Cogongrass (Imperata cylindrica) and guineagrass (P. maximum) are serious perennial weeds in small-scale farms in lowland subhumid zones of West Africa. Field studies were conducted in 2002 and 2003 at two sites in Ibadan, Nigeria (Ijaye and the International Institute of Tropical Agriculture (IITA)), to evaluate the effect of rimsulfuron on weed communities dominated by cogongrass and guineagrass in maize. At both sites, the treatments were rimsulfuron dosages of 0 (nontreated control), 10, 20, 30, 40, 50, 60, 70, and 80 g a.i./ha. Rimsulfuron did not cause any visible phytotoxicity on maize at any dosage at either site. There was a rapid increase in weed control as the dosage of rimsulfuron increased from 0 to 20 g/ha. Weed control was not improved at rates higher than 20 g/ha. Rimsulfuron was very effective against sedges, Ipomoea involucrata, Bengal dayflower, gulf leafflower, old-world diamond-flower, and wild jute providing more than 80 control at dosages between 10 and 20 g/ha at Ijaye. Rimsulfuron was less effective for cogongrass, with a maximum of only 38 control observed. At IITA, the herbicide was very effective against guineagrass, Bengal dayflower, nodeweed, coat buttons, redfruit passionflower, and waterleaf; all of which were controlled by more than 70 with any rate of rimsulfuron. Regression analysis showed that the dosage of rimsulfuron required to reduce the shoot dry biomass by 70 was 5 g/ha for guineagrass and 35 g/ha for cogongrass at 3 weeks after treatment (WAT). At crop maturity, the dosage of rimsulfuron required to reduce the shoot dry biomass by 70 was 43 g/ha for guineagrass and 200 g/ha for cogongrass. The dry biomass of cogongrass and guineagrass was higher at crop harvest than at 2 WAT regardless of herbicide dosage. Maize grain yield was 1.8 times higher at IITA than at Ijaye. At both sites, the maizegrain yield increased with increased herbicide dosage. The maximum maize grain yields were obtained at a rimsulfuron dosage of 20 g/ha.
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