A survey was conducted in the sugarcane fields of Unilorin Sugar Research Institute, Ilorin in the southern Guinea savanna agro-ecological zone of Nigeria during 2011 and 2012 cropping seasons with an objective to identify the current status of prevalent weeds in rainfed and irrigated sugarcane fields. A quantitative method was employed for the enumeration of weeds. Quadrats were laid along transects and individual weed species in each quadrat was identified and counted. Simpson's diversity index, Sorensen similarity index and relative abundance were used to determine the weed community structure. A total of 51 weed species belonging to 40 genera within 16 families were identified across the sugarcane fields. Rainfed sugarcane field was higher in weed species (41) than irrigated fields (35 weed species). Rainfed fields also had the diversity index of 28.84 % compared to 21.66% species diversity computed for irrigated fields. The result generally shows that the similarities index of weed species between rainfed and irrigated fields is about 50 %. Among the 10 abundant weed species, two grasses viz. Panicum repens L. and Imperata cylindrica (L.) Raeschel were the most abundant weeds in sugarcane fields followed by Rottboellia cochinchinensis Lour and two sedge weeds Cyperus rotundus L. and Mariscus longibracteatus Cherm. Results obtained from this study would be useful in creating a weed management programme and making informed decision on choice of herbicides. Regular weed survey to identify possible problematic weeds and weed population shifts and direct research toward new or improved weed control measures is thereby recommended.

Experiments were conducted to evaluate the performance of some sugarcane varieties against natural weed infestation. The trials were laid out in a randomized complete block design with split-plot arrangement and three replications. The sugarcane varieties were the main plots while the sub-plots consisted of weeding regimes. Recommended cultural and agronomic practices were followed to raise the crops. Data were collected on weed density, weed biomass, sugarcane tiller count and cane yield. Our results revealed that Rottboellia cochinchinensis (Lour.) W.D.Clayton, Panicum maximum Jacq, Imperata cylindrica L., Panicum repens L., Cynodon dactylon L. and Cyperus rotundus L. were the major weed problems of sugarcane in Ilorin. The monthly hoe weeded treatment had significantly higher tiller count which translated to higher cane yield (22.61 to 72.54 t/ha) than other weed control treatments. The reduction in cane yield was between 80.51 and 97.55% depending on duration of weed infestation. Association of weed parameters with cane yield was negative and significant showed that a decrease in weed infestations will result in simultaneous increase in sugarcane yield. Therefore, in selecting sugarcane variety(s) for breeding programmes, emphasis should be placed on sugarcane growth parameters that negatively and significantly correlated with weed infestation.

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