Herbicide resistance: What is it, why is it a problem, and can we solve it?
Nevin Lawrence, Integrated Weed Management Specialist
Panhandle Research and Extension Center
Since the beginning of agriculture farmers have been battling weeds. Genesis 3:18 even promises Adam “thorns and thistles” as he toils in his fields.
And while farmers of modern day have quite a few more tools at their disposal than Adam did, the weeds continue to be a problem. One of the more talked-about issues in agriculture today is herbicide resistance.
Herbicides work by interfering with very specific biochemical pathways in plants which are essential for the plant to carry out growth and reproduction. There are dozens of different pathways, each referred to as a site of action, and hundreds of herbicides which all control specific weeds in specific crops.
And while the sites of action are different for each herbicide, all herbicides need to absorb into a plant and then move through the plant to reach the site of action. No plants in a field are identical, and even neighboring plants of the same species can vary in how they absorb, move, or interact with an herbicide. If you consider the millions of weeds present in one field, it is not unexpected that a single weed may be capable of surviving an herbicide application in a given year.
Herbicides don’t cause plants to be resistant, they just remove all the susceptible weeds from the field. That surviving plant may not even be noticed, but it may be capable of producing thousands of seeds. And once several thousand resistant seeds have established into the soil, the farmer has a problem.
Herbicides-resistant weeds are difficult to manage once established. Weed seeds can stay dormant in the soil for decades. Because of the long-lived nature of seeds within the soil, it won’t work to rotate away from a specific herbicide for a season or even several years. What is often recommended instead is using multiple types of herbicides, known as mixing modes-of-action, at the same time.
The logic of this approach is that the probability of a weed within a given field being resistant to an herbicide might be one in a million, and there are millions of weeds in a field. But the probability of a weed being resistant to two different types of herbicides at the same time may be one in a billion.
This approach does work, and the research backs it up. But in the real world farmers can’t just spray two herbicides to control every weed in their field. Some herbicides control certain weeds, but not others. So famers would need to spray four, five, or six herbicides to makes sure every weed is controlled by two modes of action.
Financially, this would bankrupt a farmer and isn’t practical. And in some cases, for instance controlling kochia in sugarbeets, there is only one herbicide that works so you can’t add a second herbicide even if you could justify the costs.
What is a farmer to do? In the Panhandle our biggest current issue with herbicide resistance is glyphosate-resistant kochia. But compared to other parts of the country the problems faced in the Panhandle are not that bad. In the Eastern Nebraska alone, farmers are dealing with resistant kochia, waterhemp, palmer amaranth, ragweed, marestail and a few others. So why is the Panhandle doing a little better in respect to herbicide resistance?
A lot of weed scientist think the answer has to do with crop diversity. In the South and Midwest, farmers are growing only two or three crops (corn, soybean, and cotton) back-to-back, and all of them are RoundUp ready. While in the Panhandle we grow wheat, corn, sugarbeets, dry beans, sunflowers, millet, alfalfa, and a few others.
All of these crops in the Panhandle vary in when they are planted, how they are grown, and what herbicides are used. By growing a diverse number of crops farmers may be unintentionally keeping the weeds on their toes, making it difficult for particular species to adapt to repeated practices like using the same herbicide year after year.
So if farmers are already, unintentionally, helping to manage herbicide resistance through diverse cropping systems, can they intentionally manage resistance by making changes to their cropping system or production practices? At the Panhandle Research and Extension Center a long-term study is currently underway to answer that question. This U.S. Department of Agriculture-funded study is comparing herbicide programs, tillage practices, and crop diversity to see what practices a farmer can adopt that might help manage resistant weeds.
The end of the study is still a few years off but the initial results seem to agree that changes certain practices to target resistant weeds may be a sound option.