Supplemental N for Spring Wheat  

Excessive rainfall throughout northwest Minnesota in late May and early June has raised serious questions about loss of available N applied in the spring wheat crop. Consequently, there are questions about potential benefits from in-season application of N. Obviously, there was probably loss of N from denitrification and/or leaching. The amount that was lost, however, cannot be measured or predicted accurately. Therefore, it’s important to consider the appearance of the wheat crop and look at information collected in the past that might be appropriate for today’s situation.

There are two studies that provide appropriate information. The first study was conducted in fields of 27 cooperating wheat producers over a period of 3 years. The purpose was to identify a management practice that might possibly be used to determine the rate of in-season N that would be needed to optimize wheat production. Remote sensing technologies were not available at that time. Preplant N levels were established at 50 and 100 lb. N (soil nitrate N plus fertilizer N) per acre. The following growing season, fertilizer N was topdressed to each preplant N level at rates of 15, 20, 45, and 60 lb. N per acre (in-season N).

The average wheat yields are shown in Figure 1. Without supplemental in-season N, yields were reduced by 10% to 12% when soil preplant N was 50 rather than 100 lb. per acre. As wheat yields increase, this difference, in terms of bushels per acre increases. In-season N did not increase yield if preplant N was 100 lb. per acre. When preplant N was 50 lb. per acre, 45 lb. N/acre applied in-season was adequate for optimum yield.

Although, it’s safe to assume that there has probably been some N loss in 2005, there is no way to accurately measure the amount of loss in wet, sticky soils because of the inherent lack of accuracy in sample collection. Nevertheless, it is doubtful that soil N levels are as low as 50 lb. per acre. Therefore, potential yield losses, at the present time (June 20) are probably less than 10%.

Another trial was conducted in the summer of 2002 by Russ Severson in a production field at the Northwest Research and Outreach Center. As in 2005, excessive rainfall had created a potential for nitrogen loss. This trial was designed to look at the impact of timing of supplemental nitrogen. Wheat followed corn. Prior to the supplemental N, the field had received 80 lb. N per acre as 82-0-0 and 50 lb. 18-46-0 per acre. The supplemental N was applied at either Zadoks growth stage 14 or Zadoks growth stage 60. Measured yields are listed in Table 1.

The early application of fertilizer N increased yield by 6.8 bu. per acre (26.5%) when compared to the wheat that did not receive supplemental N. Delayed application did not increase yield.

Considering the results of both studies it would appear that there might be some economical advantage in yield for in-season supplemental N in 2005. The advantage for this practice is highest for those fields that show the most severe yellowing. The potential yield advantage for this practice diminishes for those fields that have a dark green color.

The previous discussion has focused on yield without consideration for grain protein. Past research has shown that in-season N application may benefit grain protein. When compared to the standard, this delayed N application may not lead to improved protein. It may, however, prevent dockage for low protein levels. Rumors persist that this dockage could be substantial in 2005. The physiology of the wheat plant dictates that fertilizer N applied in the vegetative stages is used primarily for growth and yield. Fertilizer N applied near flowering and pollination is used by the plant to enhance grain protein.

Considering the impact on both grain yield and protein, it appears that the application of 30 to 40 lb. N per acre prior to flowering would be a management practice that could be used to minimize potential economic losses attributed to lost N.

The supplemental in-season N can be supplied as 46-0-0 or 28-0-0. For liquid application, there may be some leaf burn. This burn should not be harmful to the plant and should not cause any loss in yield.

George Rehm, Dept. of Soil, Water, Climate
Russ Severson, UM Extension Service
and Albert Sims, NWROC