The Effects of Clover Undersown into Winter Wheat on the Yields and Soil Nitrate Concentrations of an Organic Soybean-Winter Wheat-Corn Rotation.

Red clover (Trifolium pratense) and white clover (Trifolium repens) are winterhardy cover crops with good potential to fix N and produce dry weight. In rotations with winter wheat, they can be broadcast into wheat in early spring to extend their growing season. We investigated the effects of red clover (RC) and white clover (WC) undersown into winter wheat on yields and soil nitrate concentrations in a soybean-winter wheat-corn rotation. RC and WC were terminated either in the fall or in spring before the planting of corn (Zea mays). Control treatments were post-wheat manure application (MA), post-wheat soybean (Glycine max) cover crop (SB), and no fertility amendment (NO). Soil nitrate was determined at planting and harvest for each crop. Wheat yields were not affected by treatments or year and averaged 3.76 Mg ha^-1. Soil nitrate levels at corn planting in 2012 were 66 ppm for MA and ranged from 25 to 37 ppm for the other treatments. Yields of corn were significantly higher after NO yielding 9.8 Mg ha^-1 in 2012 and 8.6 Mg ha^-1 in 2013. In 2012, corn yields were lowest after RC (2.8 Mg ha^-1) and in 2013 they were lowest after WC (4.6 Mg ha^-1). Soil nitrate levels at soybean planting ranged from 8 ppm in NO to 13 in MA. Soybean yields were significantly higher for MA (3.5 Mg ha^-1). Yield reductions after red and white clover are likely due to soil moisture deficits. Clover produces ample biomass and can have other benefits in a rotation such as weed suppression. However, farmers wanting to use red or white clover as a cover crop need to be aware of possible yield reductions and should terminate clover early to ensure sufficient soil moisture for the following crop.