studies indicate that the benefits of recharging our groundwater basins outweigh the potential risks when appropriate sites and field management strategies are implemented.
Average groundwater overdraft in California is estimated at about 2 million acre-feet per year, and from 2005 to 2010, the Central Valley alone overdrafted an estimated 1.1 to 2.6 million acre-feet(1,6). Pumping restrictions required by SGMA may cause between 750,000 and 1 million acres of agricultural lands to go fallow without new supply mitigation measures(7). Agricultural lands rated as Excellent or Good by SAGBI can percolate an estimated 1 foot of water per day, and if AgMAR were implemented on suitable wine grape acreage in the Central Valley, growers could bank 460 million acre-feet of water per day(6,8). AgMAR implementation at scale will require supply rights and infrastructure to divert excess stormwater to agricultural fields, but SGMA funding and compliance deadlines will likely motivate stakeholders to facilitate on-farm groundwater recharge efforts. Thousands of acres of wine grapes, almonds, alfalfa and tomatoes are planted on land suitable for groundwater banking, giving growers an opportunity to secure water resources for future crop production and their communities.

References
1. [CDWR]California Department of Water Resources. 2009. Bulletin 160–09: California water plan update. Sacramento (CA): California Department of Water Resources. http://www.waterplan.water. ca.gov/cwpu2009/.
2. Levintal E, Kniffin M, Ganot Y, Marwaha N, Murphy N, Dahlke H (2022): Agricultural managed aquifer recharge (Ag-MAR)—a method for sustainable groundwater management: A review, Critical Reviews in Environmental Science and Technology, DOI: 10.1080/10643389.2022.2050160
3. Dahlke H, LaHue G, Mautner M, Murphy N, Patterson N, Waterhouse H, Yang F, Foglia L. 2018. Chapter Eight – Managed Aquifer Recharge as a Tool to Enhance Sustainable Groundwater Management in California: Examples From Field and Modeling Studies. Editor(s): Jan Friesen, Leonor Rodríguez-Sinobas. Advances in Chemical Pollution, Environmental Management and Protection. Elsevier, Volume 3: 215-275. ISSN 2468-9289. ISBN 9780128142998. https://doi.org/10.1016/bs.apmp.2018.07.003.
4. Lopes et al. 2017. California Aqueduct Subsidence Study. California Department of Water Resources, Division of Engineering, San Luis and San Joaquin Field Divisions. https://water.ca.gov/-/media/DWR-Website/Web-Pages/Programs/Engineering-And-Construction/Files/Subsidence/Aqueduct_Subsidence_Study-Accessibility_Compatibility.pdf
5. Ma X, Dahlke H, Duncan R, Doll D, Martinez P, Lampinen B, Volder A. 2022. Winter flooding recharges groundwater in almond orchards with limited effects on root dynamics and yield. Calif Agr 76(2):70-76. https://doi.org/10.3733/ca.2022a0008.
6. O’Geen AT, Saal M, Dahlke H, et al. 2015. Soil suitability index identifies potential areas for groundwater banking on agricultural lands. Calif Agr 69:75– 84. https://doi.org/10.3733/ ca.v069n02p75
7. Roseman J, Lee E, Asgil L, Mountjoy D. 2021. Almond Board of California, Document #2021R0060. https://www.almonds.com/sites/default/files/2021-12/WO-6177_ABC_GroundwaterRecharge_Web_SinglePage.pdf
8. Waterhouse H, Bachand S, Mountjoy D, Choperena J, Bachand P, Dahlke H, Horwath W. 2020. Agricultural managed aquifer recharge — water quality factors to consider. Calif Agr 74(3):144-154. https://doi.org/10.3733/ca.2020a0020.

 

The Dynamic Duo: Exploring the Synergy between Irrigation and Nutrient Management

By
Scott Warr | Commercial Manager, Digital Farming, Yara
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Advancements in irrigation systems, such as drip, micro and pivot systems, have improved water distribution and incorporated fertigation (photo by Taylor Chalstrom.)

As agriculture faces increasing pressure to produce more food with less resources, the role of irrigation and nutrient management has become ever more critical. Efficient irrigation and nutrient management practices are essential not only for maximizing crop yield and quality, but also for promoting sustainability and minimizing environmental impacts. In this article, we will explore the interconnected role of irrigation and nutrient management in agriculture and how growers and advisors can implement strategies to improve their efficiency and effectiveness. By understanding the relationship between these two critical factors, we can promote sustainable agriculture while ensuring food security for generations to come.

Years ago, as I was interviewing many farm managers and their advisors to better understand their irrigation practices, I kept hearing one common statement: “The fastest way to compromise a great nutrition plan is to irrigate improperly.” Efficient irrigation management is crucial to minimize water losses, optimize nutrient use efficiency, improve soil health and increase grower profitability. Their goal is to manage irrigation by applying it at the proper time and rate for the specific crop demand and soil conditions. Excessive watering can cause waterlogging, nutrient leaching, soil erosion, disease and decreased crop yields. Conversely, insufficient watering can result in stunted growth and reduced harvest.

Agronomists have accepted and are committed to the 4Rs of Nutrient Management. While traditionally our 4Rs focus has been on the nutrients delivered with fertilizers, we can use the same paradigm to manage the equally essential, and in some crop systems more limiting, nutrients of hydrogen and oxygen delivered in the form of H2O.

