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

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 GR₅₀ 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.