ROTATION EFFECT ON SORGHUM RESPONSE TO NITROGEN-FERTILIZER UNDER DIFFERENT RAINFALL AND TEMPERATURE ENVIRONMENTS

Cropping sequence effects on sorghum (Sorghum bicolor L. Moench) respo nse to nitrogen fertilizer under different rainfall and temperature re gimes were evaluated in eastern Nebraska. The Standardized Precipitati on index (SPI) and temperature Z-scores were used to characterize the 18-yr cropping period into eight sets of contrasting cropping environm ents. Mean sorghum yields ranged from 4050 to 6260 kg/ha in continuous cropping, and from 5130 to 7120 kg/ha in rotation with soybean (Glyci ne max), a significant increase with crop rotation. Yields were lower in dry years with hot April temperatures than in dry years with cool A pril temperatures. Hot April temperatures improved yields in wet years . April temperature did not affect yield if a wet pre-season was follo wed by a dry cropping season. Sorghum responded more to fertilizer N i n dry than in wet years, reflecting less available N from soybean and the soil nutrient complex in the previous year. Sorghum grown in rotat ion with soybean did not generally respond to fertilizer N, suggesting that high fertilizer N rates are unnecessary in rotation systems. Low September temperatures reduced final yields. Given the high probabili ty of confronting adverse cropping conditions, it is strongly recommen ded that sorghum be considered as an important component of rainfed cr opping systems in eastern Nebraska to ensure minimum crop loss to farm ers. Greater economic gains can be achieved by using rotations with so ybean because of higher sorghum yields and lower fertilizer costs, and these are most likely under the favorable rainfall and temperature re gimes identified in this study. The results provide useful information for fine-tuning management options to maximize sorghum yields and red uce input costs in atypical years. (C) 1998 Elsevier Science B.V.