Response of glyphosate-resistant horseweed [Conyza canadensis (L.) Cronq.] to a premix of atrazine, bicyclopyrone, mesotrione, and S-metolachlor

A premix of atrazine, bicyclopyrone, mesotrione, and S-metolachlor has recently been commercialized for pre-emergence and early post-emergence control of broadleaved and annual grass weeds in corn in the United States. Field and greenhouse dose-response bioassays were conducted in 2015 and 2016 to evaluate the response of glyphosate-resistant (GR) horseweed to this premix applied pre-emergence or post-emergence. In field pre-emergence study, the ED90 values were 2,613 and 2,863 g ai ha─1 at 14 and 35 d after treatment (DAT), respectively, which were comparable to the labeled rate (2,900 g ai ha─1) of the premix. Under greenhouse conditions, post-emergence applications made at the labeled rate to 8 to 10 or 15 to 18-cm horseweed rosette diam provided ≥ 97% control. The ED90 values for the in-field post-emergence dose-response study were ≥ 3,431 g ai ha─1 and ≥ 6,717 g ai ha─1 for the 8 to 10 and the 15 to 18 cm tall GR horseweed, respectively. At 14 DAT, the premix applied at the labeled rate provided 85% ...