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Title: Development of a soil bioassay for triclopyr residues and comparison with a laboratory extraction

Author
item Ranft, Richard
item Seefeldt, Steven
item ZHANG, MINGCHU - University Of Alaska
item BARNES, DAVID - University Of Alaska

Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/2/2010
Publication Date: 12/20/2010
Citation: Ranft, R., Seefeldt, S.S., Zhang, M., Barnes, D. 2010. Development of a soil bioassay for triclopyr residues and comparison with a laboratory extraction. Weed Technology. 24(4):538-543.

Interpretive Summary: Triclopyr, a herbicide used to remove of woody and broad-leaf vegetation in right-of-ways and agricultural settings, has been used in Alaska, but there is little information about what happens to it after it gets in the soil. Concentrations of triclopyr in soil after application are a concern because its residues may affect growth of subsequent vegetation. The breakdown of triclopyr by bacteria may be longer in Alaska, than in lower latitudes, because of colder soils. To measure triclopyr residues in soil a bioassay that uses plants sensitive to triclopyr was developed in the laboratory. Four agricultural species: turnip, lettuce, mustard, and radish, were tested to determine sensitivity to triclopyr in a 1 week long bioassay. Mustard was the most consistent crop in the bioassay and it was used in a 4 week long bioassy. The 1 week bioassay was less sensitive to triclopyr residues than the 4 week bioassay, but it more closely matched residual amounts of triclopyr in a field trial used a chemical extraction process. The field trial used soil that had been sprayed with known amounts of triclopyr 1 to 365 days after the soil was sprayed. The bioassay often measured less residual triclopyr than the chemical extraction. The chemical extraction method may have identified a non-toxic breakdown of triclopyr as the herbicidal triclopyr acid resulting in a false positive. These methods, when analyzed together, offer a more complete picture of triclopyr breakdown and the potential for injury to other plant species in cold soils.

Technical Abstract: The use of triclopyr ((3, 5, 6 trichloro-2-pyridinyl) oxy) acetic acid for the removal of woody and broad-leaf vegetation in right-of-ways and agricultural settings has been proposed in Alaska. Concentrations of triclopyr in soil after application are of concern because its residues may affect growth of subsequent vegetation. The half-life of triclopyr may be longer in Alaska, than in lower latitudes, due to colder soils and lower microbial activity in high latitudes. 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 (Brassica campestris L.), lettuce (Lactuca sativa L.), mustard (Brassica juncea (L.) Czern), and radish (Raphanus sativus L.), were tested to determine sensitivity to triclopyr in a 1-wk bioassay. Mustard was the most consistent crop in the bioassay with a mid-range response to triclopyr and the lowest standard deviation for germination as compared to the other species. Thus, it was used to develop a 4-wk bioassay. The 1-wk bioassay was less sensitive to triclopyr residues than the 4-wk bioassay, but it more closely matched residual amounts of triclopyr in a field trial from a chemical extraction. Estimates of residual triclopyr concentrations using the bioassay method were often less than the triclopyr concentration determined using a chemical extraction. The chemical extraction method may have identified non-phototoxic 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.