Response of soybean plants to metribuzin and interaction with atrazine residues

Soybeans [Glycine max (L.) Merr. ‘Harosoy 63'] planted in Brookston clay loam soil into which atrazine [2‐chloro‐4‐(ethyl‐amino)‐6‐(isopropylamino)‐s‐triazine] had been incorporated were consistently visibly damaged at rates of atrazine greater than 0.56 kg/ha. Yield was not significantly reduced until the rate of atrazine reached 0.84 kg/ha. Residues of atrazine during the year after application were reduced by 80 to 90%. This left only small amounts of atrazine residue for uptake by soybeans planted the following year. Metribuzin [4‐amino‐6‐tert‐butyl‐3‐(methylthio)‐as ‐triazin‐5(4H)‐one] applied pre‐emergence at 142 kg/ha caused injury to these soybeans in the absence of any atrazine residues and this resulted in a reduction in plant stand, growth and yield. Metribuzin was degraded rapidly in each year with a decrease of approximately 90% in 4 months. Atrazine residues did not interact with the metribuzin treatment to cause increased soybean injury in two of three experiments. In one year, a significant interaction effect occurred which resulted in a greater reduction in soybean plant weight as the atrazine rate of the previous year increased. A yield reduction greater than that anticipated from metribuzin alone was observed for the 2.24 kg/ha prior atrazine application followed by metribuzin.