Long-term cover crop effects on biomass, soil nitrate, soil water, and wheat

Cover crops during summer fallow have been rarely researched in the semiarid northern Great Plains. This study was conducted during 2012-2019 at four Montana locations and included four functional groups (Brassica family, fibrous-rooted crops, legumes, and tap-rooted crops). Eleven treatments included sole functional groups, a Full Mix, the Full Mix minus each functional group, pea, and chemical fallow. Wheat (Triticum aestivum L.) was grown after each cover crop year with three nitrogen (N) fertilizer rates. Cover crops were terminated with herbicide at first flower stage of pea (Pisum sativum L.) 57 to 66 days after planting. Shoot biomass averaged 2.0 Mg ha(-1) over eight site-years representative of dryland farming in Montana. Using equal overall plant densities, treatments with six species averaged 13% greater biomass than two species. Measured at termination to a 0.9-m depth, Fallow held greater soil water than cover crop treatments, with Fallow averaging 57 mm greater than the Full Mix. Soil nitrate averaged 49 kg N ha(-1) greater after Fallow than the Full Mix; the Legume treatment averaged 26 kg N ha(-1) greater than the Minus Legume treatment. Wheat yield on Fallow averaged 0.85 Mg ha(-1) greater than the Full Mix in 5 of 10 site-years, mainly at the driest site-years. The Legume treatment elevated wheat protein over the Minus Legume treatment by an average of 15 g kg(-1). Cover crops grown during summer fallow reduced soil nitrate-N, soil water, and wheat yields compared with chemical fallow, especially in the major wheat growing region of north central Montana.