Source: University of Illinois

It's been a long time coming; producers, industry representatives, and university researchers have discussed it and anticipated it for years, but apparently that hasn't left us any more prepared for Roundup herbicide resistant weeds. As the growth of Roundup resistant crops have grown over the years, so, too, has the number of producers using the Roundup herbicide program as their sole defense against weeds. Jeff West, with Henry-Stark Extension, says that while it is not a noticeable issue in this part of Illinois, the number of resistant weeds being identified is growing each year. He notes that like the current economic recession, producers have been unwilling to address the issue due to the need for increased management, as well as cost. Historically, it has been one of the most economical and effective herbicide programs for row crop production; that is right up until you have to pay the price for resistant weeds.

Waterhemp that is resistant to glyphosate (Roundup) has been found in several central and western Illinois locations. We've also found waterhemp that is resistant to the diphenyl ethers (Phoenix, Cobra, Flexstar, etc.), ALS, and post-emergent atrazine. This summer, a population of waterhemp in central Illinois was found to be resistant to the "bleaching" herbicides (Callisto, Balance, etc.). It was the same story of repeated use of the same herbicide over several years, which resulted in resistant weeds.

By now you would think we would all heed the advice of resistance management. But, unfortunately, some don't learn. And now more than ever, we need to evaluate our weed control options as the price of glyphosate has been reduced to probably the lowest ever, which may cause producers to forgo a pre-emergent herbicide in favor of a glyphosate only treatment.

This past year, Patrick Tranel, University of Illinois, examined several waterhemp populations for multiple resistances (where plants are resistant to more than one herbicide family). His results were cause for concern. Of 55 glyphosate resistant plants from 10 populations, 13% were resistant to diphenyl ethers as well.

These plants exhibited resistance to both glyphosate and diphenyl ethers. So, if you're a soybean grower that has a crop in one of these fields and you apply glyphosate and find it didn't kill the waterhemp, what are the odds that an application of diphenyl ether will? What do you do then? The only option is Liberty soybeans. The problem is that you had to anticipate a resistance problem beforehand and planted Liberty beans. But you would have to plan for that before planting.

For Roundup Ready corn and soybean crops the need to use a pre-emergent herbicide is a must. Glyphosate is an excellent herbicide. It is low priced and works (usually). If we want to continue to enjoy those benefits in the future, we need to take these steps now. Talk with your local supplier about adjusting your herbicide program to avoid promotion of resistant weed species.