Crop rotation and nitrogen rate effects on major field crops in eastern Nebraska

A study was conducted near Mead, Nebraska from 1983-1986 to determine the effects of crop rotation and nitrogen fertilization on yield of corn (Zea mays L.), soybean (Glycine max (L.) Merr.), and grain sorghum (Sorghum bicolor (L.) Moench). Yield of crops in continuous monoculture were contrasted with yields from 2- and 4-year rotations. Rotations included corn/soybean, grain sorghum/soybean, corn/soybean/grain sorghum/oat+clover (Avena sativa (L.) + 80% Melilotus officianalis (L.) Lam., 20% Tifolium pratense), and corn/oat+clover/grain sorghum/soybean. Each phase of every rotation occurred every year. Main plots (crop within rotation) were split and assigned 0, low or high N treatments. Corn and grain sorghum received 0, 90, or 180 kg N ha$\sp{-1}$ while oat+clover and soybean received 0, 34, or 68 kg N ha$\sp{-1}$ for the 0, low and high N rates respectively. Rotation increased yields over continuous monocultures an average of 38%, 16%, and 15% for corn, soybean and grain sorghum, respectively. Corn and grain sorghum produced maximum yield with 90 kg N ha$\sp{-1}$ in rotation, but required 180 kg N ha$\sp{-1}$ or more in continuous monoculture. Corn yields following oat+clover were greater than other rotations when precipitation was adequate, but corn following soybean produced more grain under limited water conditions (three out of four years). Sorghum yield differences among rotations were large without applied N, but were minimized or removed with fertilizer N applications. Soybean yield following grain sorghum was higher than soybean following corn, because soybean after grain sorghum responded to N applications. Four-year rotations including oat+clover increased yields over two-year rotations, although benefits of clover must be weighed against loss of a row-crop one year out of four and its potential to deplete soil water. The two-year soybean/grain sorghum rotation produced high yields of both crops without fertilizer N (soybean averaged 2.7 Mg ha$\sp{-1}$ and grain sorghum averaged 5.1 Mg ha$\sp{-1}$)