Development of a Soil Bioassay for Triclopyr Residues and Comparison with a Laboratory Extraction

The use of triclopyr for the removal of woody and broad-leaf vegetation in right-of-ways and agricultural settings has been proposed for Alaska. Triclopyr concentrations in soil after application are of concern because residual herbicide may affect growth of subsequent vegetation. In order to measure triclopyr residues in soil and determine the amount of herbicide taken up by the plant, soil bioassays were developed. Four agricultural species, turnip, lettuce, mustard, and radish, were tested to determine sensitivity to triclopyr in a 1-wk bioassay. The sensitivity (I(50)) of turnip, lettuce, mustard, and radish was 0.33 +/- 0.05 kg ai ha(-1), 0.78 +/- 0.11 kg ai ha(-1), 0.78 +/- 0.07 kg ai ha(-1), 0.85 +/- 0.10 kg ai ha(-1) (mean +/- SE), respectively. Mustard was the most consistent crop in the bioassay with a midrange response to triclopyr and lowest standard deviation for germination as compared to the other species. Thus, it was used in a bioassay to determine triclopyr concentrations in a field trial. The bioassay of mustard closely matched residual amounts of triclopyr in a field trial determined by chemical extraction. Estimates of residual triclopyr concentrations using the bioassay method were sometimes less than the triclopyr concentration determined using a chemical extraction. These differences in concentrations were most evident after spring thaw when the chemical extraction determined there was enough triclopyr in the soil to reduce mustard growth over 60%, yet the bioassay measured only a 10% reduction. The chemical extraction method may have identified nonphototoxic metabolites of triclopyr to be the herbicidal triclopyr acid. These methods, when analyzed together with a dose response curve, offer a more complete picture of triclopyr residues and the potential for carryover injury to other plant species.