The Weed Science Society defines herbicide resistance as the ability of a weed biotype to survive herbicide application, when under normal circumstances that herbicide would have killed the weed. Resistance happens with the repeated use of the same herbicide, or herbicides with similar modes of action, on a weed population.
Herbicide resistance has been around since the 1970s and, in the United States, increased with the introduction of glyphosate-tolerant crops in 1996. Today, more that 40 percent of crop acres are infested with glyphosate-resistant weeds. Globally, about 250 weed species have been confirmed to be resistant to at least one herbicide mode or site of action. In the United States, that number is 164, according to the International Survey of Herbicide Resistant Weeds.
The Cost of Resistance
Herbicide resistance costs growers in terms of reduced yields and increased weed control demands. In Australia, herbicide-resistant weeds are increasing expenses by 27 percent per acre, due to higher input costs and yield loss. In the United States, in areas where there are no weed control options, growers are resorting to hiring workers to hand-weed -- a practice that can add up to $150 per acre or more in costs, according to the Grains Development Research Corporation.
In addition to the visible costs, there are also other implications. For example, farms that do not have a comprehensive weed-management program can also suffer from reduced land values and lost rental agreements.
Experts advise that the real cost of managing resistance will continue to increase. For example, dry bean growers in the United States could lose an average of 71 percent of their yield -- equating to $622 million per year -- if weeds are not controlled, according to a Michigan State University study.
Tools to Manage Resistance
For many years, new herbicide technologies helped keep growers ahead of resistance. If weeds started to develop resistance to a herbicide or active ingredient, there often was another discovery in the lab ready for commercialization to help take its place. For example, Group 2 chemistry helped manage triazine resistance, and glyphosate helped solve Group 2 resistance -- for a while, anyway. Today, that has all changed. While ag chemical companies continue to innovate, there are vastly fewer new active ingredients on the horizon. That's why it's so important to preserve the tools that growers already have at their disposal.
Meanwhile, the industry continues to innovate. The advent of herbicide-tolerant crops was a giant leap forward. However, experts advise that growers can't go back to the same practice of spraying the same post-emergent herbicide over the top of the crop.
New technologies such as gene editing are holding out a lot of promise, and it has been demonstrated for Group 2 tolerance in canola, dry beans and rice, and glyphosate tolerance in flax. Gene editing is different than genetically modified crops in that the genetic material of a crop can be edited without injecting anything new into it. Think of it as basically rearranging the genetic material in a plant for a desired outcome, something plant breeders have done for centuries.
RNA interference (RNAi) -- the process cells use to suppress the activity of specific genes -- is another potential technology that could help. One of the potential applications of RNAi is in thwarting a weed's processes for the development of resistance. Additionally, companies such as John Deere are exploring robotic weeding. The futuristic scene of an army of robots in a field may not be too far-fetched, but any practical applications are years away.
Preserving Our Precious Tools
Stemming the rising tide of herbicide resistance will take time and an arsenal of resources. Today, consistent application of Best Management Practices is the best strategy for preventing and prolonging resistant weeds from becoming an issue.
The Herbicide Resistance Action Committee provides helpful resources for managing the weed seedbank. Some of the top management practices include:
- Rotate your crops.
- Use mechanical weed control methods, such as cultivation, to control weeds.
- Rotate herbicides -- do not make more than two consecutive applications of herbicides with the same mode of action in the same field.
- Use tank mixtures of herbicides with differing effective modes of action.
- Scout fields after each herbicide application to identify any areas where there was a lack of control. Destroy weed escapes.
- Clean your equipment before moving to a different field to prevent the spread of resistant biotypes and save work in fields with suspected herbicide resistance for last.
When it comes to crop protection, remember to mix herbicide modes of action, particularly if you're using multiple herbicides to control the same species of weeds. Two proven solutions from UPL include:
- INTERLINE™ Herbicide is a non-selective, WSSA Group 10 contact herbicide that effectively manages hard-to-control grass and broadleaf weeds — particularly glyphosate-resistant weeds invading row crops. It's labeled for corn, canola, soybean, sugar beets and cotton and is particularly effective against marestail, ragweed, waterhemp, pigweed and kochia
- EVERESTÒ 3.0 Herbicide (Group 2) for Spring, Winter and Durum wheat is relentless on yellow foxtail and other hard to kill grass weeds while providing Flush after flush® control of green foxtail, wild oats, and other key grass and broadleaf weeds.
Growers can count on UPL for the solutions and expertise to help manage herbicide resistance and keep their fields delivering strong yields. For more information, contact your retailer or UPL sales representative.
