S‐metolachlor in pre‐emergence reduces the leaf cuticular wax and increases the efficiency of glyphosate

Abstract BACKGROUND The low glyphosate uptake capacity, possibly due to the lipophilic character of epicuticular wax on leaves, may contribute to the natural tolerance of some weed species. The use of pre‐emergence herbicides like S ‐metolachlor, which inhibits very‐long‐chain fatty acid synthesis, might enhance glyphosate's post‐emergent efficacy against hard‐to‐control weeds, such as Euphorbia heterophylla L. (milkweed) and Ipomoea triloba L. (littlebell). This study aimed to determine whether S ‐metolachlor affects glyphosate's efficacy and its impact on epicuticular wax production in these species. In Experiment 1, milkweed was 2.27 times more tolerant to S ‐metolachlor than littlebell, with GR 50 values of 1024.6 g ai ha −1 and 451.2 g ai ha −1 , respectively. In Experiment 2, S ‐metolachlor enhanced glyphosate efficacy in both species. For littlebell, S ‐metolachlor increased glyphosate efficacy by 28% at 21 days after treatment (DAT), while milkweed boosted control to 61%, compared to 45% in untreated plants. The herbicide accelerated glyphosate's action, particularly at lower doses. In Experiment 3, S ‐metolachlor reduced epicuticular wax by up to 47% in littlebell and 45% in milkweed during early growth. Wax levels remained lower in treated plants, contributing to improved glyphosate efficacy. CONCLUSION S ‐metolachlor in pre‐emergence reduces the amount of wax deposited in the leaf surface, increasing glyphosate efficiency in both species, especially at the beginning of the phenological stage. © 2024 Society of Chemical Industry.