The potential benefits of split applications of nitrogen for production of hard red spring wheat have been a topic of interest for many years. The promotion of intensive wheat-management in the early 1980’s stimulated much of the interest.

Studies at the Northwest Research and Outreach Center provided an initial evaluation of the addition of nitrogen during the growing season. These results are listed in Table 1. The intent was to strive for 110 bu. wheat per acre and determine if in-season N would help. A preplant application of 150 lb. N per acre was used. A foliar application of 15 lb. N per acre was made at the late tiller stage.

Table 1. Impact of 15 lb. N per acre in a foliar application on yield and grain protein of hard red spring wheat.

The foliar N had no positive effect on either yield or grain protein. The preplant application of 150 lb N per acre was apparently sufficient.

The use of split applications was evaluated further in 1987-89. Using liquid N (28-0-0), two equal applications were made to supply 40 lb. N per acre. The timing of the split application is listed in Table 2.

Table 2. Effect of split application of fertilizer N on yield and protein content of hard red spring wheat.

The use of foliar N in addition to the preplant N had no positive effect on yield. There was a small increase in grain protein that varied from 0.3% to 0.6%.

The evaluation of in-season fertilizer N was expanded in the early 1990’s. Trials were conducted in fields of 21 cooperating wheat producers over a period of three years. Soil samples (0 to 24 inches) were collected in the fall prior to the wheat crop. Urea (46-0-0) was broadcast and incorporated prior to planting at rates so that soil plus fertilizer N was equal to 50, 100, and 150 lb. per acre. The amount of fertilizer N used to reach each level was adjusted for the amount of NO₃-N measured in the previous fall. Then 33-0-0 was broadcast at tiller for each base level of N to supply 0, 15, 30, 45, and 60 lb. N per acre.

Usable information was collected from 15 sites. The average wheat yields for all sites are shown in Figure 1. If soil plus fertilizer N was 100 or 150 lb. per acre, N added at tiller had no positive effect on yield (see Figure 1). The use of 45 lb. N per acre at tiller had a positive effect when soil plus fertilizer N was 50 lb. per acre. Application of 45 lb. N/acre at tiller was necessary for optimum yield.

Figure 1. Yield of spring wheat as affected by rate of in-season N for three established base levels of soil plus fertilizer N.

Yields from two individual fields in this study are provided in Tables 3 and 4.

Table 3. Spring wheat yield as affected by rate of nitrogen applied in-season at three base levels of soil plus fertilizer N. MarshallCounty.

Table 4. Spring wheat yield as affected by rate of nitrogen applied in-season at three base levels of soil plus fertilizer N. RedLakeCounty.

For both fields, additional in-season N did not increase yield at the highest rate of soil plus fertilizer N. The soil plus fertilizer base of 100 lb. per acre was adequate for the high yields measured in Marshall County. But, 30 lb. N per acre in season produced about 8 bu. per acre at the lower yield levels in Red Lake County. When the base was 50 lb. N/acre, 45 lb. in-season produced optimum yield at the Red Lake County site while only 30 lb. N in-season was required at the Marshall County site.

It is important to note that adequate soil plus fertilizer N prior to planting was adequate for optimum yield. The yields measured at these individual sites agree with the average yields from the 15 sites (Figure 1).

Grain protein was also measured and average values for the 15 sites are shown in Figure 2.

Figure 2. Protein content of spring wheat as affected by rate of in-season N for three established base levels of soil plus fertilizer N.

At all base levels, grain protein was increased by the use of in-season N. As might be expected, in-season N produced larger increases when the base N level was 50 lb. per acre. In general use of 60 lb. N per acre increased grain protein by about 0.7%. A small increase was measured (about 0.4%) when the base N rate was 150 lb. per acre.

Grain protein when the base N rate was 150 lb./acre without in-season N was greater than the protein measured when 60 lb. N per acre in-season was added to a base rate of 100 lb. N per acre. Considering the information collected from 15 fields, in-season N, regardless of rate applied, had no positive effect on grain protein if the amount of soil plus fertilizer N was adequate for optimum yield.

Split applications of N for spring wheat production have also been studied in North Dakota. At Casselton, a single preplant application of 125 lb. N per acre was compared to two or three splits to equal the same rate of preplant N (see Table 5).

Table 5. Spring wheat production as affected by single and split applications of N. Casselton, ND.

Using higher rates, a similar study was conducted at Carrington under irrigation (see Table 6). At this site, use of a split application produced a higher yield. Because of the sandy soil and high potential for leaching loss, the benefit of split application would be expected. The increase of approximately 8 bu. per acre would more than pay for the cost of the extra application.

Table 6. Spring wheat production as affected by single and split applications of N. Carrington, ND.

Various studies have been conducted in the Northern Plains for several years in an attempt to accurately evaluate the impact of in-season N on yield and grain protein. The results have been consistent over time. If the nitrate-nitrogen (0 to 24 inches) combined with fertilizer N is adequate for optimum yield, in-season N will not have a positive effect on production. There might be a small increase in grain protein. The value of the protein increase, of course, will be highly dependent on the value of the premium or the discount.

As projected spring prices for fertilizer N increase, attention automatically turns to management practices that will increase the efficiency of use of fertilizer N applied. An evaluation of the information collected from several fields over several years leads to a conclusion that in-season application is one management practice that should not be considered. Adjusting fertilizer N rates for the amount of NO₃-N measured to a depth of 24 inches is one management practice that should be used for all soils that are not sandy.