The behaviour of triazine herbicides in some Manitoba soils

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Elliott, John Dyson
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Growth chamber bioassay experiments using oats (Avena sativa L. var. Russell) were conducted over a 2 year period using soil from 4 locations. The degree of herbicide movement in the soil profile, the influence of cropping systems on dissipation and the degradation rate of atrazine, SD15418 and S6115 were investigated. In all cases, the greatest amount of herbicide was retained in the upper 5 cm of soil. As the rate of herbicide application was increased, more herbicide was detected in the lower soil horizons. There appeared to be a greater downward movement of herbicide in the plots containing corn than in fallowed plots. More herbicide residue was detected at the Winnipeg and Portage la Prairie (Portage) sites (heavy clay soils) than at the Graysville and Carman sites (very fine sandy loam soils). At the Graysville and Carman sites, triazine herbicides appeared to breakdown more rapidly in fallow than in the corn plots. No difference was detected at Winnipeg or Portage between corn and fallow plots. It appeared that there was a greater percent breakdown of triazine herbicides at high rates than at low rates. Field bioassay experiments showed that there was more severe injury to oats due to triazine residue at the Winnipeg and Portage sites than at the Graysville site. At all sites atrazine and S6115 caused a greater degree of oat injury than SD15418. At the Winnipeg and Portage sites there appeared to be similar triazine injury to oats grown on either corn or fallow plots. At the Graysville site there was less injury to oats grown on the fallow plots than on the corn plots.