Fig. 1 Average normalized grain yields after 16 yr as affected by rotation and N treatment in a long-term cropping system study at Mead, NE. Rotation abbreviations: CC = continuous corn, CSB = continuous soybean, CSG = continuous sorghum, C–SB = corn–soybean, SG–SB = sorghum–soybean, OCL–SG–SB–C = oat + clover–sorghum–soybean–corn, SB–SG–OCL–C = soybean–sorghum–oat + clover–corn