Abstract

Soybean genotypes differed significantly in their ability to suppress speargrass due to their inherent traits to compete with speargrass under different management practices. Soybean genotypes and velvet bean (control) were sown in tilled and slashed at two different row spacing of 75 × 10 cm and 50 × 10 cm. The result showed that soybean plants were taller with thicker canopy cover in the tilled plots than in the slashed plots.  There was lower speargrass dry matter and higher grain yield in the tilled plots than the slashed plots. Taller soybean plants with thicker canopy cover were found in narrow row (50 cm). However, the grain yield was similar in both rows. Speargrass shoot dry matter was lower in the narrow row than the wide row (75 cm). There was significant reduction in speargrass dry matter (shoot and rhizome) at harvest in the plots sown to velvet bean compared to the plots sown to soybean genotypes. This accounted for 98% reduction in speargrass dry matter in velvet bean plots and about 70% reduction in soybean plot compared with the initial speargrass dry matter content before planting. TGX1844-18E gave highest grain yield of 985.65 kg/ha, which was similar to TGX144-2E (896.82 kg/ha). There was significantly negative correlation between soybean shoot height (-0.51 and -0.57); canopy cover (-0.68 and -0.73) and speargrass shoot and rhizome dry matter respectively. The result showed that the soybean genotypes can suppress speargrass with tall shoot, dense canopy with considerable grain yield.

Key words: Tilled, slashed, speargrass, soybean genotypes, canopy cover.