Winter Wheat Yield Depression from Legume Green Fallow

Increases in 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 will affect winter wheat ( Triticum aestivum L.) production and economic returns. The objectives of this research were (i) to determine the effect of legume green fallow on subsequent winter wheat yields and economic returns and (ii) to determine optimal legume termination dates during the legume phase. Wheat yields following three legumes [Austrian winter pea, Pisum sativum L. subsp. sativum var. arvense (L.) Poir.; spring field pea, P. sativum L.; and black lentil, Lens culinaris Medikus] 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, 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 in drier than normal years, that returns are maximized when the legume is not grown during the fallow phase. In 1996, a wet year, returns were maximized when the legume was terminated at the second termination date or after 70% of the potential maximum legume water use. We found that 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 highly weather‐dependent and inconsistent. At current fertilizer costs, legume N (in this system) was too expensive to be considered a reasonable alternative to chemical fertilizer.