A premix of atrazine, bicyclopyrone, mesotrione, and S-metolachlor has recently been commercialized for pre-emergence (PRE) and early post-emergence (POST) 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 before or after emergence (PRE or POST). In a field PRE study, the 90% effective doses (ED₉₀) were 2613 and 2863 g a.i. ha^-1 at 14 and 35 d after treatment (DAT), respectively, which were comparable to the labeled rate (2900 g a.i. ha^-1) of the premix. Under greenhouse conditions, POST applications made at the labeled rate to 8–10 or 15–18 cm diameter horseweed rosettes provided ≥97% control. The ED₉₀ values for the in-field POST dose-response study were ≥3431 and ≥6717 g a.i. ha^-1 for the 8–10 and 15–18 cm tall GR horseweed, respectively. At 14 DAT, the premix applied at the labeled rate provided 85% and 68% control of 8–10 and 15–18 cm tall GR horseweed, respectively. The root mean square error for the log-logistic model ranged from 4.2 to 9.2 and the model efficiency coefficient values were ≥0.94 (∼ 1.00), indicating a good fit for the prediction model. In conclusion, a new premix applied before emergence (PRE) will effectively control GR horseweed at the labeled rate compared with POST applications made to ≥8 cm tall plants.