Effects of relative sowing time of soybean on growth and yield of cassava in cassava/soybean intercropping

The development of yield in cassava, either as a sole crop or intercropped with quick-maturing soybean sown 1, 5 or 9 weeks after cassava planting, or a succession of two soybean sowings 1 and 14 weeks after planting, was followed at a high latitude (27°S) where the cassava growing season is limited to 9 months by winter temperatures. Competition, at least largely for nitrogen, restricted the growth of cassava, but after soybean harvest, leaf-area index increased such that there was little difference in interception of radiation among crops, and consequently growth rates and thus amounts of assimilate potentially available for tuber growth were similar. Competition from earlier-sown soybean greatly reduced branching by cassava. The reduced number of branches were sufficient to provide adequate leaf-area index, but were a reduced sink for assimilates during the main period of tuber growth. The slightly reduced assimilate supply available in early intercropped cassava was offset by the increased partitioning to tubers. As a result, soybean intercropping did not reduce tuber yield, except slightly in the case of double-intercropped cassava, but provided an additional yield of grain. Land equivalent ratio was particularly high at about 1.6 when soybean was sown within 5 weeks of cassava planting. When soybean was sown 9 weeks after cassava planting, land equivalent ratio was reduced to about 1.3 as a result of lower soybean yield