Growth responses of soybean (Glycine max) and sorghum (Sorghum bicolor) to an increase in density of Amaranthus dubius (L.) plants at two temperatures

Plants of soybean (Glycine max) and sorghum (Sorghum bicolor) were grown either alone or in combination with 1, 2, 4, 8 and 16 Amaranthus dubius plants per pot in controlled environment chambers at 31/24 and 25/18°C day/night temperatures. Above-ground dry weights, leaf areas and heights were measured at 17 and 35 days after planting. Growth of both species increased with temperature. At the second harvest, at both temperatures, even one A. dubius plant per pot had a significant effect on growth of the crop species, and the effects of the two highest densities were equivalent. Shoot dry weights of A. dubius plants were significantly lower when grown with soybean than with sorghum at the same conditions. Linear regressions showed that the dry weights of the crop species were more highly correlated with biomass than with numbers of the interfering weeds, and suggest that soybean may be more affected by an increase in abundance of A. dubius than is sorghum.