RESPONSE OF SOYABEAN (Glycine max (L.) Merill) TO INTEGRATED WEED MANAGEMENT IN A FOREST-SAVANNA TRANSITION ECOSYSTEM IN OYO STATE, NIGERIA

Weed infestation of soyabean field causes 65% yield reduction in the forestsavanna transition ecosystem of Nigeria. Single weed control method is often less effective. However, Integrated Weed Management (IWM), will ensure more effective and environment friendly control. There is little information on IWM application on soyabean field. This study was therefore aimed at investigating response of soyabean to IWM. Field study was carried out in the 2008 and 2009 planting seasons to asses the effectiveness of the integration of various weed control methods in the production of soyabean (TGx 1414E) at Apapa village, Akinyele local government area, Oyo state. The effects of Tillage (Tilled, Non-tilled), mulching (no mulch, grass, plastic) and herbicides (no herbicides, metolachlor, pendimethalin) on growth and yield of soyabean were assessed in a 2x3x3 factorial experiment arranged in randomized complete block design with four replications and at 0.05x0.6m spacing. Data were collected at 16 weeks after planting on weed diversity and abundance using 0.25m2 quadrat. The Relative Importance Value (RIV) of each weed was determined using standard procedures. Also ten soybean plants were assessed per plot for growth and yield parameters (biomass, height, number of pods and grain yield). Data collected were analysed using descriptive statistics and ANOVA at p=0.05. The most abundant weed was Spilanthes sp., with RIV of 58.5%, followed by Chromolaena odorata (L.) R.M. King and Robinson with RIV of 7.5%. The highest weed biomass of 136.4±18.5g/m2 was obtained in non tilled/pendimethalin/grass treatment, followed by non-tilled/plastic (20.0±5.1g/m2) and tilled plastic (18.2±3.0g/m2) but lowest in non tilled/pendimethelin/plastic (15.4±2.9g/m2). Tilled/metolachlor/plastic reduced weed biomass yield by 97.5%, tilled/plastic by 97.2%, tilled/grass by 82.6%, control by 72.7%. The plants grown in the control plot had a mean height of 82.0±5.4cm in 2008 and 71.6±3.3cm in 2009. The lowest height of 51.6±5.6cm was recorded in 2008 in the non-tilled/metolachlor/grass treatment and was50.8±4.6cm in 2009 in the non-tilled/pendimethalin/grass treatment. Plants grown in tilled/metolachlor/plastic had highest pod numbers of 52.4±10.5 in 2008 and 57.3±4.0 in 2009 in tilled/pendimethalin/plastic and the lowest value of 19.0±12.6 in 2008 and 24±0.8 in 2009 in the non-tilled/pendimethalin/grass. The highest grain yield of 3.9±0.3 tons/hectare (2008) and 3.7±0.9 tons/hectare (2009) were recorded in the tilled/pendimethalin/plastic treatment while the lowest value of 1.6±0.3 tons/hectare and 1.7±0.4 tons/hectare was recorded in the non-tilled/pendimethalin/grass treatment in 2008 and 2009 respectively. The treatments are significantly different with regards to weed biomass, and grain yield of soyabean in the two years. However, all plastic combined treatments were not significantly different. The tilled/plastic treatment ensured best weed suppression and highest grain yield. Adopting it as an integrated weed management alternative may reduce the amount of herbicides and labour input better than other strategies.

Key words: Integrated weed management, Metolachlor, Pendimethalin, Soyabean, Spilanthes.