Spatiotemporal Variability of Corn and Soybean Yield

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Abstract
Little is known about the spatial structure of yield across fields, nor of the temporal stability of this structure. We determined the spatial structures of grain yield for 6 yr of data and examined the stability of these structures over time. Yields were measured each year at 224 locations on an 8 × 24 grid within a 16‐ha field, using plot combines. The field was planted either to corn (Zea mays L.) or soybean [Glycine max (L.) Merr.] in an alternate‐year rotation. Soybean yields averaged 3.16, 3.14, and 3.12 Mg ha−1 in 1990, 1992, and 1994. Corn yields averaged 9.30, 9.34, and 5.02 Mg ha−1 in 1989, 1991, and 1993 with the lower yield in 1993 a result of excessive rainfall that year. The large‐scale deterministic structure of the spatial yield data was constructed for each year by median polishing. The trend surfaces were different for each year, indicating a lack of temporal stability in large‐scale structure, although the trend surfaces shared some similar features. Overall, the deterministic structure accounted for about 25% of the overall yield variance. The small‐scale stochastic spatial structure was determined by computing variograms of the yield residuals after subtracting the trends. Variograms showed strong spatial structure of the yield residuals, with zero nugget and correlation ranges approaching 150 m. Variograms differed from year to year and were not related to the crop being grown. The range was significantly correlated to precipitation, increasing with increasing total growing‐season rainfall. Long‐term monitoring of yield will be necessary to fully characterize its spatial‐temporal pattern.

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