Effect of Nitrogen Fertilization Practices on Spring Wheat Yield, Protein Content

Executive Summary. Wheat is an important commodity for California’s economy. Approximately 800,000 acres of wheat are grown every year, 75 percent of which is grown in the Southern San Joaquin Valley. Growers seek a combination of high yield and high protein content if possible to improve the profitability of wheat production. This combination can be difficult to achieve, however, especially with some of the newer, higher-yielding wheat varieties. The price that a producer receives for hard red spring wheat is determined by the grain protein content, with a premium for grain protein greater than 14% in northern California and 13% in the southern San Joaquin Valley, and a discount for wheat with less than the required grain protein (Orloff and Wright 2011). The struggle to reach both high yields and high protein percentages can have economic consequences for wheat producers, and can be a significant incentive for growers to consider higher fertilizer N applications in order to increase their chances of achieving higher grain protein percentages. However, in the past year, these applications of nitrogen (N) to cropland in the form of fertilizers or manure have been given increased attention due to groundwater nitrate studies by the United States Geologic Survey and UC Davis researchers. These studies claim that agricultural applications of N are largely responsible for the high nitrate levels observed in groundwater in the study areas (Tulare Basin and Salinas Valley), with the implication that some similar losses are also possible in other crop production regions within California.

New research is needed on commonly grown wheat varieties to evaluate N management approaches that could impact grain yield and protein levels. The objectives of this proposed study are to determine the best rate and timing of split nitrogen applications to achieve economically viable high yields, to maximize protein content, and to improve nitrogen use efficiency. Proper nitrogen fertilizer management decisions ideally should depend on: (1) the range of crop nitrogen requirements that can achieve acceptable crop yield and quality; (2) practices that consider and improve utilization of residual as well as applied nitrogen; and (3) practices that limit or prevent nitrogen losses or excess leaching below crop active root zones.

We will investigate the feasibility of different combinations of split application amounts and timing in this study. This study will also focus on the collection of soil samples to a depth of six to eight feet in the soil profiles at pre-plant or close to planting timing and again post-harvest to determine changes in soil nitrate-N status at different depths associated with wheat crops grown under different described N management schemes. This data will be used to provide a crop nitrogen utilization estimate (applied N plus estimate of residual soil nitrate-N use) during the growing season, and to describe whether or not management practices used produce net gains (additions) or net losses (use) of nitrate-N in different parts of the soil profile. These evaluations can provide estimates of quantities of nitrate-N that may be prone to leaching losses if rainfall or irrigation water applications produced deep percolation. To accomplish these research objectives a series of trials are proposed for widely different production areas, in this case including the intermountain area of northern California and the central and southern San Joaquin Valley. Several wheat varieties prominent in each respective production area will be evaluated to determine their yield and protein content with and without late-season N applications. In northern California, three varieties of hard red spring wheat will be evaluated, including Yecora Rojo and two others to be determined. In the central and southern San Joaquin Valley locations, one hard red spring, one hard white spring, and one durum spring wheat variety will be evaluated. Varieties have yet to be chosen.

This project will be considered successful upon the development of a new set of N management guidelines that can be available for field testing by growers. The success of this project would equip California wheat producers and regulatory agencies with the information on N applications needed to produce the most profitable crop while reducing the dangers of N leaching.


Justification. The primary production factors that affect protein content are cultivar selection and N fertility management. The struggle to reach both high yields and high protein percentages can have economic consequences for wheat producers, and can be a significant incentive for growers to consider higher fertilizer N applications in order to increase their chances of achieving higher grain protein percentages. With irrigation or with higher rainfall, yields generally increase and grain protein decreases somewhat with increasing water availability. As available water increases with irrigation or in higher rainfall years, conditions are created with improved potential for higher grain yields (through increases in number of heads, seed number per head, and seed size). When limited soil N reserves are spread across more seed in higher yield conditions, grain protein levels remain static or even decrease. It is often seen that under low to moderate soil N availability conditions, added nitrogen fertilizer at first improves yield and even reduces protein content until available N levels in the soil are raised to levels adequate to fully support development of all set seed. Research is required to better understand these processes and the availability of N (from residual, mineralized and applied N) and their effects on yield and protein content. Sometimes growers over-apply N to achieve both yield and protein goals in fewer applications, but this can lead to inefficient fertilizer use and reduced profitability, and could be the cause of unwanted environmental consequences such as possible nitrate leaching. Currently, there are issues concerning nitrate leaching in ground water, especially in the southern San Joaquin Valley, but eventually these concerns may be considered in other wheat production areas in the state, bringing the impact of this research to the statewide level. This project is directly linked to the Fertilizer Research and Education Program’s wish to increase the availability of information about the most efficient use of fertilizers like N.

