Abstract
We investigated the yield increase under twin-row cultivation of soybean cultivars differing in stem growth habit and high-yield factors among yield components measured in two consecutive years. ‘Toyomusume’ (determinate type) and ‘Brock’ (indeterminate type), both medium-maturing cultivars (MGI), were sown at five planting densities ranging from 8.3 to 55.6 plants/m2 with conventional row (CR) and twin-row (TR) planting patterns. The seed yield for both cultivars in the 2 years was TR ≧ CR at all planting densities. The mean yield increase of TR vs CR for Toyomusume and Brock was estimated 107% and 111%, respectively, for the 2-year period. A quadratic regression equation best fit the relationship between planting density and seed yield for both cultivars and planting patterns. The optimal planting density with the highest yield increase was 27–31 plants/m2, The yield increase at the optimal planting density for Toyomusume and Brock was 110% and 120%, respectively, and it tended to be higher for the indeterminate cultivar than the determinate cultivar. The greater yield increase of indeterminate cultivars with dense planting reflected the suppression of the decrease in branch yield associated with dense planting. This suppression of the decrease in branch yield was thought to be due to the increase in the number of branches and the small decrease in the number of pods per node on each branch with dense planting.
