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2002 Annual Report

Legume N Credits in Winter Wheat Legume Rotations

M.F. Vigil, D.C. Nielsen, R.A. Bowman

PROBLEM: With the exception of water, nitrogen (N) nutrition is the most important limiting input to profitable winter wheat production in the central Great Plains. Increases in N fertilizer costs have caused some farmers to consider alternative systems that include legumes as a source of N. Farmers need to know how these systems impact winter wheat yields, economic returns and N availability.

The two sites previously established in which the main plots consist of legume species: Austrian winter peas, spring field pea (cv. Profi), Hairy Vetch and a no-legume-summer-fallow (SF) (plot fertilized at four N rates 0, 30, 60, and 90 lb N/ac are now being used for a two year follow up study. The objectives of the follow up experiment are (1) to determine the fertilizer N response of wheat following the legumes, (2) to determine the N response of the legumes and (3) to determine the difference in N response of the legume wheat rotation as compared to wheat fallow.

APPROACH: Legumes are planted early in April or late March. Weeds growing in the fallow plot are allowed to grow and use water until the legumes are terminated. Legumes and weeds in summer fallow are terminated at the same time, usually the first or 2^nd week of June. Before planting wheat in legume stubble each fall, the legume plots are divided into 4 subplots and each subplot is fertilized with either 0, 30, 60, and 90 lb N/ac. Soil inorganic N is measured in each plot, at each termination date, in the top 3 feet of soil, and at wheat planting time to monitor changes in available N. Just after fertilizing legume stubble, wheat is planted. Grain yield is measured using standard BMP's for dryland winter wheat. Equations are fitted to the wheat-grain-yield-response to added N fertilizer for the legume-wheat plots and the wheat-fallow plots. If there is a legume N credit the N response curve for wheat following the legume plots should be higher on the vertical axis than the fallow N response curve. This is assuming that the weeds use the same amount of water as the legumes.

RESULTS: This past year (2002) wheat yields were less than previous years (Table 1). Nitrogen rates of 30 and 60 lbs/acre increased wheat biomass yields but didn’t always show a significant increase in grain yields. In the wheat plots after fallow, (SF) the highest yields were at the 60 lb N rate (49 bushel/acre). On the other hand, in wheat plots following legumes the highest yields were measured in the check plot with no fertilizer. The lowest yields were measured in wheat following Profi-pea (table 2).

Earlier in the experiment, we found that 88 % of the variability in wheat yield loss could be described by an equation based on the previous year’s legume water use (ET). Generally the highest yield reductions were with the later termination dates. Sometimes the earliest termination dates did not result in a significant reduction in grain yield. Legume green fallow increases wheat-grain-N contents similar to fertilized summer fallow. However, the increase does not increase wheat yield and or cause a large increase in grain-N-uptake when compared to traditional summer fallow. In last years’ experiment, we are seeing a similar result. It is interesting that biomass yields tended to increase with added N but didn’t always result in increased grain yield. We suspect, greater-early-season water use with the 60 and 90 lb N rate plots (bigger plants use more water than smaller ones) than with the 0 and 30 lb N rates. Perhaps less water is available in those plots later in the season during grain filling which depressed yield.

Table 2. Wheat yields as affected by fallow treatment in 2001.

FUTURE PLANS: We will conduct the follow up experiment for another year. Four publications have resulted from the study.

The authors may be reached via e-mail at: M.F. Vigil or D.C. Nielsen
or phone: 970-345-2259.