Efficient and effective use of fertilizer nitrogen is a key management practice for optimizing dollar returns from the hard red spring wheat crop.   Current fertilizer N recommendations to meet realistic expected yields are consistent for the University of Minnesota, North Dakota State University and South Dakota State University.  The research used as the base for these recommendations was conducted about 25 years ago.  At that time, the popular varieties were Era, Chris and Waldron.  Today, the most popular varieties have a higher yield potential as well as improved grain protein.  Therefore, its reasonable to ask if nitrogen recommendations should be adjusted for variety.

In order to arrive at the answer to this question, a study was initiated in northwest Minnesota in 2002 to measure the response of new and improved wheat varieties to rate of applied nitrogen.  Response was measured in terms of yield and grain protein.   The information collected during the 2002 growing season is summarized in this newsletter.  This is a progress report only.  Additional information is needed before it will be possible to reach definite conclusions.

For this study, five rates of fertilizer nitrogen (0, 40, 80, 120, 160 lb. N/acre) were applied to each of four varieties (Marshall, Alsen, Oxen, Ingot).  Four sites were selected for the study.  Excessive rain in June prevented collection of useful information from two sites in Norman County.

Soil samples were collected from the experimental sites in early November 2001.   Since weather remained warm throughout November, soil samples to 24 inches were collected in April prior to planting.   The second set of samples was analyzed for nitrate-nitrogen only.  The results of the analysis of soil samples are summarized in Table 1.

Table 1.  Results of the analysis of soil samples collected from the experimental sites used for this study.

Comparing the results from the fall and spring measurements of nitrate-nitrogen, more nitrate was measured in the spring.  The increase was substantial.  This increase is attributed to mineralization of soil organic matter associated with warmer than normal soil temperatures in November 2001.

The nitrogen, supplied as 33-0-0, was broadcast along with adequate phosphate and potash and incorporated prior to planting.  The wheat was planted in early May.   Recommended herbicides were used for weed control.

Grain was harvested in August.  Samples were collected from each experimental plot and analyzed for protein.

Grain yield at both sites was significantly affected by both rate of applied nitrogen and variety.  At the West Polk site, the response to nitrogen varied with variety (Figure 1).  With the Oxen variety, yield continued to increase as the rate of applied nitrogen increased (a linear response).  However, the application of 120 lb. nitrogen per acre produced the optimum yield of the Marshall and Ingot varieties.  The response of Alsen to applied nitrogen is difficult to interpret.  Looking at the yields for all four varieties at this site, there was apparently a different rate of nitrogen fertilizer needed to achieve optimum yield for each variety.

At the Marshall location (Figure 2), the response of all varieties to rate of applied nitrogen was similar.  Consistent with the results from the West Polk site, a nitrogen rate of 120 lb. per acre appeared to be adequate for optimum yield.

The analysis of the grain for protein produced results that were not consistent.  At the Marshall County site (Table 2), there was a general increase in grain protein as the rate of applied nitrogen increased.  When averaged over the four varieties, the increase was linear.  In other words, the increase produced by each rate of applied nitrogen was the same for each variety.

Table 2.  Protein content of the grain of four spring wheat varieties as affected by the rate of applied nitrogen.  Marshall County, 2002.

Grain protein in the wheat grain from the West Polk site is summarized in Table 3.   In contrast to the results from the Marshall County site, rate of applied nitrogen had a significant effect on grain protein; but, the values are difficult to explain.   For example, the highest protein in the Marshall variety was produced by the use of 40 lb. nitrogen per acre.  Additional nitrogen decreased protein content and this is not consistent with past research.  There is no explanation for this observation at this time.

Table 3.  Grain protein content of four spring wheat varieties as affected by rate of applied nitrogen.  West Polk County, 2002.

Although the information gathered in this study is not conclusive, there was some indication that varieties respond differently to rate of applied nitrogen.  This study will be continued in 2003 with hopes that yields are not affected by extremes in weather.

West Polk County
Wheat Study

FIGURE 1.  Yield of four wheat varieties as affected by rate of applied nitrogen.   West Polk County, 2002.

Marshall County
Wheat Study

FIGURE 2.  Yield of four varieties of hard red spring wheat as affected by rate of applied nitrogen.  Marshall County, 2002.