Influence of Herbicides on Yield and Botanical Composition of Alfalfa Hay

Perennial weeds frequently infest alfalfa (Medicago sativa L.) swards. While herbicides have been identified that control these weeds, alfalfa tolerance to these herbicides and effects on dry matter yields and botanical and chemical composition are largely unknown. Thus, field studies were conducted on a Dodgeville silt loam (Typic Argiudoll fine-silty over clayey, mixed, mesic) soil near Mt. Horeb, Wis. in 1974 and on a Plano silt loam (Typic Argiudoll fine-silty, mixed, mesic) soil near Verona, Wis. in 1976 to compare the effects of seven herbicides applied alone, or in combination with pronamide [N (1,1-dimethylpropynyl)-3,5-dichlorobenzamide] to alfalfa swards which had been colonized with quackgrass [Agropyron repens (L.) Beauv.] and several perennial broadleaf weed species. Forage yield and botanical composition were determined. Secbumeton (2-sec-butylamino-4-ethylamino-6-methoxy-s-triazine), cyanazine {2-[[4-chloro-6-(ethylamino) -s-triazin-2-yl] amino]-2-methylpropionitrile], metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one], and S-6044 [N-benzyl-N-isopropyltrimethylacetamide / 2-cyclopropanecar-boxamide-5-(2-chloro-1,1-dimethylethyl)-1,3,4-thiadiazole], applied to actively growing swards in the fall reduced alfalfa yields in at least one cutting or at one location during the following season. Pronamide increased the total alfalfa yield. In 1974, common dandelion (Taraxacum officinale Weber.) and hoary alyssum [Berteroa incana (L.) DC.] populations increased when quackgrass was suppressed with pronamide. None of the herbicides reduced common dandelion and hoary alyssum populations. Yellow rocket (Barbarea vulgaris R. Br.) populations were reduced by secbumeton at 1.8 kg/ha, terbacil (3-tert-butyl-5-chloro-6-methyluracil) at 0.9 kg/ha, metribuzin at 2.3 kg/ha, and bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4-(3H)-one 2,2-dioxide] at 1.7 kg/ha. None of the treatments increased total forage yields.