Allelopathic Potentials of Extracts of Tithonia Diversifolia (Hemsley) A. Gray in Biological C ontrol of Weeds in Cowpea Cropping System
The limitations of physical, mechanical and chemical methods of weed control coupled with the global concern about the risks associated with the use of synthetic herbicides in controlling weeds in agroecosystems has necessitated concerted efforts on promoting alternatives to synthetic herbicides. The aim of this research was to carry out field appraisal of the allelopathic potentials of aqueous extracts of Tithonia diversifolia (Hemsley) A. Gray in biological control of weeds in cowpea [Vigna unguiculata (L.) Walp.] cropping system in order to improve on the measures that have been adopted in biological control of weeds in agroecosystem. The allelochemical constituents of aqueous extracts of T. diversifolia were determined by spectrophotometric method while the phenolic compounds were identified and quantified using High Performance Liquid Chromatography (HPLC). The experiments were laid out in a randomized complete block design (RCBD) with plot size of 3 m x 3 m and three replicates. Aqueous extracts from the root, stem and leaf of T. diversifolia (10.0%, 7.5% and 5.0% w/v) concentrations were applied at 2, 21 and 35 days after planting (DAP) at the rate of 20 l/ha in the two field trials at the experimental farm located at Owode, Ede, Osun State, Nigeria. For comparison, glyphosate herbicide, hand weeding, sorghum-based bioherbicide (Sorghum bicolor L. extract) and weedy plots were maintained as the checks (controls). The allelopathic effects of aqueous extracts from the different parts of Tithonia on germination of seeds and seedling growth of cowpea were investigated by collecting data on germination and seedling growth parameters of two accessions (IT 84E-124 and Ife Brown) at seven DAP and four weeks after planting. The weed suppressive effects of aqueous extracts from different parts of Tithonia on weeds of cowpea cropping system were assessed from the data collected on weed density at 30 and 65 DAP and weed dry weight at 65 DAP. The allelopathic effects of aqueous extracts from the different parts of Tithonia on cowpea yield were evaluated from the data collected on yield parameters which include plant height at six weeks after planting, pods per plants, seeds per pod, 1000-seeds weight and grain yield. The allelochemicals detected in the aqueous extracts of Tithonia were phenols, flavonoids, tannins, saponins and alkaloids. The metabolites were more concentrated in Tithonia leaf extract than in the stem and root extracts. Results also indicated that aqueous extracts of Tithonia did not have significant stimulatory or inhibitory effect (p = 0.51) on germination of cowpea seeds. However, the seedling growth was significantly (p = 0.01) enhanced. Application of Tithonia leaf extract at 10.0% (w/v) concentration led to significant (p = 0.00) increase in shoot dry weight 44.70% and 38.67%, in the two accessions respectively. The corresponding increase in root dry weight were 62.90% and 52.30%. The reduction in weed density at 65 DAP obtained with the application of Tithonia leaf extract at 10% and 7.5% (w/v) concentrations were 65.49% and 62.05% while the weed control efficiencies (WCE) were 69.92% and 59.26% respectively. In relation to the weedy check (control), maximum cowpea grain yield increases were recorded from the application of Tithonia leaf extract at 10.0% and 7.5% (w/v) concentrations with 66.45% and 65.32% increase respectively. The yield recorded with the application of 10.0% w/v and 7.5% w/v Tithonia leaf extract at 20 l/ha was significantly (p = 0.01) higher than the yield recorded in the handweeded and glyphosate treated plots. This implies that the weeds were controlled effectively beyond the critical period of weed interference in cowpea. Thus, aqueous leaf extract of T. diversifolia is recommended for biological control of weeds in cowpea cropping systems.