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Susceptibility of Alfalfa Cultivars to Triazine Herbicides¹

While simazine [2-chloro-4,5-bis(ethylamine)-s-triazine] has been used for several years to control perennial broadleaf weeds in alfalfa (Medicago sativa L.), it occasionally fails to provide adequate weed control, or causes crop injury. Other triazine herbicides have been shown to provide more reliable weed control but also may injure the crop. Consequently, this study was conducted to determine which triazine herbicide was least injurious to alfalfa and if alfalfa cultivars differed in their susceptibilities to the herbicides. Included in the study were five alfalfa cultivars and the herbicides atrazine [2-chloro- 4-(ethylamino)-6-(isopropylamino)-s-triazine], cyanazine chloro-4- (l -cyano - l -methylethylamino) -6-ethylamino-s-triazine], secbumeton (2-sec-butylamino-4-ethylamino-6-methxy- s-triazine) and simazine.

The cultivars were established on silt loam (Typic Argin-doll Fine-Silty Mixed Mesic) and loamy sand (Typic Udipsament Sandy-Mixed) soils. Results of tests conducted on both soils indicated that the ‘Vernal,’ ‘Saranac,’ and ‘Iroquois’ cultivars had the greatest yield potential under Wisconsin conditions, but when expressed as a percentage of yield in untreated plots, the ‘Tempo’ cultivar was injured least by the herbicides. The ‘DuPuits’ cultivar, while initially responding to the triazine herbicides much like the other cultivars, exhibited greater injury after overwintering. Initial injury to the Vernal cultivar on the silt loam soil was greatest from atrazine, secbumeton, and cyanazine, but recovery was nearly complete by the spring following treatment. On the loamy sand soil, however, atrazine and high rates of simazine produced the greatest initial injury, and injury to the Vernal cultivar persisted through the following spring.

Key Words: Medicago sativa (L.) • Winterkill • Herbicide injury

² Associate professor, professor, and assistant professor, respectively, Dep. of Agronomy, Univ. of Wisconsin, Madison, WI 53706.

Received for publication September 12, 1975.