Plant Population Density and Row Spacing Effects on Soybean at Post-Optimal Planting Dates
Information on cultural practices for determinate soybean, Glycine max (L.) Merr., at post-optimal planting dates is needed in the southern USA where planting is often delayed by weather or doublecropping. The objective of this study was to examine the effects of plant population density (PPD) on determinate soybean at two postoptimal planting dates in relation to interrow spacing, plant architecture, and yield components. A 3-yr. soybean field study on Sharkey clay (very fine montmorillonitic, thermic, nonacid, vertic Haplaquept) was planted at post-optimal dates of 20 June and 3 July. ‘Davis’ (MG VI) and ‘Braxton’ (MG VII) soybean were planted in 0.5 and 1.0 m row spacing (RS) at PPD at 6.4,13, 26, 38, 51 plants m−2 under irrigated and nonirrigated conditions. Yields were higher at the 0.5 m RS than at the 1.0 m RS, and were higher at the June date than at the July date. Highest yields at the June date (2.99 and 1.97 Mg ha−1) were with PPD of 38 and 13 plants m−2 at the 0.5 and 1.0 m RS, respectively. Yields were highest at the July date (2.45 and 1.53 Mg ha−1) with PPD of 51 and 26 plants m−2 at the 0.5 and 1.0 m RS, respectively. Increase in PPD up to 38 plants m−2 increased main stem length and decreased all other plant components. Decrease in main stem yield was proportionately smaller than the increase in PPD resulting in a net yield gain from higher PPD. Decrease in branch stem yield was proportionately greater than the increase in PPD resulting in a net yield loss from PPD increase. For determinate soybean planted at a 0.5 m RS an increase in PPD above that needed at optimal dates was necessary to obtain highest yields at post-optimal planting dates. Soybean planted at the 1.0 m RS had much lower yield and was much less responsive to increase in PPD.
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