Effect of Soybean Rotation on the Response of Sorghum to Fertilizer Nitrogen

Abstract

Increasing grain sorghum [Sorghum bicolor (L.) Moench] acreage in Missouri and the subsequent soybean [Glycine max (L.) Merr.]-sorghum rotation acreage offers producers an opportunity to economize on N inputs. Soybean-sorghum rotation experiments were conducted from 1980 to 1987 to determine the amount of N that can be credited from soybeans when rotated with grain sorghum. Studies were conducted at two locations, (Columbia and Novelty) in central and northeastern Missouri on a Putnam silt loam (fine, montmorillonitic, mesic Mollic Albaqualf) and a Mexico silt loam (fine, montmorillonitic, mesic Udollic Ochraqualf). Experiments consisted of application of 0, 40, 80, 120, and 160 lb N/acre to continuous sorghum and sorghum-soybean rotation blocks. Grain yields were significantly increased from fertilizer N in both rotations in all years, but a significantly different response to fertilizer N between rotations was observed all years except for 1980 at Columbia. At the optimum N rate for each rotation, the sorghum following soybeans outyielded continuous sorghum by 439 lb/acre at Novelty and 804 lb/acre at Columbia. This higher yield can be attributed to the added benefits of rotation. Grain protein also was increased in the sorghum-soybean rotation compared with continuous sorghum. At Novelty, the 80 lb/acre N rate in continuous sorghum produced yields comparable to the 0 N rate in the rotated sorghum. At Columbia, in 1980, yield from rotated sorghum was equal to yield with 40 lb N/acre from continuous sorghum; and in 1982 and 1984, yield from 0 N rotated sorghum exceeded yield of continuous sorghum at all N rates. Based on relative yields at the maximum profit N rate, the average fertilizer N credit between the two locations attributed to soybeans was about 84 lb N/acre.