Competitive ability of sicklepod ( Senna obtusifolia (L) irwin and barneby) in two-species combinations with Chrolomaena odorata (L) K & R and Euphorbia heterophylla L.

The natural fallow through which the fertility of tropical soils is restored and the weed pressure is alleviated can no longer be sustained. However, improved bush fallow using fast-growing, deep-rooting and good biomass-accumulating shrubby legumes provides a good alternative. The suitability of Senna obtusifolia in improved fallow management was investigated by studying its competitive ability relative to Chromolaena odorata (siam weed) and Euphorbia heterophylla (poinsettia weed) in replacement series studies at four total densities (29, 58, 115 and 230 plants.m -2 ) in pure stands and at three total densities (58, 115 and 230 plants.m -2 ) in equiproportion mixture. Replacement series diagrams were drawn and relative competitive ability indices (Relative Yield, RY; Relative Yield Total, RYT; Relative Replacement Rate, RRR) were determined from the plant dry weight (PDW). In the replacement series diagrams of Senna/Chromolaena combination, the curves of the PDW/pot and those of the RY were convex for the two species. The RY values were 0.64+-0.12 and 0.59+-0.05 for S. obtusifolia and C. odorata respectively. The RYT and RRR, averaged over all densities, were 1.23+-0.08 and 1.27+-0.17 respectively. However, in the Senna/Euphorbia combination, the curves were convex for S. obtusifolia and concave for E. heterophylla . The RY values were 0.81+-0.06 for S. obtusifolia and 0.23+-0.05 for E. heterophylla respectively. The RYT and RRR values for the combination were 1.05+-0.07 and 11.07+-2.40 respectively. The results indicated that though S. obtusifolia and C. odorata attained equilibrium growth with RYT significantly deviated from unity (RYT>1) when grown in mixture, S. obtusifolia exerted greater replacement pressure than C. odorata. Conversely, in the Senna/Euphorbia combination, there was a competitive relationship (RYT+-1) in which S. obtusifolia was the aggressor and E. heterophylla the subordinate species. The study showed that while S. obtusifolia exhibited obvious competitive superiority in mixture with E. heterophylla , the superiority was less obvious in mixture with C. odorata . Keywords : Competition; fallow management; helioplastic response; weed control Moor Journal of Agricultural Research Vol. 7 (1&2) 2006 pp. 85-92