WEATHER AND MANAGEMENT IMPACT ON CROP YIELD VARIABILITY IN ROTATIONS

Crop rotation offers several advantages to improve farmers' systems wo rldwide. The positive attributes of rotations are usually dependent up on crop choices, cropping sequence, soil fertility management, and wea ther factors. Of these, weather is most uncontrollable, but its effect s can be partially manipulated through management. This study presents information on how weather affected cropping systems in a 12-yr span. The study also illustrates the use of indices of weather (composite) variables to predict yields. The composite variables are three biologi cal windows (BW) and a standardized precipitation index (SPI). Biologi cal windows based on soil temperature and soil moisture indicate the n umber of days favorable for or detrimental to crop growth. Biological window 2 (temperature > 41 degrees F+moist soil) in combination with M ay temperature explained more than 80% of the variability in corn (Zea mays) and soybean (Glycine max) yields. August temperature negatively affected corn and soybean yields, especially in continuous monocrops. Preseason 9-mo SPI (September-May) explained up to 50% of the subsequ ent season's corn yields, and this information could influence crop ch oice. Overall, yields in rotations were higher than in continuous mono cropping systems. Nitrogen fertilization increased cereal yields more in continuous monocropping than in rotations with legumes. Corn and so ybean appeared more sensitive to soil moisture and temperature variabi lity (P < 0.0001) than sorghum (Sorghum bicolor IL] Moench) (P > 0.05) . Risk as measured by standard deviation in yields or incomes did not differ significantly among systems.