Effects of growth habits of grasses on weed population and dry matter yield in grass-legume swards

A study was carried out over a period of two years to evaluate the ability of 12 grass-legume mixtures to compete with weeds over a 12-week period following planting in 2004 and after cutback in 2005. Plant height, leaf number, weed population and dry matter (DM) yields of weeds and grasses at 6 and 12 weeks after planting were recorded. Leaf number of the grasses increased slowly during the first 8 weeks after planting, then increased exponentially between 9 and 12 weeks, reaching 145 leaves/plant. The bunch-forming grasses had more (p<0.05) leaves than the Brachiaria species. In the second year, leaf number reached 329 leaves/plant. Weed populations were prominent during the first 8 weeks after planting, with broadleaf weeds the most common in both 2004 and 2005. Mixtures containing B. ruziziensis recorded the greatest (p<0.05) weed populations, and swards of B. ruziziensis-T. bracteolata consistently recorded the least (p<0.05) DM yields of 281 and 326 kg/ha at 12 weeks after planting in 2004 and 2005, respectively. Sward mixtures containing Penni-setum purpureum and Andropogon tectorum consistently had the greatest (p<0.05) yields (1100-2000 and 1500-2200 kg/ha in 2004 and 2005). The population of weeds in the 2 years declined with increase in growing period.