Effect of Herbicides on Alfalfa and Subsequent Crops

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 sativa L. ‘Ranger’), effectively controlled downy brome (Bromus tectorum L.), tumble mustard (Sisymbrium altissimum L.), and prickly lettuce (Lactuca serriola L.). 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 mays L. ‘Rugosa’) > potatoes (Solarium tuberosum L. ‘Norgold’) > winter wheat (Triticum aestivum L. ‘Nugaines’) > bush beans (Phaseolus vulgaris L. ‘Humilis’) ≅ sugar beets (Beta vulgaris L. ‘U. & I. hybrid No. 8′).