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.
