Effect of Atrazine on Soybean Tolerance to Metribuzin

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Abstract
Atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] at 0.14 kg/ha and metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)one] at 0.56 kg/ha applied under field conditions interacted synergistically to reduce soybean [Glycine max (L.) Merr. ‘Harsoy-63′] growth. In the greenhouse, a number of combinations with atrazine at 0.07 kg/ha or greater and metribuzin at 0.56 kg/ha and greater interacted synergistically to reduce soybean fresh and dry weights 30 days after planting. Over a soil pH range of 4.6, 5.6, and 6.7, atrazine–metribuzin interactions were more apparent as the soil pH values increased. Atrazine at 10-5 and 10-6 M concentration in the sand culture nutrient solution during the early growth of ‘Swift’ soybean seedlings decreased 14C-metribuzin uptake and movement into the 12-day-old soybean shoots during the 12-hr treatment period. However, 10-7 M atrazine increased 14C-metribuzin in the shoot by increasing stomatal aperture and subsequent transpiration. Conditions favoring the synergistic interaction were low atrazine residue levels which increase soybean transpiration, high metribuzin rates, and high soil pH levels.