A regional analysis of the response of soybean yield to planting date

Planting date is a critical aspect of all soybean [Glycine max (L.) Merr.] production systems, but the response of yield to planting date fluctuates widely among environments. A combined analysis of many planting date experiments will provide a better estimate of the average response. Data from 28 nonirrigated planting date experiments that were conducted for more than 1 yr with at least three planting dates (a minimum of one in June) were combined by regions (Midwest, 9; Upper South, 10; Deep South, 9) for analysis. Experiments using the early soybean production system (ESPS) were not included. A segmented-linear regression model with two segments was used to determine when yield began a rapid decline as planting was delayed. The rapid decline began on 30 May in the Midwest, 7 June in the Upper South, and 27 May in the Deep South. The rate of decline was larger in the Upper South (1.1) and Deep South (1.2) than it was in the Midwest (0.7% points per day). Yield trends before the rapid decline began showed no significant advantage for early plantings. Only 23% of the April or early (first week) May plantings exhibited higher yields than later plantings and the average advantage was 7%. The average response to planting date was remarkably similar across all regions and there seems to be no consistent advantage for ultra-early planting dates with traditional cultivars, but there was a significant penalty for planting after the critical date in late May or early June.

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