There’s general agreement that an adequate supply of nitrogen either from the soil and/or fertilizer is essential for optimum yield of hard red spring wheat. This nutrient, when deficient, can also have a negative effect on grain protein.

Currently, nitrogen fertilizer recommendations are the same for all wheat varieties. Yet, some varieties have an inherent higher protein content than others. Since nitrogen is a key component of protein, it’s reasonable to think that the inherent differences in grain protein could translate to differences in nitrogen uptake. Differences in absorption of (uptake) nitrogen could mean that specific rates of fertilizer nitrogen should be matched to individual varieties. Therefore, a study was initiated in 2002 and continued in 2003 to evaluate the impact of five rates of nitrogen on four varieties of hard red spring wheat.

In 2003, the study was conducted at four locations in northwestern Minnesota. Three locations were in the fields of cooperating wheat producers. The fourth was at the Northwest Research and Outreach Center (NWROC).

Soil samples from three of the four locations were collected in late fall of 2002. These samples were analyzed for pH, P (Olsen), K, and nitrate-nitrogen (NO₃-N). The results of these analyses are presented in Table 1.

Table 1. Results of analysis of soil samples collected from three experimental sites. Collected in Fall 2002.

Soil Property	Location
	NWROC	East Polk Co. (Hove)	West Polk Co. (Cameron)
pH	8.1	8.1	8.1
P (Olsen), ppm	9.9	6.3	13.5
K, ppm	167	136	255
NO3-N (0 to 24 in), lb./acre	42.7	34.5	37.7
Previous Crop	wheat	soybeans	wheat

The various rates of nitrogen along with adequate amounts of phosphate and potash were broadcast and incorporated before planting. Of the four varieties, P2375 was considered to be an old variety and was used as a standard for comparison. The Olsen, Knudson, and Oxen varieties are relatively new and were suggested for use in the study by several wheat growers.

The wheat was planted with equipment designed for research plots. All treatments were sprayed with the appropriate herbicide for weed control and fungicide for control of leaf diseases and scab.

The grain was harvested with a combine designed for research plots. Yields were corrected to 13.5% moisture.

The grain yields are summarized in Figures 1-4. Use of fertilizer nitrogen produced a substantial increase in yield at three of the four sites. Use of fertilizer nitrogen had no effect on yield at the Bradow site (Figure 2). The lack of a yield increase resulting from the use of fertilizer nitrogen puzzling. It was obvious from visual observations early in the growing season that fertilizer nitrogen had no effect on early growth. Therefore, soil samples (0 to 2 feet) were collected at early heading and analyzed for nitrate-nitrogen. The results of that analysis are summarized in Table 2.

Table 2. Measured nitrate-nitrogen (0 to 24 inches) at the Bradow site at early heading.

The measured amount of NO₃-N increased as the rate of fertilizer nitrogen increased and this was expected. Measured NO₃-N in the control treatment was low (24.1 lb. per acre). So, the source of nitrogen for a yield of 100 bu. per acre was a mystery. The wheat at the Bradow site followed an excellent crop of sugar beets in 2002. The sugar beet tops are, obviously, an excellent source of N (slow release) for wheat production.

The amount of fertilizer nitrogen needed for optimum yield did not change with variety at the other locations. The optimum rate at the NWROC and Cameron sites was 120 lb. nitrogen per acre. At the Hove site, the optimum rate appears to be about 100 lb. nitrogen per acre (Figure 4). This difference in optimum rate is attributed to the previous crop of soybeans at the Hove site.

The yield information collected from this study in 2003 provides good evidence that the same rate of fertilizer nitrogen should be recommended for all new, improved varieties of hard red spring wheat.

The results of this study also illustrate the importance of nitrogen to economic wheat production. At NWROC, the optimum rate of 120 lb. N per acre nearly doubled yield of all varieties when compared to the control (Figure 1). The optimum rates of nitrogen fertilizer produced yield increases of 30 and 25 bu. per acre at the Cameron and Hove sites respectively. If wheat is priced at $3.50 per bushel and the cost of N is $.20 per pound, the return on investment was $67.50 per acre and $101.00 per acre at the Cameron and Hove sites.

Even though the amount of nitrate-nitrogen measured before planning was low, a relatively low rate of nitrogen produced excellent yields. This observation verifies the concept that fertilizer nitrogen is used very efficiently when there are weather conditions that are conducive to high yields.

Sincere appreciation is expressed to the Minnesota Wheat Growers for providing financial support for this research. A special “thank you” also goes out to the cooperating wheat growers who were kind enough to allow us to conduct the research in their fields.

Figure 1. Response of 4 wheat varieties to rate of fertilizer N at the Northwest Research and outreach Center.

Figure. 2. Response of 4 wheat varieties to rate of fertilizer N at the Bradow farm in West Ottertail County.

Figure 3. Response of 4 wheat varieties to rate of fertilizer N at the Cameron farm in West Polk County.

Figure 4. Response of 4 wheat varieties to rate of fertilizer N at the Hove farm in East Polk County.