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Persistence and Factors Affecting Dissipation of Molinate under Flooded Rice Culture¹

ABSTRACT

Pesticides, essential for the economical production of rice (Oryza sativa L.), could pose a serious problem if transported to surface impoundments and estuaries along the Gulf Coast via the return flow associated with flooded rice culture.

Field experiments were conducted under flooded rice cultivation to determine persistence and half-life of molinate (S-ethyl-hexahydro-1-H-azepine 1-1carbothioate). Persistence and half-life were evaluated with respect to intermittent and continuous flow irrigation schemes at normal and excessive application rates of 3.4 and 11.2 kg/ha molinate, respectively. Persistence at statistically significant levels ranged from 96 to 384 hours following the application, and generally was more a function of the application rate than irrigation scheme. Half-life values averaged 96 ± 22 hours in intermittent flow plots, and 54 ± 17 hours in continuous flow plots over the 3-year experiment. Application rate had little effect on half-life.

Best fit analysis of field data to the first order biological decay equation and laboratory studies under flooded soil conditions suggested that biological degradation was the principle mode by which molinate was dissipated in the field experiment.

Key Words: Ordam • pesticide • irrigation return flow • half-life • biological degradation • redox potential

¹ Contribution of the Texas Agric. Exp. Stn. and the Texas Agric. Ext. Serv. This work was sponsored in part by EPA Grant no. S802008.

² Research Scientist, Dep. of Soil and Crop Sci., Texas A&M University, College Station; Assistant Professor, Texas Agric. Exp. Stn. at Beaumont, Tex.; Associate Professor, Dep. of Soil and Crop Sci., Texas A&M Univ., College Station; and Agric. Chemist, Texas Agric. Ext. Serv., College Station, TX 77843.

Received for publication February 7, 1977.