Enhanced biodegradation with repeated

Of the three insecticides studied, enhanced biodegradation has been definitively associated with repeated applications of carbofuran ( ). In our studies, the chemical assay data from 1987 only weakly suggested the development of enhanced biodegradation, and the data from 1988 indicated that biodegradation rate was not affected by multi-year carbofuran treatments. Since a severe drought occurred in 1988, a lack of soil moisture may have slowed the rate of carbofuran degradation. 

Reduced weed control with butylate in butylate history fields was reported in 1983 (7,8). Laboratory, greenhouse and field trials demonstrated that repeated annual use of butylate may cause enhanced butylate biodegradation and result in reduced weed control. Reports of reduced weed control with repeated use of butylate were much less frequent than with EPTC (Stauffer Chemical Company unpublished). Since 1986, the potential for enhanced biodegradation of butylate has been documented and described more completely by researchers in the Southeastern U.S. (9,10,11,12). 

Enhanced biodegradation. Four annual applications of EPTC resulted in a significant reduction in the recovery of EPTC by GC analysis following the fifth annual application. At 5 and 7 DAT, EPTC recovery was 1.40 and 0.28 ppm, respectively. Adjacent plots which had been treated with EPTC for 2 years and rotated out to alternate herbicides for 2 years had 2.99 and 1.99 ppm at 5 and 7 DAT, respectively. These results demonstrate enhanced biodegradation of EPTC with repeated annual use (Table I). Many other researchers have demonstrated that repeated annual applications of EPTC may lead to reduced control of weeds (2-3,14-19) due to enhanced biodegradation. 

An adjacent plot which had been treated for two years, then left untreated for one year prior to subsequent buytlate application had 2.11 ppm butylate recovered at 7 DAT. This demonstrated enhanced biodegradation of butylate with repeated annual use (Table II). Repeated annual applications of butylate do not lead to reduced weed control as frequently as is the case with EPTC, although butylate persistence may be reduced by annual repeated applications (7-12). 

The rate of biodegradation of the second addition may be the same as the final rate evident during the active phase of breakdown of the first addition [106, 107, 118-120]. However, it is far more common to have a greater rate of biodegradation, which is usually measured as the loss of parent compound or the formation of 14C02 from labeled compound, following the second rather than the first application. The rate is further enhanced with still more additions. This enhancement of rate upon repeated additions of chemical substrate has been reported frequently for several pesticides and surfactants [14,15, 99,100,113, 123-126] as follows ... 

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