Standardized precipitation index and nitrogen rate effects on crop yields and risk distribution in maize

Crop performance in rainfed cropping systems generally is dependent on rainfall amount and distribution. The objective of this study was to analyze the long-term consequences of rainfall expressed as a standardized precipitation index (SPI) and fertilizer nitrogen (N) on yields and risk probabilities of maize in the udic-ustic moisture regimes in the Great Plains in Nebraska. The SPI is a precipitation index for classifying drought stress conditions. The study was conducted on a Kennebec silt loam (Cumulic Hapludoll) over an 11-year period, 1986-1996, using monoculture maize (Zea mays L.) and maize in rotation with soybean (Glycine max. (L.) Merr.) in combination with N fertilizer levels between 0 and 160 kg ha(-1). Maize yields in monoculture ranged from 4.8 to 5.7 Mg ha(-1), and from 6.4 to 6.8 Mg ha(-1) in rotation. The differences in yields between monoculture and rotation were larger at low N rates and decreased as N fertilizer increased above 40 kg ha(-1). Current year's maize yields either exhibited a weak or no response to N fertilizer in years when the preceding preseason (October-April) and the previous growing season (May-August) were dry (negative SPI value). Regression of yield as the dependent variable and the 12-month April SPI as the independent variable explained up to 64% of yield variability in a curvilinear relationship. Optimum SPI values were in the range of -1.0 to 1.0, substantiating the adaptability and performance of crops under mild stress as proposed by Ether scientists, Prediction of subsequent yields using past SPI data was relatively better in rotations (R-2=41-50%) than in monoculture (R-2=15-40%). Risk, calculated as the lower confidence limit of maize returns over variable cost of fertilizer, was less in rotations than in monoculture, and in both cropping systems returns were maximized with the application of N fertilizer at 40 kg ha(-1). Used with other criteria, the SPI can be a practical guide to choice of crops, N levels, and management decisions to conserve water in rainfed systems. (C) 2000 Elsevier Science B.V. All rights reserved.