Palmer amaranth. Waterhemp. Marestail. If you’re a no-tiller in the Corn Belt, you’ve likely encountered at least one of these weeds on your farm. Chances are, they’re resistant to one or more herbicides.

Bryan Young, a weed scientist at Purdue University, says these three are the most problematic weed species confronting corn and soybean growers, which is a direct result of the evolution and wide-scale spread of resistance to glyphosate. In the FMC white paper, “Overlapping Residual Herbicides,” Young says the reason they’re the toughest to battle is because of their:

  1. Prolific seed production
  2. Gaps in weed emergence throughout the growing season
  3. Ease of seed dispersal (carried by wind, water, contaminated livestock feed or wildlife)
  4. Ease of overcoming or escaping herbicide-based management tactics

With tillage out of the question for no-tillers, using soil residual herbicides may be an effective solution for controlling these weeds. According to Young, residual herbicides provide long-term control of weeds with extended emergence periods, reduce the density of weeds that may require a post-emergence herbicide application, offer a wider window for any necessary post-emergence applications and provide different herbicide sites of action compared to glyphosate.

But the key to using soil residual herbicides to fight herbicide-resistant weeds, particularly where options are extremely limited for post-emergence control, is to select the most effective soil residual herbicides and overlap them. This means applying a residual herbicide before the residual activity of the first herbicide dissipates to the point where weed emergence occurs, Young says.

What qualifies as an effective soil residual herbicide? Young says it must:

  1. Have the potential to control the target weed species, even if applied alone
  2. Be applied in a manner to optimize herbicide efficacy (i.e., proper application rate, application timing and application methods)
  3. Represent a different herbicide site of action than the resistant traits found in the targeted weed

No-tillers can find a few examples of effective overlap programs for control of resistant weeds in soybeans in the white paper.

Do you overlap residual herbicides to control an herbicide-resistant weed? Share your weed-control program in the comments below and let us know how it’s working.