Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/13/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: Waterhemp (Amaranthus tuberculatus) is an extremely problematic weed for farmers throughout the Midwestern United States because it has evolved resistance to many different herbicides. Glufosinate is a herbicide that still works to control waterhemp in most places. However, glufosinate should be applied with other effective herbicides to minimize the risk of waterhemp evolving resistance to it. Tank-mixing the PPO-inhibiting herbicide saflufenacil with glufosinate has been reported to dramatically enhance control of a closely related species, Palmer amaranth (Amaranthus palmeri), but saflufenacil may not be applied to soybean foliage (postemergence). This research was conducted to determine if tank-mixing glufosinate with PPO-inhibiting herbicides that can be safely applied to soybean foliage would improve control of waterhemp without increasing injury to a soybean crop. In field studies conducted in Wisconsin and Illinois in 2020 and 2021, we observed no increase in soybean injury and no increase in waterhemp control when PPO-inhibiting herbicides were tank-mixed with glufosinate and applied to actively growing waterhemp and soybean plants. Although we did not see increased control of waterhemp with tank-mixtures in our studies, tank-mixing fomesafen, lactofen and 2,4-D with glufosinate may be effective at reducing the risk of waterhemp evolving resistance to glufosinate in some areas. Research results will benefit the agri-chemical industry, producers, university extensionists, and crop consultants.

Technical Abstract: Glufosinate is an alternative or complement to glyphosate and other commonly used POST herbicides to manage herbicide-resistant weed species. Enhancing broadleaf weed control with glufosinate through effective tank-mixtures may mitigate further herbicide resistance evolution in soybean cropping systems. Two field experiments were conducted in 2020 and 2021 across locations in Wisconsin (Arlington, Brooklyn, Janesville, and Lancaster) and one location in Illinois (Macomb) to evaluate the impact of glufosinate tank-mixed with PPO-inhibitors (flumiclorac, fluthiacet-methyl, fomesafen, and lactofen), bentazon (Group 6), and 2,4-D choline (Group 4) on waterhemp control, soybean phytotoxicity, and yield. The experiments were established in a randomized complete block design with four replications. The first experiment focused on soybean phytotoxicity 14 days after treatment (DAT) and yield in the absence of weed competition. All treatments received a PRE-emergence herbicide, with POST-emergence applications occurring between the V3-V6 soybean growth stages, depending on the site-year. The second experiment evaluated the impact of herbicide treatments on waterhemp control 14 DAT and on soybean yield. Lactofen, applied alone or with glufosinate, presented the highest phytotoxicity to soybean 14 DAT, but this injury did not translate into yield loss. Tank-mixing PPO-herbicides with glufosinate did not increase waterhemp control, nor affect soybean yield compared to solo applications of glufosinate but may be an effective practice to reduce selection pressure for glufosinate-resistant waterhemp.