Rice allelopathy for weed management in aquatic ecosystems

Lake Veeranum and it's distributaries form the major irrigation source for rice crop in Tamilnadu state in South India. Now its distributaries have been infested with water hyacinth (Eichhornia crassipes. Mart. Solms), but this invasive alien weed has not entered the rice fields. Based on this observation, residues of 60 rice cultivars were screened for their allelopathic inhibition on water hyacinth in laboratory bioassays as well as in micropond tests. Cultivars whose extracts 3 days after application, caused >40% reduction in weed biomass and chlorophyll were grouped as highly allelopathic and those causing 20–40% reduction as moderately allelopathic. The highly and moderately allelopathic cultivars were later tested in concrete microponds. The residues of selected rice cultivars were soaked in water of the micropond to make concentrations of 2, 5 and 10% and compared with floating weeds of a micropond with normal untreated water. Reduction in weed bio-mass and chlorophyll content were recorded at 24 h interval. Three days after transplanting, the cultivar BPT proved highly allelopathic to water hyacinth and caused reduction 47.68% in weed biomass in Lab bioassay and 45.61% in microponds. Rice cultivar ADT-36 was moderately allelopathic, it reduced the weed bio-mass by 33.40 and 32.0% in laboratory bioassay and micropond, respectively.