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Agronomy Day 2008

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Tour C

Waterhemp Notches Another Victory Against Illinois Farmers

Aaron Hager
Aaron Hager
Assistant Professor of Weed Science
Department of Crop Sciences
217-333-4424
hager@illinois.edu
Pat Tranel
Pat Tranel
Associate Professor of Molecular Weed Science
Department of Crop Sciences
217-333-1531
tranel@illinois.edu

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 the University of Illinois have developed specific recommendations to help farmers better manage this problem in soybean. The progressive four-step program 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.

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.

Considerations:

  • Why invest in a soil-residual herbicide? 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 achieve maximum or near-maximum soybean yield.

Step 2: The initial post-emergence application of glyphosate (alone at 0.75 to 1.0 pound acid equivalent per acre) must be made when waterhemp is 3 to 5 inches tall.

Considerations:

  • Why use glyphosate alone instead of tankmixed with a PPO-inhibitor? Limited data, the potential for significant antagonism with these tankmixes, and the occurrence of PPO-resistant waterhemp populations suggest 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 2007 field research trials (see table).
  • Susceptible waterhemp less than 5 inches tall is very sensitive to 0.75 lb ae glyphosate. Waterhemp plants that survive 0.75 or 1.0 lb glyphosate acid 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.

Considerations:

  • In 2007 field research, glyphosate-resistant waterhemp plants continued to grow at near-normal rates following treatment with glyphosate. If 7 to 10 days elapse before lack of control becomes obvious, glyphosate-resistant waterhemp plants might grow an additional 6 to 8 inches.
  • PPO-inhibiting herbicides are the only remaining herbicide options for control of glyphosate-resistant waterhemp. Waterhemp control with PPO-inhibitors is optimized when full rates are applied to small plants (5 inches or less).

Step 4: Re-scout the treated field within 10 to 14 days. If plants treated with a second herbicide application might survive, rogue these surviving plants from the field before they reach a reproductive growth stage.

Response of a glyphosate-resistant waterhemp population to labeled in-crop application rates of glyphosate (University of Illinois, 2007).
Glyphosate
rate (fl oz)
Waterhemp
size (inches)
Days after treatment
7 14 21
--------------Percent control-------------
22 4 30 43 33
44 4 40 47 50
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