The effects of phenological and meteorological factors on soybean yield

The influence on yield of variation in phenology and the prevailing temperature/radiation conditions was studied for ten sowings of 'Lee' soybeans under irrigation at a latitude of 34 degree S. The durations of the preflowering period, the flowering period, and the pod filling period, the mean daily screen temperature and total daily solar radiation during the experiment were observed for each sowing date and appropriate mean values of a series of environmental variables were calculated for the phases of development applicable to each sowing date. Three major yield components were taken separately; bean density (beans m(-2), bean growth rate (mg bean(-1) day(-1)) and the duration of bean filling days (days). The variation in the first two of these parameters across sowing dates was explained in terms of either further components of yield or the calculated meteorological variables. A stepwise regression procedure was used to select the best predictors from the sets of possible causal factors. Bean growth rate was best predicted by the temperature during flowering and the quotient, radiation per day during the pod filling period divided by bean density. Bean density was further divided into components of the number of reproductive nodes and the number of beans per node. There was a strong negative correlation between these components. Reproductive node number was best predicted by the total number of nodes/m(2) which in turn was best predicted by a function of the duration of the preflowering period and plant density. Beans per node was best predicted by temperature during flowering and bean filling, radiation during bean filling and by the number of pod bearing nodes. The relative importance of the various factors correlated with yield potential in soybeans was evaluated. The importance of the duration of the preflowering and bean filling phases and of temperature during flowering was demonstrated for this set of conditions. eng