Information is lacking on the performance of soybean [Glycine max (L.) Merr.] cultivars grown under lengthening rather than shortening days. Soybean cultivars in a range of maturity group (MG) were planted in March at Gainesville, Florida (29° 40' N) during 1976 through 1979. The soil type was Kendrick sand (loamy, siliceous, hyperthermic Arenic Paleudult) and Gainesville sand (hyperthermic, coated Typic Quartzipsamment). Cultivars were evaluated with two objectives in mind: I) to determine yield and reproductive performance of cultivars in a range of MGs under short but lenthening days, and 2) to identify high yielding cultivars which matured by late June so that a second warm-season crop could be planted.
Cultivars in MG 000 to I were uniformly early flowering, early maturing, short, and low yielding, but those in MG II, III, and IV were each slightly slower in reproductive development, of later maturity, taller, and higher yielding. Although MG II, III, and IV cultivars flowered within 33 days after emergence, their postflowering development was slower which resulted in more vegetative growth, a longer seed filling period, and better seed yields. With March planting at this location, MG II, III, and IV cultivars normally matured by late June. ‘Williams’ (MG III) was one of the better cultivars, yielding 3,000, 3,200, 2,760, and 2,800 kg/ha in the four seasons. ‘Amsoy 71’ (MG II), ‘Calland’ (MG III), ‘Union’ (MG IV), and ‘Cutler 71’ (MG IV) produced acceptable yields; however, the latter three cultivars had poorer seed quality, matured later, and often retained green stems and a few green leaves at pod maturity.
Cultivars of MG V, VI, VII, and VIII flowered about 10 days later than MG 000 to IV and progressed slowly into pod development. Cultivars of MG V matured most of their pods by 1 August but retained many green leaves and exhibited very poor quality seed. Those in MG VI, VII, and VIII were so adversely affected by the long days of May, June, and July that they did not complete reproductive growth until September and October.
With selection of soybean cultivars of appropriate maturity, March planting with maturation by late June was Eeasible at this latitude, thus permitting double cropping of soybeans after soybeans.