Environmental fate of herbicides trifluralin, metazachlor, metamitron and sulcotrione compared with that of glyphosate, a substitute broad spectrum herbicide for different glyphosate-resistant crops

The introduction of crops resistant to the broad spectrum herbicide glyphosate, N-(phosphonomethyl)glycine, may constitute an answer to increased contamination of the environment by herbicides, since it should reduce the total amount of herbicide needed and the number of active ingredients. However, there are few published data comparing the fate of glyphosate in the environment, particularly in soil, with that of substitute herbicides. The objective of this study is to compare the fate of glyphosate in three soils with that of four herbicides frequently used on crops that might be glyphosate resistant: trifluralin, alpha,alpha,alpha-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine, and metazachlor, 2-chloro-N-(pyrazol-1-ylmethyl)acet-2',6'-xylidide for oilseed rape, metamitron, 4-amino-4,5-dihydro-3-methyl-6-phenyl-1,2,4-triazin-5-one for sugarbeet and sulcotrione, 2-(2-chloro-4-mesylbenzoyl)cyclohexane-1,3-dione for maize. The distribution of herbicides between the volatilized, mineralized, extractable and non-extractable fractions was studied, along with the formation of their metabolites in laboratory experiments using 14C-labelled herbicides, over a period of 140 days. The main dissipation pathways were mineralization for glyphosate and sulcotrione, volatilization for trifluralin and non-extractable residues formation for metazachlor and metamitron. The five herbicides had low persistence. Glyphosate had the shortest half-life, which varied with soil type, whereas trifluralin had the longest. The half-lives of metazachlor and sulcotrione were comparable, whereas that of metamitron was highly variable. Glyphosate, metazachlor and sulcotrione were degraded into persistent metabolites. Low amounts of trifluralin and metamitron metabolites were observed. At 140 days after herbicide applications, the amounts of glyphosate and its metabolite residues in soils were the lowest in two soils, but not in the third soil, a loamy sand with low pH. The environmental advantage in using glyphosate due to its rapid degradation is counterbalanced by accumulation of aminomethylphosphonic acid specifically in the context of extensive use of glyphosate.     

Mobility and persistence of four herbicides in soil of a South Australian vineyard

The use of persistent herbicides has increased the potential for contamination of soil, soil water and groundwater. The mobility, dissipation and fate of four herbicides, norflurazon, oxadiazon, oxyfluorfen and trifluralin, used in South Australian viticulture, have been studied in a typical sand‐over‐clay vineyard soil. Following herbicide application at field rates to plots up‐slope of miniature lysimeters, surface soil and soil water were sampled regularly over the period of annual rainfall. The concentration of each herbicide in the soil cores, surface soil and soil water was determined by GLC‐NPD following solid‐phase concentration procedures where necessary. Oxadiazon dissipated more quickly than the other three herbicides in the soil. Norflurazon was the most mobile of these herbicides in this soil. However all four herbicides were found in the soil water within the first year, though only norflurazon was found in the soil water in the subsequent year. Norflurazon moved laterally to a greater extent than the other herbicides. © 2000 Society of Chemical Industry