Factors affecting the efficacy of seed safeners against chloroacetamide herbicides in sorghum

Abstract

Seed treatments using cyometrinil, oxabetrinil, and flurazole have effectively safened grain sorghum against chloroacetamide herbicides such as alachlor and metolachlor at rates of 2.24 or 3.36 kg/ha under field and laboratory conditions. Safeners were less effective in protecting sorghum from herbicide injury when soil was continuously wet (110% FC) from time of planting to emergence (3 to 5 days) than when soil was wet for only 1 or 2 days prior to emergence. In contrast, if the surface soil remained dry until the coleoptile reached the soil surface, the herbicides had little effect on unsafened sorghum. Safeners exerted a competitive effect on the absorption of metolachlor. The absorption of metolachlor from solution by emerging coleoptiles and roots decreased in the presence of cyometrinil. The rate of seedling respiration was decreased by seed treatments. Polarographic measurements of oxygen uptake by sorghum seed during imbibition and early stages of germination indicated that respiration was decreased due to seed treatment within 2 h and remained affected through 48 h. Oxabetrinil and flurazole had less effect on oxygen uptake by sorghum seedlings than did cyometrinil. Certain oximes, including cyometrinil, oxabetrinil, 2-pyridinealdoxime-O-benzyl ether, and 2- pyridinealdoxime-O-phenethyl ether, which are known to protect sorghum from chloroacetamide herbicides, and metolachlor, acted as synergists to insecticidal effects of propoxur on a resistant strain of housefly. Flurazole, a nonoxime protectant for grain sorghum, had less effect on the toxicity of propoxur than did oximes. In addition, sorghum coleoptiles were positive for a standard test for oxidative metabolism in insects. Like insects, the enzyme system necessary to oxidize aldrin to dieldrin was active in coleoptiles. Both of these responses indicated that the mixed function oxidase enzyme system may have been active in degradation of the herbicides and/or safeners used in this study.