Crop rotation is reported to increase grain sorghum [Sorghum bicolor (L.) Moench] yield in some, but not all cases. This study compares yield of continuous grain sorghum with that of sorghum grown (i) in a 2-yr rotation with soybean [Glycine max (L.) Merr.]; (ii) in a 4-yr oat + clover [Avena sativa (L.) + 80% Melilotus officinalis (L.) Lam., 20% Trifolium pratense]-corn (Zea mays L.)-soybean-grain sorghum rotation; and (iii) in a 4-yr soybean-corn-oat + clover-grain sorghum rotation. Interactions between crop rotation and N rate were also determined. The study was conducted for 4 yr on a Sharpsbury silty clay loam (fine, montmorillonitic, mesic Typic Arguidoll). Continuous grain sorghum showed a greater response to fertilizer N than sorghum in rotation. Sorghum following legumes in rotation did not respond to applications of more than 90 kg N ha-1, while continuous sorghum increased with all levels of N applied. Yield of continuous sorghum was 3.4, 5.0, and 5.3, Mg ha-1 with 0, 90, and 180 kg N ha-1, respectively, while sorghum in rotation averaged 5.0, 5.4, and 5.3 Mg ha-1 at the same N rates. Differences between continuous and rotated sorghum were large without applied N, but were minimized or removed with N applications. Sorghum in the 2-yr sorghum-soybean rotation produced sustained high yields (5.0 Mg ha-1) without additions of N fertilizer.
