Optimizing winter wheat grain yield and protein concentration with weather-responsive nitrogen management in semi-arid dryland systems

Winter wheat production in eastern Colorado’s semi-arid environments is constrained by highly variable precipitation, heat stress, and nutrient limitations, particularly nitrogen (N). Nitrogen is the primary driver of both grain yield and grain protein concentration (GPC). However, the inverse relationship between yield and GPC, commonly known as the “dilution effect,” creates a management challenge under variable weather (Simmonds, 1995; Bogard et al., 2010). As yields increase in years with sufficient-to-above-average precipitation, starch accumulation in the grain typically outpaces protein deposition, causing GPC to decline. This can push GPC below market thresholds (commonly 11.5–12.5% for hard winter wheat), resulting in price discounts despite higher yields (Lollato et al., 2019).

Consistent GPC is essential for milling and baking quality because gluten proteins (gliadins and glutenins) determine dough strength and loaf volume (Khalid et al., 2023). Lower protein levels also reduce the nutritional contribution of wheat-based foods to human health, which provide a significant portion of dietary protein worldwide (Shewry & Hey, 2015).

Modern winter wheat varieties have higher yield potential due to improved drought tolerance, harvest index, and disease resistance (Graybosch & Peterson, 2010; Battenfield et al., 2013; Boehm et al., 2023). These genetic advances increase N demand, yet many regional recommendations have not been fully updated to account for both higher yield potential and weather variability (Maeoka et al., 2020; Lollato et al., 2021).

This study evaluated N response in three Colorado hard winter wheat varieties, Canvas and Langin (hard red) and Snowmass 2.0 (hard white), across 14 diverse environments in eastern Colorado during the 2019 and 2020 growing seasons. 

The objectives were to:

1. Determine agronomic optimal N rates (AONR = soil N [0–12 inches] + fertilizer N) that balance high grain yield with minimal protein dilution;
2. Assess how soil properties and weather conditions modulate performance at optimum N management.

Methodology 

Trials were conducted at 14 environments (site-year combinations) in eastern Colorado during 2019–2020 across eight locations (Arapahoe, Burlington, Genoa, Julesburg, Lamar, Roggen, Sheridan Lake, and Yuma). Two sites (Akron and Orchard) were excluded due to sawfly damage compromising the study. These environments encompassed a range of soil types, tillage practices (no-till, minimum-till, vertical tillage, and conventional tillage), crop rotation, and climatic conditions representative of the West-Central Great Plains dryland wheat belt (Table 1). 

^a All sites included fallow prior to winter wheat planting.

Growing season precipitation (April to harvest) ranged between 5.7 and 9.1 inches (2019) in the “wet year” and 1.6–6.4 inches in the “dry year” (2020) (Figure 1). Growing degree days (GDD) from planting to harvest ranged from 2194–2784 °C in 2019 and 2238–2939 °C in 2020. Weather data were collected from on-site automated weather stations installed at each trial location and supplemented with data from the Colorado Agricultural Meteorological Network (CoAgMET).

Three winter wheat varieties (Canvas, Langin, and Snowmass 2.0) were assessed and four nitrogen fertilizer rates (0, 45, 90, or 135 lb N/ac) were applied as urea ammonium nitrate and top-dressed in early spring. Pre-plant N applications were determined by the farmer (generally urea applied pre-plant in the fall and an additional 8 lb N/ac at planting as 8–28–0).

Plots were harvested using a modified Case IH 1620 small plot combine equipped with a Harvest Master H2 GrainGauge to collect experimental unit grain weight, grain moisture, and grain test weight. Grain samples were collected from each plot at harvest to assess protein values post-harvest using a Foss grain analyzer.

Total available N was calculated as the sum of applied N, residual soil nitrate (NO₃⁻) from 0–12 in depth, and organic matter OM credit (estimated at 30 lb N/ac per 1% soil organic matter). When considering AONR calculations, fertilizer N rates (including N pre- and at-plant) plus soil N (0–12 in) were considered. 

For each location–year–variety combination, linear and quadratic regression models were fitted to assess grain yield responses to total available N. The relationship between grain yield (bu/ac) and grain protein concentration (GPC, %) was modeled using linear regression. To assess crop performance levels (10, 50, and 90%), we modeled conditional distribution of wheat grain yield at AONR to total available N using quantile linear regression. 

