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Soybean Sowing Date: The Vegetative, Reproductive, and Agronomic Impacts

^a Dep. of Agronomy and Horticulture, Univ. of Nebraska, Lincoln, NE 68583-0915
^b Dep. of Agronomy, Iowa State Univ., Ames, IA 50011. Contribution of the Dep. of Agronomy and Horticulture, Univ. of Nebraska, Lincoln, NE 68583. Published as Paper no. 15201, Journal Series Nebraska Agric. Exp. Stn. Project No. 12-194. Funding for this research was received from the Nebraska Agricultural Research Division, Nebraska Soybean Development, Utilization, and Marketing Board, United Soybean Board, and Fluid Fertilizer Foundation

^* Corresponding author (jspecht1{at}unl.edu).

The sensitivity of soybean [Glycine max (L.) Merr.] main stem node accrual to ambient temperature has been documented in greenhouse-grown plants but not with field-grown plants in the north-central United States. Biweekly V-node and R-stage, stem node number, internode length, and other traits were quantified in an irrigated split-plot, four-replicate, randomized complete block experiment conducted in Lincoln, NE, in 2003–2004. Main plots were early-, mid-, late-May, and mid-June sowing dates. Subplots were 14 cultivars of maturity groups 3.0 to 3.9. Node appearance was surprisingly linear from V1 to R5, despite the large increase in daily temperature from early May (10–15°C) to July (20–25°C). The 2003 and 2004 May planting date regressions exhibited near-identical slopes of 0.27 node d^–1 (i.e., one node every 3.7 d). Cold-induced delays in germination and emergence did delay the V1 date (relative to planting date), so the primary effect of temperature was the V1 start date of linearity in node appearance. With one exception, earlier sowings led to more nodes (earlier V1 start dates) but also resulted in shorter internodes at nodes 3 to 9 (cooler coincident temperatures), thereby generating a curved response of plant height to delayed plantings. Delaying planting after 1 May led to significant linear seed yield declines of 17 kg ha^–1 d^–1 in 2003 and 43 kg ha^–1 d^–1 in 2004, denoting the importance of early planting for capturing the yield potential available in soybean production, when moisture supply is not limiting.

Abbreviations: CI, confidence interval • DAP, days after planting • Ddf, denominator degrees of freedom • DOY, day of year • ET, evapotranspiration • MG, maturity group • Ndf, numerator degrees of freedom • R, reproductive • V, vegetative

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Received for publication May 4, 2006.