Response of established alfalfa to herbicides applied in the winter

Herbicide injury to established alfalfa (Medicago sativa L.) was studied to determine the effects of single and repeated herbicide applications, dates of herbicide application, alfalfa dormancy, and alfalfa cultivars.

Metribuzin [4-amino-6-tert-butyl-3(methylthio)-as-tria2in-5(4H)- one] at 0.56 and 1.12 kg/ha, simazine [2-chloro-4,6-bis(ethylamino)-striazine] at 1.12 and 2.24 kg/ha, and terbacil (3-tert-butyl-5-chloro- 6-methyluracil) at 0.84 and 1.68 kg/ha were applied to dormant alfalfa for three years. The low rate of each herbicide did not cause alfalfa injury. At high rates for a single season, alfalfa injury was greatest from terbacil and less from simazine. Metribuzin caused the least injury. After three annual applications simazine at 2.24 kg/ha caused the most alfalfa injury. A bioassay to detect herbicide residues revealed that alfalfa injury on simazine treated plots resulted from toxic soil residue.

Date of herbicide application was studied in two experiments. High rates of metribuzin, simazine, and terbacil were applied in December, January, and February to dormant alfalfa. Alfalfa was injured more at the later application dates. In a second experiment simazine at 2.24 kg/ha and terbacil at 1.68 kg/ha were applied to dormant alfalfa at six winter dates for three years. The trend of more alfalfa injury with later applications was not apparent in this experiment. Injury from terbacil was more dependent on climatological conditions than injury from simazine. Precipitation for ten days following terbacil application correlated best with alfalfa injury.

The effect of alfalfa dormancy on herbicide injury was studied by stimulating alfalfa to break dormancy by using plastic covered frames. Terbacil was applied at 1.68 kg/ha to alfalfa plots both in side and outside the frames when 10% of the alfalfa plants inside the frames appeared to have broken dormancy. Terbacil injury on dormant and non-dormant alfalfa was not significantly different in 62% of the comparisons.

Thirty alfalfa cultivars were screened against several herbicides for injury as measured by alfalfa height. Herbicides included: chlorpropham (isopropyl m-chloro-carbanilate) at 3.4 kg/ha, metribuzin at 0.84 kg/ha, pronamide [3,5-dichloro (N-l,l-dimethyl-2-propynyl) benzamide] at 2.5 kg/ha, simazine at 1.7 kg/ha, terbacil at 1.7 kg/ha, and hexazinone [3-cyclohexyl--6-(dimethyl ami no)-1-methy1-1,3,5- triazine-2,4(lH,3H)-dione] at 3.4 kg/ha. Alfalfa cultivars did not vary significantly in susceptibility to herbicide treatments.