High Summer Temperature in the Warm Region of Japan Negatively Affects Soybean Yield.

The yield of soybean recorded in the contest for good practice farming (contest yield) varies from 2t/ha to over 4t/ha within each of major cultivars in Japan. These records are from relatively well controlled fields and free from major failure of crop establishment and pest damages. Thus the variation of yield among the contest yields reflects constraints on seed production by the mid-season weather condition and the high-yielding techniques specific to each farmer. An analysis on the meteorological factors of yield variation was attempted to identify how the contest yields are affected by solar radiation, precipitation and temperature. The meteorologically possible yield (Ymax) was calculated for each sample by a simple model based on sowing date, air temperature and solar radiation. Ymax was compared with actually reported yield (Yac) and the factors to differentiate Yac/Ymax is being analyzed. The model-assisted analysis on meteorological impact on soybean production supports the observation in the TGC study, the negative effect of high temperature on seed yield at Kyoto. The correlation between Ymax and Yac was significant but weak, indicating that other factors than catchment of solar energy cause variation of the 'good practice' soybean yield. Drought occurrence during early seed filling and high daily minimum temperature during pod setting and mid seed filling tended to reduce yield in terms of Yac/Ymax in the early cultivar Enrei (MG IV). But it was not the case for the late cultivar Fukuyutaka (MG VI). The difference between two cultivars may reflect the different meteorological conditions the two cultivars experience. The result suggested the high summer temperature in the warm region of Japan may be affecting negatively the soybean yield.