The same paradigm of nutrient management can be used for irrigation management when thinking about right source, right place, right time and right rate. Efficient irrigation management is crucial to optimize nutrient use efficiency.

Right Source
Choosing the right source of water for irrigation is crucial. Water quality can vary significantly, and it is essential to consider factors such as salinity, alkalinity and potential contaminants. Testing the water source and ensuring it meets the required quality standards will help prevent adverse effects on soil health and plant growth. Growers and advisors should consider the following factors regarding water quality:

Salinity
High salt concentration can harm plants, reduce crop yield and quality, and affect soil health. Use electrical conductivity (EC) or total dissolved solids (TDS) meters to measure salinity and manage it through leaching, salt-tolerant crops or water treatment.

pH
Water acidity or alkalinity affects nutrient availability, uptake and soil health. Maintain a pH range of 6.0 to 7.5 through pH-adjusting chemicals or selecting pH-tolerant crops.

Nutrient content
Nitrogen, phosphorus, and potassium levels in water impact plant growth and nutrient management. Adjust fertilizer rates or choose crops suitable for specific nutrient levels.

Pathogens and contaminants
Water may contain harmful bacteria, viruses and heavy metals that affect plant and human health. Implement water treatment, testing and monitoring practices.

Water availability
Consider the source, quantity and timing of water for irrigation. Implement water management practices to ensure availability throughout the growing season.

Water quality is vital for agricultural irrigation. Though growers cannot control the quality of their water source, they can monitor and adjust it as needed. Consider all relevant factors to ensure suitable water for crop growth without posing risks to plants or human health.

Right Place
The “right place” in irrigation management involves effectively delivering water and nutrients to the plant’s effective root zone. Advancements in irrigation systems, such as drip, micro and pivot systems, have improved water distribution and incorporated fertigation (applying fertilizers through irrigation.) Fertigation increases nutrient efficiency, reduces waste and promotes soil health. Regular maintenance ensures high distribution uniformity, avoiding uneven irrigation. To evaluate distribution uniformity, contact your local Natural Resources Conservation Department or refer to this resource: ucanr.edu/sites/farmwaterquality/files/156399.pdf. Proper installation, maintenance and monitoring optimize the right place for uniform water and nutrient distribution, maximizing crop yield and sustainability.

Right Time
Knowing how much and when to turn on irrigation is crucial for maximizing water efficiency, promoting healthy plant growth, and optimizing crop yield. Consider the following factors:

Crop water needs
Understand the specific water requirements of each crop, considering different growth stages and their corresponding water demands. This knowledge helps determine when irrigation is necessary for optimal crop development.

Soil moisture monitoring
Regularly monitor soil moisture levels using sensors or visual inspection techniques. This information identifies when the soil has dried sufficiently to require irrigation, avoiding both overirrigation and underirrigation.

Weather conditions
Monitor weather forecasts and local climatic patterns. Factors like temperature, humidity, wind and solar radiation influence evapotranspiration rates, affecting water loss from the soil and plants. Adjust irrigation timing based on anticipated water loss.

Plant stress indicators
Observe signs of water stress, such as wilting, leaf rolling and changes in leaf color, to determine irrigation needs. Providing water at the right time prevents water stress, promotes optimal plant growth and minimizes crop yield losses.

Remote plant stress monitoring
Innovative technologies using sensors, aerial imagery or satellite data enable real-time monitoring of plant stress levels. Adjust irrigation timing based on these insights, improving water efficiency and crop performance.

Irrigation scheduling techniques
Utilize techniques like soil moisture-based scheduling, crop evapotranspiration (ET) data or plant water demand. These tools guide when to irrigate, considering crop needs and environmental conditions.

Water conservation considerations
In water-limited regions, time irrigation to maximize water use efficiency. Avoid peak water demand periods, applying water during cooler, less evaporative periods to minimize water loss and optimize utilization.

By considering these factors, growers can determine the appropriate timing for irrigation, ensuring crops receive adequate water when needed the most. This approach maximizes water efficiency, conserves resources and promotes healthy plant growth and optimal crop yield.

Right Rate
Once we know the amount of water the plant needs and when, we need to determine how frequently and how long to apply the water so that we do not have runoff or infiltration below the effective root zone. This might be an area for most improvement. By determining the appropriate rate, we can ensure that water and nutrients remain within the effective root zone, where plants can efficiently utilize them. This minimizes leaching and evaporation, reducing loss and waste.

To determine the right rate of irrigation:

  • Understand soil characteristics, including type, infiltration rate and water holding capacity
  • Determine the irrigation application rate specific to your system
  • Consider the water demand of the crop

Several tools can aid in developing an effective irrigation schedule. These include evapotranspiration models, soil moisture monitoring and plant-based sensors that track water and nutrient uptake. By utilizing these approaches, farmers can align irrigation events with actual plant and soil water needs, maximizing water use efficiency.

As agriculture strives to meet the growing global food demand while conserving resources, the proper management of irrigation and nutrients has emerged as a critical aspect. This article has emphasized the importance of efficient irrigation and nutrient management practices for achieving optimal crop production, maintaining high-quality harvests and reducing environmental harm. By adopting the 4Rs of Irrigation Management that improve the efficiency and efficacy of these practices, growers and advisors can contribute to sustainable agriculture. Through a comprehensive understanding of the interplay between irrigation and nutrient management, we can pave the way for a future where agriculture meets the needs of the present while safeguarding the needs of future generations.

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