Nitrogen response of grain sorghum in rotation with soybean

The grain sorghum [Sorghum bicolor (L.) Moench] and soybean [Glycine max (L.) Merr.] rotation is the major sorghum production system in Nebraska. Fertilizer N needs for rotations are commonly determined by adjusting the N rate for continuous sorghum by a fertilizer nitrogen replacement value (FRV). The FRV due to rotation with soybean varies widely and, given the importance of the soybean-sorghum rotation, a basis for direct determination of N rates for grain sorghum following soybean is needed that includes the cost of fertilizer N. Thirty-nine N rate trials for grain sorghum following soybean were conducted in southern Nebraska on medium to fine texture soils. The treatment structures varied but generally included five or more N rates in increments of 35 kg ha^-1 or less. Grain sorghum yield response to applied N and the economically optimum nitrogen rate (EONR) increased as yield level increased. The agronomic efficiency of applied N increased with increased yield level and, within each yield category, decreased with increased N rate. Agronomic N efficiencies were <6 kg grain kg N^-1 applied at sites with maximum yields of <6 Mg ha^-1, indicating presence of severe constraints other than N. The EONR decreased and the range of profitable N rates decreased as the N price-grain price (P_N:P_G) ratio increased. Expected sorghum yield, as well as P_N:P_G, was therefore important for the determination of EONRs. Soil organic matter (SOM; 17-37 g kg^-1 in the 0- to 20-cm depth) and soil nitrate concentration (1.3-6.7 mg kg^-1 in the 0- to 120-cm depth) were positively correlated with grain yield without N application, but showed no correlation with the yield response to applied N. Within the ranges represented by these trials, soil information was less essential for determining the EONR for grain sorghum following soybean than setting a realistic yield goal (YG).