Soybean [Glycine max (L.) Merr.] seed yield is influenced by planting date, pattern, and density of seeding, but cultivars differing in growth habit may vary in response to cultural treatments. Narrow‐row compared to conventional wide‐row plantings have consistently produced higher seed yields in the northern USA, where early maturity groups (MG) and indeterminate (INDT) types are commonly used. Positive responses to narrow rows have been less consistent in the southern USA, where late MG and determinate (DT) cultivars are common. Therefore, we hypothesize that this disparity in seed yield response to narrow‐row culture between the two areas is due to inherent differences in DT‐ and INDT‐type canopies resulting from their growth habits. This study, conducted in Gainesville, FL (29 ° 38′N) in 1984 and 1985, employed ‘Duocrop’ (INDT) and ‘Kirby’ (DT), May and July planting dates, 0.91‐, 0.61‐, and 0.30‐m interrow spacings, and 0.18‐ and 0.08‐m intrarow spacings in a Randomized Complete Block (RCB) design. Node and pod numbers, leaf area index (LAI), crop growth rate (CGR), total biomass, and seed yields were significantly increased (per unit land area) with increasing plant population density (PPD) up to a certain PPD, depending on spatial arrangement. The greatest seed yield of both INDT and DT types was from the May planting, narrow‐row culture (0.30 m), and high PPD, but response to PPD was confounded with squareness (ratio of intra‐ to interrow distance among plants) of planting pattern. High PPD (18 to 42 plants m−2 and high squareness values gave higher seed yields than combinations of lower PPDs and lower squareness values. We conclude that seed yield of both DT and INDT soybean in subtropical latitudes is optimized by May seeding, high PPD (40 plants m−2), and use of square planting patterns as approximated by narrow‐row culture.

Daylength limits adaptation of soybean [Glycine max (L.) Merr.] genotypes to a narrow range of latitudes and sowing dates by affecting vegetative and reproductive growth and development. Photoperiod and sowing‐date experiments were conducted in Gainesville, FL (29.38°N) during 1984 and 1985 to assess the vegetative and reproductive growth of soybean genotypes previously observed to differ in sensitivity to change in latitude and sowing date. ‘Kirby’ and ‘Improved Pelican’ and two breeding lines, F82‐7656 and F84‐1220, selected for adequate vegetative development under short days (“juvenile” trait), were used. Photoperiods included 14‐h, 18‐h, and natural‐winter daylengths between 22 Jan. and 30 Mar. 1984 in a greenhouse experiment, and 10‐h (covered 14 h) and natural‐summer days in 1985 in a field experiment (Jonesville Taxajunct Soil classified as loamy, mixed, thermic Arenic Hapludalf). Sowing dates in the 1984 and 1985 field experiments were monthly from 1 April through 1 August. Improved Pelican, Kirby, F82‐7656, and F84‐1220 ranged from highly to only moderately responsive to the photoperiod and sowing date treatments. In 1984, under natural‐winter days and 14 h, all plants averaged the R1 stage at 30 and 60 days after planting (DAP), respectively. Plants under 18 h were vegetative at 68 DAP, the termination date. Under 10 h in the 1985 field experiment, Kirby, F82‐7656, Improved Pelican, and F84‐1220 were in the R1 stage in 48, 52, 54, and 57 DAP, respectively. Plant height, node number, and DAP to R1 for F84‐1220 were most constant over the range of photoperiods and sowing dates, indicating efficacy of the juvenile trait in extending adaptability.Improved Pelican in the April and May sowings grew excessively tall (2 m) and aborted its flowers and pods, which indicated not only pre‐, but also postflowering daylength effects. Kirby and F82‐7656 at the August sowing date flowered too early, resulting in severe reduction of vegetative growth and pod number. In the 1 April and 1 August sowings and the 10‐h photoperiod, F84‐1220 was near optimum height and produced more branches, racemes, and pods than the other genotypes. We conclude that the juvenile trait, as demonstrated in line F84‐1220, can improve climatic adaptation of soybean cultivars by buffering photoperiod effects incident to suboptimal latitudes and/or sowing dates.