Invasive Alien Weeds of Water Hyacinth (Eichhornia Crassipes) in Tamil Nadu Irrigation System and the Impact of Herbicidal Control on Aquatic Environment

In India almost 80 percent of the lakes are already infested with this weed. Even in Tamil nadu, very big tanks like, Chembarabakkam tank, Dusi-Mamandur tank, Kaveripakkam tank, Tamiraparani tanks and Veeranam tank etc., are also affected by this weed. The Veeranum Lake, it has been connected through 27 distributary channels form the major irrigation source that covers a large proportion of the rice tract of the state with a command area of 18,000 ha. Recent survey on the infestation of aquatic weeds in these channels indicated that many of these channels were infested heavily with water hyacinth. A research programme supported by Ministry of Water Resources, Government of India was taken up to study the impact of herbicides on the control of water hyacinth, to optimize their doses and time of application and to study their impact on aquatic environment, through a series of green house and water- shed studies. Among the different herbicides tried viz., 2, 4-D Na salt, glyphosate and paraquat, glyphosate proved more efficient with higher magnitude of plant height reduction, weed mortality (cent percent) and reduction in bio-mass. The doses of 1.5 kg ha-1, 2.5 kg ha-1and 1.5 kg ha-1 were found to be optimum for glyphosate, 2, 4-D Na salt and paraquat, respectively. Different seasons did not vary in influencing the impact of herbicides on the weed. Glyphosate also caused least fish mortality of 23.30 per cent after 32 days. Different organs like gills, brain, liver and kidney of the fishes were studied for histopathology and the observations showed tissue distractions in glyphosate treated fish. In the watersheds also, glyphosate proved more efficient and safe on water quality in terms of pH, EC, DO, COD and mineral content, of treated water. The glyphosate treated water was also found to be safe for irrigating crops with 87, 58 and 62 per cent germination of paddy, cotton and ladysfinger.