Introducing green manures in an organic soybean-winter wheat-corn rotation: Effects on crop yields, soil nitrate, and weeds

In organic soybean– winter wheat – corn rotations, animal manure is a common choice to maintain high yields, but leguminous crops grown as green manures after wheat harvest and incorporated into the soil before corn planting, can be an alternative when animal manure is not accessible. Forage legumes with high dry matter (DM) production and high biological N fixation have been shown to meet corn N demand. However, in Eastern Nebraska, lack of precipitation can reduce green manure growth and N fixation, leading to an insufficient N supply for corn, but corn growth can also be impacted by green manure soil water use. Our objectives were 1) to determine the green manure potential of four forage legumes, and 2) to evaluate management methods that optimize green manure benefits. We conducted an experiment at the ARDC near Mead, NE, from 2011 - 2014. Red clover, white clover, alfalfa, and sweet clover were undersown into winter wheat in early spring. After wheat harvest, they were either mowed or not mowed, and terminated in the fall or the next spring. We measured green manure DM, weed DM, soil nitrate concentrations, and crop yields throughout the rotation. We compared green manure effects to effects of cattle manure, post-wheat soybean green manure, and a control (no fertilizer). Red clover produced the most DM, up to 5.5 Mg ha-1 and showed excellent weed control, especially when mowed. Green manures did not increase soil N compared to the control. Corn yields were always significantly higher after cattle manure (7.6 to 8.1 Mg ha-1) than after undersown green manures, and were lowest after red clover in 2012 (2.8 Mg ha-1) and after white clover in 2013 (4.6 Mg ha-1), because of the clovers’ high soil water use and insufficient N production. In our study, green manures established well, but increased corn yields compared to a control in only one of three years. Cattle manure was the most reliable method to maintain high crop yields. Future research should investigate combinations of cattle and green manure to increase N availability to corn and decrease N leaching losses after corn harvest. Advisor: James R. Brandl