Expanding production of early‐maturing soybean [Glycine max (L.) Merrill] cultivars in the southeastern USA has exposed such cultivars to a wide range of environmental conditions during seedfill. Temperatures during this growth stage influence levels of specific fatty acids, particularly of the unsaturated fatty acids. Our objective was to evaluate the grain quality responses of early‐maturing cultivars to the wide range of planting dates in the southeastern USA. Protein and oil contents, along with fatty acid profiles, were assessed for cultivars from Maturity Group (MG) 00 through IV using late April, mid‐May, early June, and late June planting dates on a well‐drained Maury silt loam (fine, mixed, mesic Typic Paleudalf) in 1990 through 1993. Across years and cultivars, delayed planting increased protein content and linolenic acid levels and reduced oil content and oleic acid levels, but had little or no influence on palmitic, stearic, or linoleic acid levels. The higher seed‐fill temperatures associated with early planting were strongly correlated with increased oil content and oleic acid levels and reduced linolenic acid levels. Increasing seed‐fill temperatures were closely associated with reduced linolenic acid levels for all six cultivars. However, the oleic acid response to seed‐fill temperatures strongly depended on cultivar maturity. Oleic acid levels of early‐maturing cultivars were much more sensitive to seed‐fill temperatures than were those of later maturing cultivars. While overall effects of environment on grain quality characteristics may be relatively small, perhaps the quality of new low linolenic acid cultivars could be amplified through culture under the warmer conditions of the southeastern USA.

An early planted, early maturing soybean [Glycine mar (L.) Merr.] cropping system could stagger harvest maturity and increase the number of options available for subsequent crops. However, premature flowering of early maturing cultivars could reduce canopy closure at flowering (CCR1) or at beginning seed fill (CCR5) as well as vegetative mass at R5 (VMR5). This study was conducted to determine the Maturity Group (MG) best suited for this early planted cropping system, and to relate growth, development, and environmental parameters to yield. Two cultivars each from MG 00,0,I,II,III, and IV were planted in six Kentucky field environments ranging from 36°40' to 38°7'N lat from 1986 to 1989. Vegetative, pod‐set, and seed‐fill durations were generally shorter in early‐maturing cultivars, but also interacted with environment. Maturity groups 00 and 0 had inadequate canopy closure at R1 and R5, inadequate vegetative mass at R5, and produced lowest overall yields. While MG I and II appeared to be most sensitive to vegetative and pod‐set rainfall and seed‐fill temperatures, MG II produced the best mean yield. Seed‐fill duration and rainfall were apparently more important for MG III and IV than for earlier MGs. Maturity Group II appears to be more adaptable than other MGs in these early planted, early maturing soybean cropping systems in Kentucky.

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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 the 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 seedfill. 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.

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.