Determining the optimum planting date × maturity group combination for soybean produced in South Carolina

Previous research has shown that delayed planting of soybean (Glycine max L. Merr.) can reduce yield by as much as 30 lb ac⁻¹ day⁻¹ when planted after mid-June. In South Carolina, soybean is often planted in rotation with other crops or double-cropped behind cereal grains, which can lead to delayed planting and potential yield-loss. In this study, our objective was to determine the optimum planting date (PD) × maturity group (MG) combination on non-irrigated soybean yield in South Carolina, and to determine yield results for the entire planting window ranging from March through August. Four MGs (IV, V, VI, and VII) were planted on six PDs (March–August) in 2021 and 2022 in Florence and Blackville, SC. Data collection consisted of stand counts to determine final plant populations, end-of-season plant height and node counts, and yield/moisture content at harvest. The April, May, and June PDs resulted in the highest grain yield in Blackville in 2021 (averaging 76 bu ac⁻¹) and 2022 (averaging 42 bu ac⁻¹). The April and May PDs had the highest grain yield in Florence when averaged over both years (53 bu ac⁻¹). MG alone did not influence yield in Florence. However, MGs V, VI, and VII produced the highest yields in Blackville. The optimum PD × MG combination for yield was the May planted MG V in Blackville (88 bu ac⁻¹ in 2021 and 49 bu ac⁻¹ in 2022) and the April planted MG VII in Florence (65 bu ac⁻¹). Plant heights and node counts were highest when soybean was planted in April and May, and MG IV had the tallest plants overall due to its indeterminate growth habit. Results from this study suggest that planting soybean as early as late-March and as late as late-June may not reduce soybean yield in South Carolina as some late-March and late-June MG combinations met or exceeded the state yield average of 37 bu ac⁻¹ and did not differ statistically from April and May yields. This research has already impacted soybean growers in South Carolina as the crop insurance window for full coverage has been extended to include earlier and later PDs as of 2023.