NITROGEN-FERTILIZER RESPONSE POTENTIAL OF CORN AND SORGHUM IN CONTINUOUS AND ROTATED CROP SEQUENCES

Crop management systems need to be designed to maintain economic profi tability and minimize negative environmental impact. The objective of this study was to determine the effects of previous crop, yield potent ial, and residual soil nitrate (RSN) on grain yield response to N fert ilizer of sorghum [Sorghum bicolor (L.) Moench.] and corn (Zea mays L. ). Trials were conducted on 38 farms in 14 counties in eastern Nebrask a during 1988 to 1990 and separated by previous crop into three groups , including (i) cereal [either sorghum, corn, oat (Avena sativa L.), w heat (Triticum aestivum L.), or rye (Secale cereale L.)], (ii) soybean [Glycine max (L.) Merr.], and (iii) forage legume [either alfalfa (Me dicago sativa L.), sweet clover (Melilotus officinalis Lam.), or red c lover (Trifolium pratense L.)]. The potential for a response to N fert ilizer was described by relating initially available N to yield level (N/Y). Initially available N included RSN to a depth of 40 in., prepla nt and starter fertillizer N, and NO3-N in irrigation water. Yield lev el was assumed to be equal to the maximum predicted yield from regress ion analysis in individual trials. The critical level of N/Y at which 95% of maximum predicted yield was attained without N fertilizer appli cation, was 0.80 lb initial N/bu grain for sorghum following cereal, 1 .44 lb initial N/bu grain for corn following cereal, 0.65 lb initial N /bu grain for corn following soybean, and zero lb initial N/bu for cor n following forage legume. Knowledge of initially available N relative to expected yield for individual fields will help farmers make sound economic and environmental decisions on the need for N fertilizer in c ontinuous and rotated crop sequences.