Allelopathic effects of Chromolaena odorata (L.) King & Robinson and Mikania micrantha H.B.K. on three selected weed species

The use of chemical pesticides leads to negative impacts such as pesticide residue problem and weed resistance. Thus, an economical and environmentally safe method such as allelopathy needs to be explored for controlling weeds in agricultural areas. Therefore, a study was conducted to determine the allelopathic effects of the aqueous leaf extract and leaf debris (incorporated into the soil) of Chromolaena odorata and Mikania micrantha on three bioassay weed species namely Eleusine indica, Cyperus iria and Ageratum conyzoides under laboratory and greenhouse conditions. Three concentrations of each of the aqueous leaf extracts (12.5, 25.0 and 50.0 g/L) and leaf debris (2.5, 5.0 and 10.0 g/500 g soil) of C. odorata and M. micrantha were used in the experiment. The experimental design used was the complete randomized design (CRD) with three replications. The experiment was conducted twice. Results showed that the leaf extracts of C. odorata and M. micrantha significantly reduced all seedling growth parameters of the three bioassay species with the exception of the effect of C. odorata on the shoot length of C. iria. Leaf extracts of both species significantly inhibited growth parameters of E. indica and A. conyzoides at 50.0 g/L by more than 96 % compared to the control. The speed of germination index (S) was the most sensitive compared to that of the other indices but the Days required for 50% germination of total germinated seeds (T50) and the Days required for 50% germination of the total seeds (T’50) showed the least difference. At 50.0 g/L of C. odorata leaf extract, the “speed of germination index” of E. indica and A. conyzoides was 0, which is the lowest possible result. The leaf debris of C. odorata and M. micrantha significantly inhibited A. conyzoides and C. iria at all seedling growth parameters. The shoot length and dry weight of A. conyzoides was greatly reduced by M. micrantha leaf debris at 50.0 g/L by 87% and 90.4% of the control respectively. Whilst, at the same concentration, the fresh weight of C. iria was reduced by 93.5% compared to that of the control. Therefore, C. odorata and M. micrantha show allelopathic properties when used as cover crop and mulch by controlling the growth of A. conyzoides, followed by C. iria and E. indica. Further studies need to be conducted to investigate the type of inhibition mechanisms involved.