One approach that could be important in improving use of late-season N applications is to try to match late season N availability to meet yield goals or yield history for the production area. It is important to make enough nitrogen available for crop needs without applying excess nitrogen. Applications should be adjusted for yield goals or likely yield potential. The following examples illustrate the impact of yield potential on the amount of actual N contained in the grain protein if protein levels were raised from 13 percent up to 15 percent. The higher the yield, the more applied N would be needed late-season to affect protein percentages. Raising the yield and protein content through the efficient application of N would provide the most advantageous economic impact for California’s growers.

Past research projects have estimated that the amount of total available N required for favorable yields and protein ranges for hard red spring wheat are at least 1.6 lbs of total available N per bushel at lower yield levels to over 2.0 lbs total available N per bushel (or about 3 lbs N per 100 lbs of grain). In good production years, grain yields in CA can often be in the range of 3.5 to nearly 5 tons/acre. Research conducted in the 1980’s by University of California researchers to improve grain protein focused primarily on moderate yielding / higher protein wheats such as Yecora Rojo (Wright et al. 2008, Munier 2006). Newer varieties can often yield about a ton/acre higher than varieties such as Yecora Rojo, but most are lower in protein. Late-season N has been shown to improve protein but the levels needed to consistently achieve higher protein and yield combinations are not well established for most areas of the state. Research is needed to evaluate the effectiveness of late-season N application to improve the protein content of a range of varieties with different yield and N accumulation characteristics. Wheat N studies conducted in 2012 by Orloff, Wright, and Hutmacher were consistent with earlier studies in that they demonstrated that varieties differed in N uptake and yield potential, heavily depending on the soil type of each particular location.

As an indication of how recommendations derived from N management research projects can be utilized as guidelines for adjusting N fertilizer management practices, we can direct attention to a similar research project, Field Evaluations and Refinement of New Nitrogen Guidelines for California Cotton that was conducted between 1996 and 2001 by UC Cooperative Extension personnel, including two of the project investigators for this proposed project. The cotton commodity group funded that work between 1996 and 2001, and a follow-up study was partially sponsored by FREP from 2001 through 2003. The results of that cotton research, which concerned the uptake of N in cotton, soil nitrate uptake and accumulation patterns with different N management practices, and cotton growth and yield responses to supplemental N, have since been integrated into the general practices for many California cotton growers. Much of this research was communicated to the public in several professional society conferences (Hutmacher et al. 2000, Roberts et al 2001), and in publications such as “Response of Acala Cotton to Nitrogen Rates in Energy Production and Environmental Protection” in 2002 (Hutmacher et al. 2002). The same research approach applied to wheat could provide similar benefits and refined guidelines for wheat producers. The information provided by this project could improve the knowledge base for California wheat producers and regulatory agencies, better defining N applications needed to produce the most profitable crop (in terms of yield and quality) while reducing the dangers of N losses associated with possible nitrate leaching.

There also are economic incentives for growers to adopt and test the guidelines that result from this study. Wheat production is only economically viable if growers maintain high yields with low-input costs.

With the high cost of fertilizers and their application, growers need to maximize N use efficiency while at the same time minimize the number of fertilizer applications. This project will address the issue of input costs of such fertilizer applications while evaluating the trade-off between protein content and yield. By the end of the project, researchers will hopefully have identified some economically favorable applications, which can then be integrated into the practices of California’s wheat industry.


Objectives

The objectives of this research are to:

1. Compare the yield and protein content responses of the most popular hard red, hard white, and durum spring wheat varieties to a range of nitrogen application treatments that provide N at specific times in the production season (pre-plant, tillering, boot and flowering growth stages). 2. Evaluate the effectiveness of different split applications of N for increasing grain protein in two distinct production regions (northern CA intermountain region, central San Joaquin Valley region).
2. Measure residual pre-plant soil nitrate-N and use that information to adjust pre-plant nitrogen fertilizer application amounts in each field trial location.
3. Determine quantities of nitrate-N occurring at different depths in the soil profile as a function of location/soil type and time of season, focusing on data collection to estimate potential for nitrate accumulation or movement below the root zone and susceptible to deep percolation losses.
4. Compare results of soil nitrogen quick test for in-season soil nitrate-N evaluations in the 0 to 2 foot zone in the soil profile with those of standard air-dried samples for nitrate-N using laboratory methods.
5. Measure response of lower stem nitrate content, flag leaf total N and total N of the leaf one below the flag leaf to late season available and applied N across some of the major N application treatments at study sites, focusing on sampling 1-2 weeks prior to heading, at heading and at flowering and determining if tissue N values can be used as indictors of likely protein responses to split N applications as done in these study sites.

Methodology

Work Plans and Methods
Trial sites, fertilizer treatments and data collection will now be described.

