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 rain fall amount and distribution. The objective of this study was to analyze th e long-term consequences of rainfall expressed as a standardized precipitat ion index (SPI) and fertilizer nitrogen (N) on yields and risk probabilitie s of maize in the udic-ustic moisture regimes in the Great Plains in Nebras ka. The SPI is a precipitation index for classifying drought stress conditi ons. The study was conducted on a Kennebec silt loam (Cumulic Hapludoll) ov er an 11-year period, 1986-1996, using monoculture maize (Zea mays L.) and maize in rotation with soybean (Glycine max. (L.) Merr.) in combination wit h N fertilizer levels between 0 and 160 kg ha(-1). Maize yields in monocult ure ranged from 4.8 to 5.7 Mg ha(-1), and from 6.4 to 6.8 Mg ha(-1) in rota tion. The differences in yields between monoculture and rotation were large r 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 f ertilizer in years when the preceding preseason (October-April) and the pre vious growing season (May-August) were dry (negative SPI value). Regression of yield as the dependent variable and the 12-month April SPI as the indep endent variable explained up to 64% of yield variability in a curvilinear r elationship. Optimum SPI values were in the range of -1.0 to 1.0, substanti ating the adaptability and performance of crops under mild stress as propos ed 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=1 5-40%). Risk, calculated as the lower confidence limit of maize returns ove r variable cost of fertilizer, was less in rotations than in monoculture, a nd 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 pr actical guide to choice of crops, N levels, and management decisions to con serve water in rainfed systems. (C) 2000 Elsevier Science B.V. All rights r eserved.