Biologically effective rates of a new premix (Atrazine, bicyclopyrone, mesotrione, and S-metolachlor) for preemergence or postemergence control of common waterhemp [Amaranthus tuberculatus (Moq.) Sauer var. rudis] in corn

A premix of atrazine, bicyclopyrone, mesotrione, and S-metolachlor was recently approved for broadspectrum weed control in corn in the United States. Greenhouse and field experiments were conducted in 2015 and 2016 to evaluate the response of common waterhemp to various rates of the premix applied before emergence (preemergence, PRE) or after emergence (postemergence, POST) in corn. In greenhouse dose-response bioassays, PRE application of the premix at 975 g a.i. ha⁻¹ provided 90% control (visual estimates) of common waterhemp at 28 d after treatment (DAT). The POST doses to control 90% (ED₉₀) of common waterhemp at 21 DAT were estimated as 1157 and 1838 g a.i. ha⁻¹ for the 8-10, and 15-18 cm tall common waterhemp, respectively. Under field conditions, the premix applied PRE at the labeled rate (2900 g a.i. ha⁻¹) provided 98% and 91% control of common waterhemp at 14 and 63 DAT, respectively. The ED₉₀ values for the in-field POST dose-response bioassay were 680 and 2302 g a.i. ha⁻¹ at 14 DAT for the 8-10 and 15-18 cm tall common waterhemp, respectively. The root mean square error and the model efficiency coefficient values indicated a good fit for the prediction models. Spearman’s correlation coefficient (r_S) showed that corn yield was positively correlated (rS ≥ 0.55; P < 0.001) with common waterhemp biomass reduction. The premix applied PRE provided higher corn yields compared with the premix applied POST. The new premix will provide an additional herbicide option with multiple effective modes of action to control common waterhemp in corn.