Long-term tebuthiuron content of grasses and shrubs on semiarid rangelands

Abstract

Perennial plants collected from 5 north-central Arizona semiarid locations were assayed for tebuthiuron [N-5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl-N,N'-dimethylurea] and its metabolites using gas chromatography with flame photometric detection. Tebuthiuron was applied at rates ranging from 0.9 to 6.7 kg active ingredient (a.i.)/ha in 1975 through 1979. Plants were harvested in 1980 through 1986, 2 to 11 years after applications. Tebuthiuron was detected in sideoats [Bouteloua curtipendula (Michx.) Torr.] and blue grama [B. gracilis (H.B.K.) Lag. ex Griffiths] 10 years after application of 6.7 kg/ha. Metabolites of tebuthiuron were detected in blue grama 11 years after applications of 2.2, 4.5, and 6.7 kg/ha. The ratios of tebuthiuron to metabolites varied widely. The highest concentrations of tebuthiuron plus metabolites were 25 microgram/g in blue grama 10 years after application of 4.5 kg/ha, and 21 and 23 microgram/g in sideoats grama 9 and 10 years, respectively, after application of 6.7 kg/ha. Only these 3 samples of 120 samples assayed exceeded the legal limit of 20 microgram/g of tebuthiuron plus metabolites in forage plants. No samples from plots treated with 4.0 or less kg/ha exceeded 10 microgram/g of tebuthiuron plus metabolites, and only 10% of them exceeded 5 microgram/g.