Planting dates for early-maturing soybean cultivars

Early planting dates may not be required for successful culture of early-maturing soybean [Glycine max (L.) Merr.] at midlatitudes in the USA. The objectives of this research were to evaluate early-maturing soybean cultivars in southern environments, to try to identify cultivar characteristics contributing to the fit of these cultivars into this cropping system, and to determine the range of planting dates that best discriminate among northern cultivars. Field trials, involving 12 Maturity Group (MG) II cultivars, were planted in late April to mid-May, late May to early June, mid-June, and mid-July on a Maury silt loam (fine, mixed, mesic Typic Paleudalf) near Lexington, KY, in 1993 and 1994. Yield response to planting date was consistent across years, showing best yields for mid-June plantings. Pioneer 9273 produced significantly more grain than all other cultivars. Across years and planting dates, the lowest yielding cultivar (IA 2008) produced 19% less than did Pioneer 9273, emphasizing the importance of cultivar selection. Canopy closure ratings at reproductive growth stages R2 or R5 were somewhat better related to yield than were plant mass (at R2 or R5) or plant height. The shortest cultivars had the greatest stubble losses; producers may want to avoid such cultivars. Identification of high-yielding cultivars was clearest for the first two planting dates. Perhaps cultivars that performed well under cool, moist soil conditions were favored by the earlier planting dates. Selection of early-maturing cultivars for production south of their traditional full-season areas appears to be best done from early planting dates in the southerly zone.