Wheat producers are always searching for management practices that will consistently increase grain protein in hard red spring wheat. Since nitrogen (N) is a key component in grain protein, it’s logical to look to improved management of N fertilizer as a means of increasing grain protein.

An understanding of the physiology of the spring wheat plant may also provide some clues that will help. For example, researchers have found that a high percentage of N applied late in the season was directed to the grain to increase grain protein rather than yield.

However, the logistics of applying fertilizer N after early grain fill would be difficult to organize for those who grow spring wheat on large acreages. Current technology in fertilizer manufacture can produce slow-release N fertilizers which will delay the release of N for various periods of time. If release of the major portion of the N in the fertilizer material is delayed until after initial grain formation, that N may not go to support plant growth and grain yield. Instead, the N released later in the growing season may be used for increasing grain protein. Therefore, there is justification for evaluating the application of slow-release N fertilizer in spring wheat production systems.

Recently, application of slow-release N fertilizer was evaluated in five fields of cooperating wheat producers in northwest Minnesota. The evaluations were completed in 2000 and 2001. Soil samples (0 to 24 inches) were collected in the fall of 1999 and 2000 and analyzed for nitrate-nitrogen (NO 3-N). The following spring, before planting, three base levels of fertilizer N were supplied as urea (46-0-0). These were: 1) recommended N rate, 2) 0.5 x recommended N rate, and 3) 1.5 x recommended N rate. The base rate of fertilizer N applied at each site was determined by expected yield and the amount of NO 3-N measured in the previous fall. The N fertilizer along with adequate phosphate and potash was incorporated before planting.

Superimposed over the three base levels of N fertilizer was the application of slow release N fertilizers (coated urea) at rates to supply 30 lb. N per acre. Slow release N fertilizer was either broadcast and incorporated prior to planting or topdressed about 30 days after wheat emergence. Grain yields and grain protein were measured.

The measured NO 3-N and other soil properties for the respective sites are listed in Table 1. A N credit was used when wheat followed soybean in the rotation.

Table 1. Relevant soil test properties for the test sites used for the evaluation of slow-release N fertilizers.

Neither the slow-release N product nor the time of application affected grain yield in 2000. So, the yields shown in Table 2 are averages for both products applied preplant and topdress. Even though yields were good, use of fertilizer N broadcast and incorporated before planting had no effect on yield at the East Polk site (Table 2). For the Norman site the application of 65 lb. N per acre was adequate for optimum yield.

Table 2. Wheat yield in 2000 as affected by use of slow-release N fertilizer.

The 60 bu. per acre yields measured in 2000 were better than the yield goal of 50 bu. per acre. Yield was increased by the slow-release N fertilizers at the Norman site when the base N rate was 65 lb. per acre. Otherwise, the slow-release N products had no positive impact on yield in 2000.

Table 3. Wheat yield in 2001 as affected by use of slow-release N fertilizers.

The base rate of fertilizer N had a substantial impact on yield at all sites in 2001. A rate of 40 lb. N per acre was adequate at the East Polk site where wheat followed soybean. The rate needed for optimum yield was 135 and 75 lb. N per acre at the Norman and West Polk sites respectively. There was slightly more carryover NO 3-N following wheat at the West Polk site (see Table 1).

Except for the 135 lb. N per acre rate at the Norman site and all rates of applied N at the East Polk site, use of slow-release N fertilizer increased wheat yield. The average increase was 6.0 bushels per acre when the recommended rate of fertilizer N was used. This increase was measured at two of five sites. Slow-release N fertilizers are usually more expensive than urea. The added cost varies with the slow-release product used. Considering the projected cost of urea next spring plus the extra cost of the slow-release characteristic, it is doubtful if use of slow-release N fertilizer would be consistently profitable for wheat yields in the range of 50 to 60 bushels per acre.

The potential impact of slow-release N fertilizers on wheat protein cannot be ignored. Results from 2000 are summarized in Table 4.

Table 4. Wheat protein in 2000 as affected by use of slow-release N fertilizers.

The relatively good yields in 2000 were accompanied by wheat protein values that would deliver a premium or eliminate the possibility of dockage. Grain protein at both sites used in 2000 increased as the base rate of fertilizer N without the use of slow release increased (see Table 4). There was only one instance where slow-release N increased grain protein. The economic value of that increase must still be determined.

Grain protein values for the three trials conducted in 2001 are summarized in Table 5.

Table 5. Wheat protein in 2001 as affected by use of slow-release N fertilizers.

At all locations, grain protein increased as the base rate of fertilizer N increased. In all situations, the highest base N rate produced the highest grain protein values. There is no apparent explanation for the relatively low values for all treatments at the Norman County site.

The use of the slow-release N products, regardless of time of application increased grain protein for all base N levels used in Norman County. Increases ranged from 0.4 to 1.0%. In East Polk County, application of the slow-release N increased grain protein when the base N rate was 40 and 80 lb. N per acre. At this site, the base N rate of 120 lb. N per acre was apparently adequate for both optimum yield and grain protein.

The impact of slow-release N on grain protein at the West Polk site was similar to the impact measured at the East Polk site. Except for the lowest base N rate, increases in grain protein produced by the use of slow-release N fertilizers was small.

When compared to 46-0-0, there is an added cost for the slow-release N products having a similar analysis. Dictated by the premium paid for the protein, the value of the added protein may not compensate for the added cost of the slow-release N products that are available. The cost will vary with product. For any wheat producer who is thinking about using slow-release N fertilizers, it would be wise to check the cost before making a decision to purchase.