Effect of Herbicides on Alfalfa and Subsequent Crops

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Abstract
Three herbicides, 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine), 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine), and 3-tert-butyl-5-chloro-6-methyluracil (terbacil), applied annually at 0.45, 0.90, and 1.80 kg/ha for 2 or 3 years to establish forage alfalfa(Medicago sativaL. ‘Ranger’), effectively controlled downy brome(Bromus tectorumL.), tumble mustard(Sisymbrium altissimumL.), and prickly lettuce(Lactuca serriolaL.). However, downy brome control decreased to less than half and broadleaf control ceased 18 months after the final herbicide application. Atrazine injured alfalfa most. Usually, annual applications of 0.45 kg/ha did not reduce yield. Greenhouse and field bioassay data indicated that the herbicides leached below the 0 to 32.5 cm soil zone and persisted. Residues were absorbed by roots of crops planted 2 to 3 years after the last application. Injury was similar whether the last application was made 18 or 30 months previously. The order of crop tolerance was sweet corn(Zea maysL. ‘Rugosa’) > potatoes(Solarium tuberosumL. ‘Norgold’) > winter wheat(Triticum aestivumL. ‘Nugaines’) > bush beans(Phaseolus vulgarisL. ‘Humilis’) ≅ sugar beets(Beta vulgarisL. ‘U. & I. hybrid No. 8′).

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