Cereal Grains Fertility

Neil Christensen

Wheat Yield and Protein Response to Nitrogen

Few would dispute the importance of N fertilizer in Pacific Northwest (PNW) cereal grain production. While N rates, timing, placement and source may vary; nearly all PNW wheat production systems require N fertilizer to reach potential yields. Producers have learned that the most profitable strategy for soft white winter wheat is to add just enough N to reach the yield potential set by the most limiting variable - usually water. The challenges are to accurately estimate yield potential based on stored soil water and precipitation and to estimate the amount of soil N that will become available to the crop.

A new challenge faces PNW growers as emphasis shifts toward production of diverse, superb quality wheat for specific markets. Consistent production of high quality hard wheat requires management of N fertilizer for both grain yield and grain protein content. Fortunately, response to increased N supply is similar for all classes of wheat and this knowledge can be used to guide N management decisions. This article presents both general and specific aspects of wheat yield and protein responses to N fertilizer.

Typical wheat yield and grain protein responses to N fertilizer are illustrated in Figure 1. Yield often doubles with modest rates of fertilizer N when N is deficient and yield is not limited by water supply. Grain protein content usually decreases with the initial increase in grain yield because protein is "diluted" in a larger quantity of grain. Consequently, grain protein percentage is often higher in unfertilized controls than in modestly fertilized plots. At higher rates of N, grain yield response begins to taper off and grain protein content increases. Grain protein continues to increase as N supply exceeds the rate required for maximum yield.

 The relationship between yield and protein responses is such that there is a range or "window" of grain protein concentrations associated with near maximal grain yield. Knowledge of this protein window, spanning as much as three percentage points, can be used to manage N fertilizer for protein concentrations acceptable in different classes of wheat. With soft white wheat, for example, one would strive to be in the lower-left corner of the window - near maximal yield with low protein. In contrast, with hard red wheat one would strive for the upper-right corner of the window - near maximal yield with high protein.

Yield and protein responses to N fertilizer for two hard white spring wheat cultivars ('Winsome' and 'ID 377S') and two hard red spring wheat cultivars ('Jefferson' and 'WPB 936') are illustrated in Figure 2. Data are from an OWC funded trial conducted under irrigation at the Central Oregon agricultural Research Center in 2001.

All four cultivars exhibited a typical grain yield response to applied N. Yields of three cultivars ('Winsome', 'ID 377s', 'Jefferson') were comparable and significantly greater than the yield of 'WPB 936'. Cultivars differed significantly from one another in grain protein percentage, with three cultivars ('Winsome', 'ID 377s' 'Jefferson') exhibiting typical patterns of protein response to N. Mean grain protein concentrations decreased in the order 'WPB 936' > 'Jefferson' > 'ID 377s' > 'Winsome', reflecting both wheat class and cultivar-within-class differences.

The interaction between genotype (cultivars) and management (N fertilizer rates) must be taken into consideration when managing N fertilizer for grain protein concentration. The importance of this interaction can be illustrated by comparing protein responses by the two hard white cultivars, 'Winsome' and 'ID 377s'. Both cultivars exhibited classic yield and protein responses to N with a minimal grain protein contents of ~ 9% at 80 lb N per acre. As N rates increased beyond 80 lb N per acre, however, the rate of increase in grain protein was much less for 'Winsome' as compared to 'ID 377s'. Consequently, 'Winsome' wheat contained only 12% protein as compared to more than 14% protein for 'ID 377s' when both were fertilized with 320 lb N per acre. The take-home message is that growers need to understand the characteristics of individual cultivars when managing N to reach specific grain protein percentages.

Applying additional N fertilizer to increase grain protein is not without risk. Considerations include added production costs, possibility of yield reduction from excess N, and possibility of not achieving class-acceptable grain protein. Data collected in Oregon over the past few years suggest that N fertilizer rates may need to be increased by 25 to 75% in order to achieve 14% grain protein in hard red spring wheat. Some have questioned the magnitude of these N rate increases based on the amount of N contained in grain protein. Our research, however, shows that plants become less efficient in taking up fertilizer N as N rates increase. For example, in the experiment illustrated in Figure 2, N fertilizer recovery by the crop decreased from 62% to 19% as N rates increased from 80 to 320 lb N per acre. Thus, at N rates needed to maximize yield, plants took up only one-third to one-fifth of the fertilizer N applied to increase grain protein. Fertilizer N not recovered by the crop remains in the soil and may be subject to leaching loss in irrigated or high rainfall regions.

Emphasis on production and marketing of diverse, high quality wheat for specific markets brings with it the need for more specific and precise crop management information. With the support of the Oregon Wheat Commission, Oregon State University scientists are working to provide the knowledge and materials wheat growers will need to accomplish their objectives.

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