Effect of Seasonal Variation on the Persistence and Dissipation Behaviour of the Herbicide Mixture of Fomesafen + Quizalofop-Ethyl in Tropical Soybean Agroecosystem and Safety Risk Assessment

Weeds are the major limiting factor for optimum soybean production in India. The herbicide mixture of fomesafen and quizalofop-ethyl provides effective control of a broad spectrum of weeds, but its fate in the tropical soybean ecosystem is unknown and also the risks involved to the consumer and the environment are still unexplored. Hence, a supervised field trial was conducted following the post-emergence application of fomesafen 12% + quizalofop-ethyl 3% in two consecutive seasons. The dissipation of fomesafen followed biphasic double first order in parallel kinetics, whereas quizalofop-ethyl dissipation followed first order kinetics. A significant difference in the persistence of fomesafen was observed due to seasonal variation of meteorological parameters. However, the variation was significant only in plant, but non-significant in soil, in case of quizalofop-ethyl. The overall shorter persistence of both fomesafen and quizalofop-ethyl was recorded in warmer climatic conditions of Season I than Season II. The results thus indicated that care must be taken during application of this herbicide mixture in cold climatic regions, since both the herbicides may exhibit higher stability. The absence of end-point residues at harvest concluded that the formulation is safe for application in tropical agroclimate. The low chronic dietary toxicity and low soil ecological toxicity indicated that the herbicide mixture will offer no threat against consumer health and soil ecosystem. However, there was a concern about the toxicity against soil algal population which needs to be reconfirmed by further studies.