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.