Cover Crops, Fertilizer Nitrogen Rates, and Economic Return of Grain Sorghum

Leguminous cover crop systems have been envisaged as a critical component of sustainable agriculture because of their potential to increase soil productivity by cycling C and N in agricultural systems. Our objectives were to: (i) determine the effects of including summer cover crops and N rates in the cropping system on the growth and yield of the succeeding grain sorghum [Sorghum bicolor (L.) Moench] crop, (ii) calculate the N fertilizer replacement value (NFRV) and evaluate economic returns, and (iii) determine the most cost-effective cropping system. Field experiments were conducted for two seasons in Kansas. Leguminous summer cover crops and double-cropped soybean [Glycine max (L.) Merr.] residues improved soil N availability; N was subsequently used by the succeeding crop. Across years and cropping systems, the mean increases in grain yield as a result of including cowpea [Vigna unguiculata (L.) Walp.], pigeonpea [Cajanus cajan (L.) Millsp.], sunn hemp (Crotalaria juncea L.), double-cropped soybean, and double-cropped grain sorghum in the rotation compared with a fallow system with 0 kg N ha(-1) were 56, 62, 43, 32, and 3%, respectively, and NFRVs across the years were 53, 64, 36, 27, and -3 kg N ha(-1), respectively. Across years, grain sorghum in a double-cropped soybean system and a fallow system with 90 kg N ha(-1) gave profitable economic net returns. We conclude that including leguminous cover crops in a cropping system has the potential to reduce N requirements and improve the N availability and grain yield of the succeeding grain sorghum crop.