Sterile oat (Avena sterilis subsp. ludoviciana) and turnipweed (Rapistrum rugosum (L.) All.) are two important weed species in wheat fields, and their chemical control is affected by low temperatures at the time of spray. Both weed species were exposed to a freezing treatment (−4 °C) before and after spraying for three nights from 7:00 PM to 5:00 AM, and their response was compared with the control plants that were not subjected to any freezing treatments. In this study, the efficacy of mesosulfuron-methyl plus iodosulfuron-methyl-sodium plus mefenpyr-diethyl (hereafter, MIM) (1.2% OD) was affected by the gradual decrease in temperature. At the recommended dose (18 g ai ha−1: 15 g ai ha−1 mesosulfuron-methyl +3 g ai ha−1 iodosulfuron-methyl-sodium), sterile oat in the freezing before spray (FBS), compared to the no freezing (NF) and the freezing after spray (FAS), had a 58% and 47% higher survival rate, respectively, and turnipweed in these conditions had a 50% and 40% higher survival rate, respectively. The LD50 (dose required to control 50% of individuals) and GR50 (dose required to cause a 50% growth reduction of plants) in sterile oat and turnipweed rankings were similar; NF < FAS < FBS. The selectivity index for this herbicide in NF, FBS, and FAS treatments for wheat in sterile oat was 1.83, 0.14, and 0.99 and in turnipweed was 2.44, 0.20, and 1.40, respectively. Our results suggest that the efficacy of MIM will be reduced if applied during freezing stress and growers need to use alternative herbicides or other weed management tools.

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