Help Stop Glyphosate Resistance
Illinois farmers have another adversary to consider in their annual battle against weeds. Glyphosate-resistant waterhemp is poised to bring new challenges to agronomic cropping systems in Illinois, and it might be especially troubling for soybean producers. Weed scientists at Southern Illinois University and the University of Illinois have developed specific recommendations to help farmers better manage this problem in soybean. The progressive four-step program described inside was developed using data describing waterhemp biology, ecology and control. The steps are specific to herbicides, but weed management practitioners should strongly consider utilizing other cultural practices that increase the competitive ability of the soybean crop. Be sure to control any existing waterhemp plants before planting.
Step 1: Apply a full rate (according to label guidelines for soil type and organic matter content) of a soil-residual herbicide no sooner than 7 days before planting.
- Why invest in a soil-residual herbicide? Soil-residual herbicides may represent the only herbicide option for certain waterhemp populations. A waterhemp population resistant to both PPO inhibitors and glyphosate would not be controlled by any postemergence soybean herbicides. Waterhemp is competitive with soybean; previous research has shown that waterhemp can cause up to 40-percent soybean yield loss.
- Why use a full rate instead of a reduced (“set-up”) rate? Waterhemp germination and emergence extend late into the growing season. The later into the season that waterhemp emergence can be delayed, the greater the potential to maximize soybean yield.
Step 2: The initial postemergence application of glyphosate (alone at 0.75 to 1.0 lb acid equivalent (ae)/ acre) must be made when waterhemp is 3 to 5 inches tall.
- Why use glyphosate alone instead of tankmixing with a PPO inhibitor (such as lactofen, fomesafen, or acifluorfen)? Unless the herbicide sensitivity/resistance profile of the particular waterhemp population is known, the potential for significant antagonism with these tankmixes suggest glyphosate be applied alone. If the waterhemp population is confirmed to be:
- glyphosate-resistant: apply a PPO inhibitor followed 7 to 10 days later with glyphosate
- PPO-resistant: apply glyphosate alone
- Increasing the glyphosate application rate from 0.75 to 1.5 lb ae/acre (the maximum rate allowed by label) did not consistently improve control of a confirmed glyphosate-resistant waterhemp population in field research trials.
- Susceptible waterhemp less than 5 inches tall is very sensitive to 0.75 lb ae/acre of glyphosate. Waterhemp plants that survive 0.75 or 1.0 lb ae/acre glyphosate when treated at 5 inches or less should be closely monitored.
Step 3: Fields must be scouted 7 days after the initial glyphosate application to determine treatment effectiveness. If waterhemp control is inadequate and retreatment is necessary, consider applying a PPO-inhibiting herbicide (lactofen, fomesafen, or acifluorfen) at a full labeled rate (with recommended additives) as soon as possible.