WEED-CONTROL IN FLOODED RICE WITH VARIOUS HERBICIDE COMBINATIONS IN THE SOUTHERN GUINEA SAVANNA ZONE OF NIGERIA

Field trials were conducted during the 1990 and 1991 wet seasons at Badeggi to assess the efficacy of various herbicide combinations for post-emergence and pre-emergence control of a weed flora dominated by Digitaria debilis (Desf.) Willd., Leptochloa caerulescens Steud., Ludwigia spp., Sphenoclea zeylanica Gaertn. and Cyperus difformis L. in direct-seeded and transplanted lowland rice cv. Faro 29. In 1990 post-emergence application of piperophos + propanil at 1.45 + 2.5 kg a.i./ha, oxadiazon + propanil at 1.0 + 3.0 g a.i./ha and fluorodifen + propanil at 1.8 + 1.2 kg a.i./ha resulted in comparable weed control and grain yield to hand-weeding twice in both trials. These herbicide mixtures increased rice grain yield by 344.5-388.9% and 433.3-533.4% over the unweeded controls, which yielded 0.9 t/ha and 0.6 t/ha in the transplanted and direct-seeded lowland rice respectively. All the pre-emergence herbicides resulted in a lower number of panicles and a lower grain yield than hand-weeding twice and the weed-free control because of herbicide phytotoxicity or poor weed control efficacy in the direct-seeded lowland rice. However pre-emergence oxadiazon at 1.0 kg a.i./ha increased rice grain yield by 520.0% above the unweeded control yield of 0.6 t/ha. Pretilachlor + dimethametryn at 1.5 + 0.5 kg a.i./ha, 1.88 + 0.63 kg a.i./ha and oxadiazon at 0.75 and 1.0 kg a.i./ha applied pre-emergence resulted in comparable weed control, number of panicles and grain yield to handweeding twice in the transplanted lowland rice. In 1991 pretilachlor + dimethametryn at 1.88 + 0.63 kg a.i./ha recorded the highest grain yield, which was a 318.2% increase over the unweeded control yield of 1.1 t/ha in the transplanted lowland rice. Post-emergence application of piperophos + propanil at 1.45 + 2.5 kg a.i./ha resulted in the highest net return of Naira 13650.0 and a marginal benefit/cost ratio of 23.3.