Early-maturing soybean cropping system: I. Yield responses to planting date

An early-planted, early-maturing soybean [Glycine max (L.) Merrill] cropping system has been quite successful across the southeastern USA. This cropping system was intended to use early planting to improve yield by avoiding late season drought stress. However, producers in this region use planting dates ranging from late April through late June. Our objective was to characterize the yield response of early-maturing cultivars to environmental conditions over a range of planting dates. Field studies were conducted on a well-drained Maury silt loam (fine, mixed, mesic Typic Paleudalf) in 1990 through 1993 using cultivars from Maturity Group (MG) 00 through IV planted in late April, mid-May, early June, and late June each year. Modest moisture stress was observed over tile years tested; however, temperatures during soybean growth stages depended strongly on planting dates. When averaged across years and cultivars, yields were similar for the first three planting dates. Best overall yields were obtained using the MG IV cultivar for late April and mid-May planting; however, its advantage over MG II and III cultivars diminished for the early June planting date and disappeared for the late June planting date. Early-maturing cultivars (MG 00 to I) were less likely to achieve competitive yield levels. Lower yields of early-maturing cultivars for the late April planting date were associated with low temperatures during vegetative growth and with high temperatures during seed-fill. In the absence of pronounced moisture stress, low or high temperatures may become the key environmental factors limiting yield. Early planting, a centerpiece of previous successful early-maturing soybean cropping systems in the southeastern USA, appears to be less important under more favorable moisture conditions.