Crop rotation and nitrogen effects on normalized grain yields in a long-term study

Effects of year-to-year variability in agricultural production systems have always been a concern, but few studies are conducted for a long enough period of time where management system evaluations and assessments can be made. Given this limitation, questions about whether management systems are effective at reducing temporal variability remain in production agriculture. These questions prompted investigation of a long-term crop rotation study to determine effects of crop rotation and N fertilization practices in a rainfed environment on normalized grain yields. Sixteen years of grain yield data from an experiment with seven cropping systems (three monoculture, two 2-yr rotations, and two 4-yr rotations) and three N fertilizer rates are included in the study. Grain yields from 1983 through 1998 for each crop and N fertilizer treatment were normalized and then relative grain yield within a cropping system and N fertilizer treatment were combined, which resulted in relative yields for each cropping system and N fertilizer treatment combination in each year. Using the normalized yields, overall analyses of the 16 yr of data were conducted to assess what effects cropping systems and N fertilizer have on yield variability. These analyses demonstrated that crop rotation systems are more effective at reducing long-term yield variability than monoculture systems, even with N fertilizer. As expected, N fertility, obtained from either fertilizer or legumes in monoculture or rotation systems, is probably one of the most, if not the most important aspect in reducing yield variability. Analyses of normalized yields also demonstrated that reductions in yield variability could be obtained in many of our cropping systems with proper management.