Waterhemp Outsmarts Herbicide Rotation

Researchers have discovered that waterhemp, and possibly other weeds don’t respond to rotating herbicides each year as they originally thought.

After three years of research and six generations of waterhemp, Illinois weed scientists confirm that once waterhemp is resistant to a herbicide you’ll always be fighting that resistance in your field. These genetic changes within the plant haven’t caused adverse effects on yield or seed production as once assumed.

“It goes back to our first major case of herbicide resistance in triazine, which inhibit photosynthesis, plants with triazine resistance have a large fitness cost—they grower slower than those without resistance,” says Pat Tranel University of Illinois Ainsworth professor in the Department of Crop Sciences. “That’s what lead weed scientist to thinking that rotating herbicides would help, but as new cases of resistance came along we learned that triazine was the exception, not the rule.”

A fitness cost assumes that when you change one part of the gene you make a sacrifice somewhere else like they found in triazine resistance. With waterhemp, there is a very low fitness cost which means plants with resistance are just as competitive as those without resistance in terms of reproduction and plant vigor.

“What we’ve found in this case is that most of the [waterhemp] resistances had very little cost,” Tranel says. The group started their test with 45,000 seeds and simulated six years of herbicide rotation to come to this conclusion.

“This study tells us that fitness cost isn’t going to help you much in terms of the herbicide resistance, so even long rotations aren’t going to work,” Tranel says. “It gives us that much more incentive to do the right things to avoid resistance in the first place.”

Follow weed best management practices to avoid spreading resistance as long as possible:

  1. “You need to use multiple, effective modes of action every year, not just one, but multiple,” Tranel, says. “Kill every weed twice, if you rely on only one control that’s how you get resistance.” Use two effective modes of action each time you apply herbicides, with special emphasis on post-emergent applications since that’s when some of the vilest offending weeds germinate.
  2. Scout to catch weeds before they’re too tall for herbicides and stop weeds before they start with a pre emerge. “Spray weeds when they are less than 4” tall,” says Joyce Tredaway Ducar, Auburn University Extension weed scientist. “Starting clean at planting is also very important—make sure you have a good pre-emergent [herbicide] down.”
  3. Use the right rate. Lower-than-recommended herbicide rates have helped spread resistance across the U.S. It might seem like it saves money, but weed escapes steal from your bottom line in the current and in future seasons.
  4. Consider mechanical options such as tillage or chopping. Tilling weeds under can bury seeds deep enough they are unable to germinate and chopping crews can stop late-season misses from putting on large seed heads.