Applied Fertilizer Nitrogen Treatments:

a. The following nitrogen fertilization treatments will be established to investigate impacts of different total N applications and timing of split applications on grain yields, grain protein, and impacts on soil nitrate utilization and zones of use and accumulation in the soil profile.
b. Preplant applications of nitrogen will be adjusted to account for residual soil nitrate in the preplant soil sampling for each site. N application adjustments will consider estimates of available soil N – measure soil nitrate-N pre-plant in upper 2 to 3 feet of soil profile (and ammonium-N, if reasonable for how samples have to be handled), and use these values to modify baseline initial fertilizer.
c. Applications may be adjusted in consideration of possible yields, yield goals (adjust fertilizer N rates to supply adequate amounts for high yields for the combination of season length, soil conditions and available suitable varieties).

Varieties to be tested:
Varietal differences in responses to different timings and amounts of nitrogen applications will be evaluated within each of the nitrogen treatments imposed in this study. Varieties will be chosen that are appropriate for and suited to the different production areas. Prominent varieties in the Intermountain area are different from the commonly used varieties in the San Joaquin Valley production area. In the northern California location, three varieties of hard red spring wheat will be evaluated, including Yecora Rojo and two others to be determined. In the central and southern San Joaquin Valley locations, one hard red spring, one hard white spring, and one durum spring wheat variety will be evaluated. Varieties have yet to be chosen.

Plot Size, Layout:
Research trials will be established in the southern San Joaquin Valley and the Intermountain Region. All treatments utilized (variety by nitrogen treatment combinations) will be replicated four times. Each plot will be approximately 10 by 25 feet in size and will be arranged in a randomized complete block design. An analysis of variance will be performed on all data.

Plant Tissue Sampling:
The focus of plant tissue sample collection to clearly identify degree to which can detect if there are treatment effects on plant tissue N content that are consistent enough to use as relative indicators of plant N status (lower stem nitrate levels, flag leaf, leaf below flag leaf, focusing on sampling 1􀍲2 weeks prior to heading, at heading and at flowering). Sampling methods will be similar to those described by Tindall et al (1995).

Deep Soil Sampling for Nitrates – Pre-plant and post harvest:
This study will focus on collection of soil samples at pre-plant or close to planting timing and again post-harvest to allow determination of changes in soil nitrate-N status with depth in the soil profile associated with growing wheat crops under the different described N management schemes. We will use a Giddings soil sampler to collect samples to a depth of eight feet below grade in order to closely monitor the presence of N in the soil at and below the crop root systems. This data will be used to provide a crop nitrogen utilization estimate (applied N plus estimate of residual soil nitrate N use) during the growing season, and to describe whether or not management practices used produce net gains (additions) or net losses (use) of nitrate􀍲N in different parts of the soil profile. These evaluations can provide estimates of quantities of nitrate􀍲N that may be prone to leaching losses if rainfall or irrigation water applications produce deep percolation.

Soil Residual Nitrate Quick Test:
The quick-test method has been used previously and effectively in a number of crops and situations to measure residual soil nitrate-N in surface soils in vegetable crops in California (Hartz 2000) and in multiple agronomic crops in the Midwestern and eastern U.S. (Roth et al. 1991; Magdoff 1991). The basic approach in these tests includes: (a) collection of soil samples in the field; (b) extracting the nitrate in the soil using a weak salt solution; (c) use of a nitrate sensitive test strip placed into the extracted soil solution; and (d) use of a colorimetric test / reflectometer to evaluate color development, which has been shown to be correlated with solution nitrate concentrations. Part of the reason to emphasize evaluations of performance of the soil nitrate quick test versus laboratory analysis methods on air dried soil samples is to determine if these tests could provide an additional low-cost method to estimate residual soil nitrate that could be recommended for use by wheat growers in California. The emphasis of these soil test evaluations will be to: (a) assess the degree of correlation between quick tests and laboratory nitrate analyses; and to (b) analyze the relative utility of the quick test method as a readily available and relatively low cost method to double-check soil nitrate-N status at key times during the growing season to improve grower capabilities to assess and revise nitrogen management recommendations. If the performance of the tests is acceptable and costs relatively low, the approach will be to encourage growers to increase the number of times and distribution in the field of soil nitrate monitoring for improving management decisions.

Experimental Sites:
Field trials will be located in two distinctly different production locations in the state: (a) the intermountain area of northern CA (Siskiyou County and/or Modoc County); and (b) the central San Joaquin Valley wheat production area (Kings, Fresno Counties). The selection of the sites will be focused on moderate to high yield potential sites for each of the production areas. In area (a), experiments are planned at the University of CA Intermountain Research and Extension Center in Tulelake, CA and in one grower field located in Scott ¡¿À¡¿Valley, Siskiyou County; and in area (b) experiments are planned at the University of CA West Side Research and Extension Center and in one grower field in either Kings or Tulare County.