Agronomic studies on irrigated soybean in southern New South Wales. I. Phenological adaptation of genotypes to sowing date

Serial sowing date studies were used to examine the response of a diverse range of soybean genotypes to sowing date in the Murrumbidgee Irrigation Area (MIA). The aim was to explore the scope to improve the flexibility for rotating irrigated summer soybean crops with winter cereals by broadening the range of potential sowing dates. Serial sowings of diverse genotypes were made in small plots at intervals of similar to 7 days (2006-07) or 10 days (2007-08) from late November to late January (2006-07) or mid-February (2007-08) and the dates of flowering and maturity recorded. Simple linear models relating rate of development towards flowering to photo-thermal variables indicated that large differences in time to flowering between genotypes, sowing dates, and years could be explained in terms of differences in genotype sensitivity to mean photoperiod and/or mean daily temperature between sowing and flowering. In general, warmer temperatures hastened and longer days delayed flowering, consistent with quantitative short-day photoperiodic response. The earliest flowering genotypes were insensitive to the prevailing photoperiods, and their smaller variations in time to flower over sowing dates and years were related to temperature. Conversely, later flowering genotypes were progressively more sensitive to photoperiod, with flowering occurring later and being more responsive to sowing date. In both seasons, late maturing genotype x sowing date combinations suffered cold temperature damage and frosting. For those genotype x sowing date combinations that were physiologically mature before the first frost, crop duration was a linear function (r(2) = 0.86**) of time to flowering. In 2007-08, measurements were also made at maturity of total standing dry matter (TDM), seed yield, and seed size. For those genotype x sowing date combinations that matured before the first frost, TDM was largely a linear function (r(2) = 0.83**) of crop duration, while seed yield was strongly related (r(2) = 0.86**) to TDM. Exposure to cold temperatures before physiological maturity reduced seed size and harvest index. Using the generalised relations developed in these studies, it was concluded that commercial yields may be possible for irrigated soybean crops in the MIA sown in December or possibly later. These options are evaluated in greater detail in the companion paper, using large-scale agronomic trials of a subset of adapted genotypes.