A growing concern for farmers and crop scientists is the proliferation of weeds that have evolved resistance to widely used herbicides, such as glyphosate, the active ingredient in Roundup. Aaron Hager, a faculty member in the department of crop sciences in the College of Agricultural, Consumer and Environmental Sciences and expert in weed science and plant protection spoke recently with News Bureau reporter Sharita Forrest about the potential impact of herbicide-resistant weeds on agriculture in the U.S.
How widespread is the problem of glyphosate resistance?
We have three weed species in Illinois with documented resistance to glyphosate: horseweed, also called marestail; Palmer amaranth; and waterhemp.
We estimate in the southern three-quarters of Illinois you’ll find glyphosate-resistant waterhemp, and when you add in the counties with documented resistance to protoporphyrinogen oxidase (PPO) inhibitor herbicides, you have a significant resistance problem statewide.
Palmer amaranth is a species we’re very concerned about because it’s such a competitive, rapidly growing species and is dramatically changing farming methods in the Mid-South and southern states. Glyphosate-resistant Palmer has been confirmed in two or three locations in the southernmost regions of Illinois, and we are accumulating evidence that it can survive and thrive even in Central Illinois.
Marestail is one of the winter annual species that farmers often find in their no-till fields that they have to control before they plant. It’s a bit more challenging in Southern Illinois because a greater proportion of the marestail in that region actually emerges in the spring and behaves more like a summer annual. Farmers have to rely on soil-residual herbicides and be careful what they use for post-emergence treatments.
Waterhemp appears on the widest geographic scale and is arguably the most resistant. It began changing how farmers go about weed management within the last five years. We now have waterhemp populations in Illinois such that if a farmer were to plant a glyphosate-resistant soybean variety and waterhemp were to emerge, there are no herbicide options available that would be effective.
We frequently find waterhemp populations that have multiple resistances, at the field level or the plant level. In Illinois, waterhemp has evolved resistance to five different herbicide classes, and we’ve found as many as four types of resistance in a single plant. If you add in the auxinic herbicides, waterhemp has developed resistance to six different herbicide families across the U.S.
When multiple resistances develop, managing the species becomes very difficult.
It’s a daunting challenge because our data would suggest that the occurrence of multiple resistances is going to become the norm.
What options are available to farmers for managing resistant species?
If glyphosate is no longer effective, farmers have several different post-emergence herbicide options to use with corn, but fewer options with soybeans. They can tank-mix another herbicide in with the glyphosate specifically to manage resistant waterhemp.
Glyphosate was used to control marestail for many years, but now that resistance has developed, farmers have to be careful that their “burndown” herbicide includes a different herbicide besides glyphosate. That might be a tank-mix combination with another product, another herbicide altogether or they might till to control the
over-winter population before planting.
How did this resistance problem develop?
When the Roundup-Ready technology came into the marketplace in 1996, and usage expanded in the 1990s, farmers began moving away from most other herbicides.
Before then, we had treated a high percentage of our soybean acres with soil-residual herbicide, and many of those were very effective on waterhemp.
As we moved to post-emergence applications, glyphosate became one of the only herbicides used on a large percentage of our acres. Farmers are slowly going back to using residual products in soybeans as weeds become resistant to glyphosate and the spectrum shifts to weed species less sensitive to glyphosate.
Roundup-Ready technology and glyphosate allowed us to control weeds as we never had before. But there’s a difference between simply controlling weeds and managing weeds, and we didn’t manage them very well in that system. Some of the issues that we’re seeing now with populations of Palmer and waterhemp are the outcome of simple weed control.
It’s predictable that we would have gotten to this point eventually, even if we’d had a more integrated weed-management system in place all these years, but it would have taken a lot longer to get here if we hadn’t relied on one product to control the whole spectrum of weeds.
How quickly have weeds such as waterhemp developed multiple resistances?
The first report of stacked resistance to triazine herbicides and ALS inhibitors in waterhemp was published in 1998.
In 2001, we identified a waterhemp population in Western Illinois that had resistance to three different herbicide families.
In 2009, we found the first-ever documented waterhemp population with resistance to four herbicide families, including glyphosate.
When did you begin talking to crop producers about the problem?
We’ve talked to producers about herbicide resistance for many years, since the 1980s when the first herbicide-resistant weed in Illinois was discovered. We came out with our first set of recommendations for managing glyphosate-resistant waterhemp in 2008.
Resistance wasn’t very common or widespread then, but we knew that resistant populations were eventually going to spread across the state. Many farmers didn’t see a need to address a problem that they didn’t see as affecting them yet. That’s unfortunate; it was inevitable, and we were trying to be proactive and help them minimize the consequences when resistance did occur.
The most costly year to have a resistance problem is the first year that it has built up to a significant enough level that the herbicides you’ve been using suddenly don’t work. You can try to re-treat the field with another product, but if you have a population with stacked resistance that may not work either. Then you may be facing a growing season with a weed you can’t get rid of, and it will lower your crop yield.
We haven’t seen a lot of novel chemistry for weed management in the last 10 years.
And while these resistant populations continue to increase, there’s not a fix on the horizon any time in the foreseeable future. Ultimately someone will develop something new, but farmers can’t count on it happening in the next season or so.
How much can resistant weeds decrease yields?
Studies have documented waterhemp reducing soybean yield by as much as 42 percent. Losses could be worse in corn. Depending on a crop’s market value, farmers could lose hundreds of dollars per acre.
If farmers are proactive, their weed-management costs may increase somewhat, but the potential cost won’t be anywhere near the financial loss they’ll experience if they allow resistance to build up.
How many Illinois acres are being treated now with glyphosate?
Glyphosate is the most widely used herbicide in Illinois right now. It’s used on more than 90 percent of soybean acres. I would estimate that it’s used on 80 percent or more of corn acres. As resistance increases, farmers will have to apply other herbicides in conjunction with it or pre-treat the soil with something else.
