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 ~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 × sowing date combinations suffered cold temperature damage and frosting. For those genotype × sowing date combinations that were physiologically mature before the first frost, crop duration was a linear function (r2 = 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 × sowing date combinations that matured before the first frost, TDM was largely a linear function (r2 = 0.83**) of crop duration, while seed yield was strongly related (r2 = 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.
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.
Powdery mildew—caused by the fungus Erisyphe diffusa (syn. Microsphaera diffusa)—was first observed in commercial soybean crops in southern New South Wales (NSW), Australia, in 2011. Its detection raised concerns that soybean production might be constrained if the severity of the disease reached the levels observed in northern Australia. Field experiments were conducted over four consecutive seasons to examine the response of three soybean cultivars—Djakal, SnowyA and the breeding line N005A-80—to two fungicides and two fungicide application regimes. The cultivar Djakal was identified as having a high level of resistance to powdery mildew. The severity of infection symptoms varied between seasons. The most severe symptoms were observed during the 2014–2015 season which resulted in the largest grain yield reduction of 20% for the cultivar SnowyA. All fungicide treatments provided a significant reduction in the severity of symptoms, with the split application of tebuconazole and both the single and split applications of tebuconazole + prothioconazole providing the most effective control of the disease. Few other grain yield effects were found, even when strong disease control was achieved. This was a suspected result of the consistent late-in-the-season onset of the disease. Few differences were observed among the treatments in terms of lodging severity, date of physiological maturity, or grain oil and protein concentrations. It was concluded that both fungicides provided effective control of powdery mildew. However, when disease pressure is low, application might not be warranted in southern NSW.
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