AVAILABILITY OF THE HERBICIDE TRIFLURALIN FOR CONTROL OF WILD OATS AS INFLUENCED BY SOIL CHARACTERISTICS IN FOUR MANITOBA SOILS

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Abstract
The effects of soil properties on available phytotoxic residues and total extractable residues of the herbicide trifluralin were investigated in four Manitoba soils. The four soils, Red River heavy clay, Gladstone clay loam, Newdale loam, and Almasippi loamy sand, were treated with trifluralin at levels in the range 0.2–3.0 ppmw (dry weight soil), and the soils maintained at or near field capacity in the growth chamber. Available residues were determined at 0, 2, and 5 wk using a wild oat (Avena fatua) bioassay. Total residues were measured at regular intervals to 16 wk using ultrasonic extraction and gas–liquid chromatography analysis. Biological results showed that wild oat control expressed as ED50 required much less applied or residual trifluralin in Almasippi loamy sand than in Red River heavy clay, with the other two soils being intermediate. Total residues, on the other hand, remained highest in Red River heavy clay, although the differences between soil types were not great. Net loss of trifluralin was best explained by a first-order kinetic expression. Availability of phytotoxic residues was related mathematically to clay content of the soils in combination with time from application. It was further shown that bioassay is inadequate for determination of total residues in soils unless time from application is held constant.

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