Results 

The concept of grain protein dilution was confirmed, in which a negative relationship exists between grain yield and GPC (Figure 2, left panel). However, the intensity of this relationship was modified by N treatment; higher N application rates (90 and 135 lb N/ac) were associated with a lessened severity of protein dilution at higher yields. This pattern was consistent across the three varieties, which all had similar responses in yield-protein trade-off and benefit from N fertilization. Considering the economic input of additional N fertilization, the protein premium needed to cover N fertilizer cost decreases as yield increases (Figure 2, right panel). At low yields, the required protein premium needed to cover N fertilizer cost is larger. However, at higher yields, the required premium is decreased; thus at higher yields, increased N applications are more economically justified, even with a smaller protein increase. 

The results of this study demonstrate that N management influences grain yield and GPC, but crop responses vary depending on environmental conditions, particularly precipitation and baseline soil conditions. The N level that maximized GPC to the commercially desirable 12% protein was on average 15.5% higher than that needed to maximize yield (232 lb N/ac for 12% protein vs. 201 lb N/ac for yield at AONR), indicating a higher N input needed to avoid protein dilution in higher-yielding years (Figures 3 and 4). To push GPC to its absolute maximum, the agronomic optimum N rate (GPC AONR) averaged 272 lb N/ac—35% more than the yield at AONR (Figure 4). These findings indicate that achieving commercially acceptable GPC levels often demands more N than maximizing yield alone, especially in years with adequate growing conditions. 

In the drier 2020 season, soil clay content and soil organic matter (SOM) emerged as the most important factors for maintaining higher wheat yields at the agronomic optimum N rate. Under drought conditions, higher clay percentages and greater SOM helped buffer yield losses, while high residual soil nitrate-N reduced yields. This contrasts with the wetter 2019 season, where rainfall and pre-plant soil nitrate were the dominant drivers of grain yield grains. These findings underscore the value of building soil health, particularly through practices that increase SOM, to provide additional soil N supply and to improve resilience during dry years common in the semi-arid Great Plains.

Grain yield at AONR varied annually with values ranging from ~60–130 bu/ac in the wetter year (2019) and ~28–78 bu/ac in the drier year (2020), reflecting substantially lower average grain yield (32% on average) in the drier year (Figure 5). In drier cropping seasons (2020), the benefits of additional N fertilization are limited, while favorable environmental conditions (higher rainfall, 2019) enhance response to N. However, under N-limited conditions (low SOM and soil nitrate), higher rainfall intensifies the risk of the dilution effect (Figure 2), potentially resulting in price discounts. These results highlight the importance of adapting N management to seasonal environmental conditions to ensure effective N application. 

Figure 6 provides a practical decision-support tool for CCAs and producers. It shows recommended fertilizer nitrogen (N) rates (lb/ac) for winter wheat at three common yield goals (50, 70, and 90 bu/ac) based on pre-plant (no fertilizer) soil nitrate-N concentration (0–12 inches) and SOM percentage. These recommendations are derived for non-drought years (2019 basis) using the median yield–N response (τ = 0.50) observed in the study (Figure 5). When pre-plant soil nitrate-N is high (>30 ppm) and SOM is 1% or greater, little to no additional fertilizer N is required to achieve even a 70 bu/ac yield goal. In contrast, at low soil nitrate levels (<10 ppm) and low SOM (<0.5%), fertilizer N requirements increase sharply, reaching 194 lb N/ac for a 90 bu/ac target. This decision aid reinforces the need to adapt N management to both current soil conditions and seasonal weather outlook. In years with favorable moisture prospects, producers can confidently apply higher N rates to capture yield potential while protecting grain protein. In drier years or when soil N supply is already strong, more conservative rates improve nitrogen use efficiency and reduce economic and environmental risk.

Conclusion

This study demonstrates the importance of weather-adaptive nitrogen management for balancing grain yield and grain protein concentration in semi-arid winter wheat production systems of the West-Central Great Plains. Results confirmed a consistent negative relationship between yield and grain protein (the dilution effect), which can be substantially mitigated by higher N rates under favorable weather conditions. On average, achieving commercially desirable protein levels (~12% protein) required 15.5% more nitrogen than rates that maximized yield alone. Nitrogen response was strongly modulated by seasonal precipitation: additional N provided limited benefit in dry years but drove significant gains in yield and protein under favorable, wetter conditions. The three varieties evaluated (Canvas, Langin, and Snowmass 2.0) showed similar responses to N, indicating that variety selection had less influence than N rate and environment. Economically, higher yield environments lowered the protein premium needed to offset fertilizer costs, supporting more intensive N programs when conditions are productive.

These findings underscore the value of flexible, site- and season-specific N strategies that integrate soil testing (inorganic N and SOM), yield goals, and weather outlook to optimize productivity, grain quality, and profitability while reducing risk in variable climates. This is preliminary data from 14 environments over two seasons. Further on-farm collaboration is needed to strengthen these recommendations by capturing a wider range of soil types, weather conditions, wheat varieties, and management histories.