Resistance of Common Groundsel to Simazine and Atrazine

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Abstract
Failure to control common groundsel (Senecio vulgaris L.) was observed in 1968 in a nursery where 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine) or 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) had been used once or twice annually since 1958. Seedlings from seed collected at that location were not controlled by pre-emergence applications of simazine or atrazine at rates up to 17.92 kg/ha. Seedlings from a seed source where triazine herbicides had not been in continuous use were completely controlled by 1.12 or 2.24 kg/ha of either chemical, and partially controlled by 0.28 to 0.56 kg/ha. Postemergence applications of atrazine at 1.12, 2.24, or 4.48 kg/ha did not affect common groundsel from the nursery source, but killed small seedlings from the other source. Preemergence applications of four other herbicides were equally effective against seedlings from both sources.