LEGUMES IN WINTER WHEAT ROTATIONS

Authors: Vigil, Merle; Nielsen, David

Submitted to: Colorado Farmer Stockman
Publication Type: Popular Publication
Publication Acceptance Date: 10/8/1999
Publication Date: 12/1/2000
Citation: N/A

Interpretive Summary: Increases in nitrogen (N) fertilizer costs have caused some farmers to consider the use of a legume during the summer fallow phase of a wheat- summer-fallow system. This legume would be used as an alternative N source for dryland wheat. With legumes there exists a symbiotic relationship between the legume plant and the soil bacteria rhizobium. This symbiotic relationship results in the fixation of atmospheric N into plant available N. Farmers need to know how the use of this system will impact winter wheat production and economic returns. The objectives of this research were to: (i) determine the effect legume-green fallow has on subsequent winter wheat yields and economic returns, and (ii) to determine optimal legume termination dates. Wheat yields following: Austrian winter peas, spring field pea, and black lentil were compared with wheat yields following fertilized traditional summer fallow. Legume biomass, biomass N, and water use were measured at four termination dates during the green-fallow phase of the rotation. Wheat yields following the annual legume were reduced when compared with traditional summer fallow: by 6 bushels /acre at the earliest legume termination date and by at least 16 bushels/acre at all other dates. Economic analysis indicates that most years, that returns are maximized when the legume is not grown during the fallow phase. We found 88 % of the reduction in winter wheat yield, could be explained by legume water use the previous year. At current fertilizer costs, legume N (in this system) was too expensive to be considered a reasonable alternative to chemical fertilizer.

Technical Abstract: Increases in nitrogen (N) fertilizer costs have caused some farmers to consider the use of a legume during the fallow phase of a wheat-fallow system as an alternative N source for dryland wheat. Farmers need to know how this system impacts winter wheat production and economic returns. Our objectives were to: (i) determine the effect legume-green fallow on subsequent winter wheat yields and economic returns, and (ii) to determine optimal legume termination dates. Wheat yields following: Austrian winter peas, spring field pea, and black lentil were compared with wheat yields following fertilized summer fallow. Legume biomass, biomass N, and water use were measured at four termination dates during the green-fallow phase of the rotation. Wheat yields following the annual legume were reduced when compared with traditional summer fallow: by 400 kg ha-1 at the earliest legume termination date and by at least 1050 kg ha-1 at all other dates. Economic analysis indicates that most years, that returns are maximized when the legume is not grown during the fallow phase. We found 88 % of the variability in winter wheat yield, could be explained by legume water use the previous year. In general, the competitiveness of legume-green fallow with winter wheat fallow is inconsistent. At current fertilizer costs, legume N is too expensive to be considered a reasonable alternative to chemical fertilizer.