Effect of seed moisture content following hand harvest and machine threshing on seed quality of cool tolerant soybean (Glycine max (L.) Merrill.)

Cool tolerant soybean (Glycine max (L.) Merrill) cultivars are a potential new crop for Canterbury, New Zealand, but cool temperatures during seed maturation may prevent seed desiccation to the 'safe' harvest moisture content of ca. 15%. In an experiment in the 2000/2001 season with cv. Northern Conquest and March, the effect of seed moisture content (SMC) at harvest on seed quality (germination and vigour) was determined. Seeds were hand harvested to avoid mechanical damage. At the first harvest (54-56% SMC) the mean germination was 96%, accelerated ageing (AA) germination was 94%, and conductivity was 17.06 muS/cm/g. At 'harvest ripeness' (11-14% SMC) the corresponding data were 93%, 82% and 21.57 muS/cm/g. Germination had therefore not changed but vigour had declined. In an adjacent experiment in the same season the effect of seed moisture content at machine threshing was examined. Germination was reduced by machine threshing at SMC > 16-19%. Seed vigour was highest at a SMC of 13-15%, and was reduced (lower AA germination and higher conductivity) when seed was harvested at higher SMC. Machine threshing at 10-12% SMC also reduced seed vigour. For these two cultivars in this environment, soybeans can be machine threshed at ca. 15% SMC without dramatically impairing seed quality, provided desiccation in the field can continue long enough for them to reach this seed moisture content.