The influence of fallow period on weed vegetation and rice yields in Sierra Leone

Field trials were conducted at three sites in Sierra Leone near Njala University College, to compare the weed vegetation before and after clearing operations and dryland rice yields on the same sites after zero, five‐year and 10‐year fallow periods respectively. The weed flora during the cropping period at all three sites bore some relation to the vegetation before clearing. Though there were no plant species common to all sites before farming began, the weed shift during the cropping period produced a number of such common species; Rottboellia exaltata L., Pennisetum subangustum Stapf & C. E. Harbbard, Calopogonium mucunoides Desv., Ageratum conyzoides L. and Panicum laxum Sw. were the most important. The five‐year fallow site had the greatest weed cover, height and biomass; next in order were the site under continuous cultivation and the 10‐year fallow site. Thus the latter site produced the highest rice yields, followed by the zero fallow site. Stam F‐34 (propanil + fenoprop) application was shown to be more economical than hand weeding on the zero‐ and five‐year fallow sites, but less so on the 10‐year fallow site.