SPATIOTEMPORAL VARIABILITY OF CORN AND SOYBEAN YIELD

Little is known about the spatial structure of yield across fields, no r of the temporal stability of this structure. We determined the spati al structures of grain yield for 6 yr of data and examined the stabili ty of these structures over time. Yields were measured each year at 22 4 locations on an 8 x 24 grid within a 16-ha field, using plot combine s. The field was planted either to corn (Zea mays L.) or soybean [Glyc ine max (L.) Merr.] in an alternate-year rotation. Soybean yields aver aged 3.16, 3.14, and 3.12 Mg ha(-1) in 1990, 1992, and 1994. Corn yiel ds averaged 9.30, 9.34, and 5.02 Mg ha(-1) in 1989, 1991, and 1993 wit h the lower yield in 1993 a result of excessive rainfall that gear. Th e large-scale deterministic structure of the spatial yield data was co nstructed for each year by median polishing. The trend surfaces were d ifferent for each gear, indicating a lack of temporal stability in lar ge-scale structure, although the trend surfaces shared some similar fe atures. Overall, the deterministic structure accounted for about 25% o f the overall yield variance. The small-scale stochastic spatial struc ture was determined by computing variograms of the yield residuals aft er subtracting the trends. Variograms showed strong spatial structure of the yield residuals, with zero nugget and correlation ranges approa ching 150 m. Variograms differed from year to year and were not relate d to the crop being grown. The range was significantly correlated to p recipitation, Increasing with increasing total growing-season rainfall . Long-term monitoring of yield will be necessary to fully characteriz e its spatial-temporal pattern.