Evaluation of the Efficacy of Different Rates of Herbicides on Weed Growth and Grain Yield of Two Rice Varieties in Two Rice Ecologies in Sierra Leone

Two rice varieties, NERICA L19 (weed competitive) and ROK10 (high yielding) were subjected to two pre-emergence (RiceForce and ButaForce) and two post-emergence (Stam and RiceForce) with different active ingredients and rates of application during the wet seasons of 2009 and 2010 at Rokupr Mangrove Associated Swamp and at Gbomsamba in the Boliland to determine (i) the effectiveness of different rates of application of pre-and post emergence herbicides on weed growth and yield of rice varieties, (ii) identify suitable herbicides for the two ecologies and (iii) the cost benefit of herbicides application. The results showed significantly reduction in weed population with herbicides application (P=.05). Percentage reductions over control ranged from 65 to 91 percent in 2009 and from 68 to 87 percent in 2010. The herbicides Stam (PE), RiceForce (Pe) and ButaForce (Pe) were the most effective in reducing weed population (P=.05). All herbicides treatments gave significantly higher grain yields than control plots. Grain yields increased with increased rates of application but yields from full and three-quarter doses were similar. Yields of the two varieties at Rokupr were similar to Gbomsamba. The three-quarter doses for the herbicides RiceForce (Pe) and ButaForce (Pe) could be considered for higher marginal returns. The herbicide, Stam (PE) though effective in reducing weed population gave low marginal returns. The results suggest that growing a weed competitive variety and a high yielding variety in conjunction with reduce herbicide rates of RiceForce (Pe) and ButaForce (Pe) could be an effective and economical weed management strategy for rice in the Mangrove Associated Swamp and Boliland ecologies in Sierra Leone.