Agronomic studies on irrigated soybean in southern New South Wales. II. Broadening options for sowing date

Abstract

The response of irrigated soybean to sowing date and to plant population was evaluated in field experiments over three years at Leeton, in the Murrumbidgee Irrigation Area (MIA) in southern New South Wales. The aim was to explore the options for later sowings to improve the flexibility for growing soybean in double-cropping rotations with a winter cereal. The experiments were grown on 1.83-m-wide raised soil beds, with 2, 4, or 6 rows per bed (years 1 and 2) or 2 rows per bed only (year 3). Plant population, which was manipulated by changing either the number of rows per bed (years 1 and 2) or the within-row plant spacing (year 3), ranged from 15 to 60 plants/m2 depending on the experiment. Two sowings dates, late November and late December, were compared in years 1 and 3, while in year 2, sowings in early and late January were also included. Three genotypes (early, medium, and late maturity) were grown in years 1 and 2, and four medium-maturing genotypes were grown in year 3. In general, machine-harvested seed yields were highest in the November sowings, and declined as sowing was delayed. Physiological analyses suggested two underlying causes for the yield decline as sowing date was delayed. First and most importantly, the later sown crops flowered sooner after sowing, shortening crop duration and reducing total dry matter (TDM) production. Second, in the late January sowings of the medium- and late-maturing genotypes, harvest index (HI) declined as maturity was pushed later into autumn, exposing the crops to cooler temperatures during pod filling. Attempts to offset the decline in TDM production as sowing was delayed by using higher plant populations were unsuccessful, in part because HI decreased, apparently due to greater severity of lodging. The studies indicated that, in the near term, the yield potential of current indeterminate cultivars at the late December sowing date is adequate, given appropriate management, for commercially viable double-cropping of soybean in the MIA. In the longer term, it is suggested that development of earlier maturing, lodging-resistant genotypes that retain high HI at high sowing density may allow sowing to be delayed to early January.