Asulam for tansy ragwort control in alfalfa : field efficacy and selectivity factors Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/d791sk85r

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  • Tansy ragwort (Senecio jacobaea L. ) is a poisonous weed which infests alfalfa fields in western Oregon. Studies were conducted to evaluate the effectiveness of asulam (methyl sulfanilylcarbamate)for control of tansy ragwort in alfalfa and to elucidate factors responsible for its selectivity. In seven replicated field trials in 1973 and 1974, 2.2 kg/ha of asulam gave 70% to 90% control of tansy ragwort. No visible injury or yield reductions were recorded in alfalfa treated with rates of asulam up to 9. 0 kg/ha. Varying carrier volume from 38 to 151 L/ha or adding 0. 2% surfactant to the spray solution did not influence tansy ragwort control. In the greenhouse, rates of foliar applications of asulam sodium salt causing growth reductions of 50% (GR₅₀) were calculated by using the least-squares method. Estimated GR₅₀ values for alfalfa were 109 and 55 kg/ha, without and with surfactant, respectively. GR₅₀ values for tansy ragwort were 0.30 kg/ha without surfactant and 0.38 kg/ha with added surfactant. Spray retention by alfalfa and tansy ragwort was measured by using 2% acid red dye solutions and asulam sodium salt solutions. Increasing carrier volume increased spray retention in both species. Tansy ragwort retained an average of 2.5 times as much as alfalfa when no surfactant was added to the spray solution. When surfactant was added, the retention per unit leaf area was the same for each species. Retention by tansy ragwort was not influenced by surfactant but retention by alfalfa was 2.3 times as much with surfactant as without surfactant. The effects of surfactant and time of exposure on the uptake of ¹⁴C asulam solution into foliage of both species was measured. The treated portions were washed, activity in the wash water was measured, and uptake was calculated. Uptake of ¹⁴C asulam in both species was at a maximum 48 hr after treatment. Uptake by tansy ragwort was 1.9 times as high as in alfalfa when no surfactant was added to the spray solution. When surfactant was added, alfalfa absorbed 1.6 times as much asulam as tansy ragwort. The addition of surfactant increased uptake by alfalfa 2.7 times and did not significantly influence uptake by tansy ragwort. Tansy ragwort translocated 2.5 times as much ¹⁴C activity from the treated area as alfalfa when values were expressed as a percentage of the amount taken up. The amount translocated in alfalfa was independent of the amount of uptake; similar amounts of activity were found outside the treated area, even though 2.7 times as much was taken up when surfactant was added. Trans location by tansy ragwort was not affected by addition of surfactant. Metabolism of ¹⁴C asulam applied with surfactant was studied in both species. Plants were harvested 48, 96, and 144 hr after treatment and extracted with acetone. The aqueous residue of the acetone extract was partitioned with ethyl acetate and the ¹⁴C activity in the ethyl acetate phase, the aqueous phase, and the plant residue was determined. The total non-extracted activity was higher in alfalfa residues than tansy ragwort, but when the values were converted to a percentage of the amount of uptake, there were no significant differences between species. Alfalfa had significantly higher percentages of the ¹⁴C activity present in the aqueous phase than did tansy ragwort. There were significantly lower amounts of free ¹⁴C asulam in alfalfa than in tansy ragwort as determined by thin-layer chromatography. An unknown asulam metabolite was present in the ethyl acetate extracts of alfalfa while more than 75% of the ¹⁴C activity in ethyl acetate extracts of tansy ragwort was asulam. The aqueous extracts of both species apparently contained an acid-labile conjugate(s) of asulam. Results of the studies conducted do not completely account for differences in sensitivity found in the greenhouse. Other possible explanations for selectivity are discussed.
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