Response of soybeans to sowing date during the wet season in the Ord Irrigation Area, Western Australia

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Abstract
The responses of 3 soybean cultivars to sowing date during the wet season in the Ord Imgation Area (OIA) in northern Western Australia are described. The cultivars, Buchanan, Ross and Durack, are classified as early, medium and late maturity respectively, when grown during the wet season in the OIA. The cultivars were grown 10 times between 22 December and 18 April during 2 wet seasons and we investigated the patterns of phenology, growth and seed production. Sowing date had no significant effect (P> 0.05) on the time to flowering (26-36 days) with cv. Buchanan, but the duration of flowering was curtaiied so that the period from sowing to maturity declined from 120 to 95 days as sowing was delayed from December to April. In contrast the time to flowering of cv. Durack declined in response to photoperiod from about 70 to 40 days and the time from sowing to maturity declined from about 160 to 100 days. The responses of cv. Ross were intermediate between those of Buchanan and Durack. The phenological responses to sowing date were consistent with responses to photoperiod rather than to temperature. Sowing date also affected plant morphology and yields and quality of seed. Delay in sowing after December led to declines in above-ground dry matter yields at flowering, in number of nodes on the main stem at flowering, crop height at maturity and seed yields. Mean individual seed weights increased with delay in sowing. Oil concentrations in the seed declined (from 23 to 17%) and protein concentrations increased (from 32 to 45%) as the period of pod development occurred later in the season. Phenology is a major determinant of the suitability of a cultivar for specific cropping systems. The early maturing cultivar, Buchanan, most closely meets the requirements for a system of double cropping in which the wet season soybean crop is followed by a May sown dry season crop. The late maturing cultivar, Durack, is suitable for a system involving a single wet-season crop.
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