With glyphosate-resistant weeds already an issue in many Corn Belt states and throughout the South this growing season, experts at Pioneer Hi-Bred, a DuPont business, urge growers to scout regularly and carefully for resistant weeds that can seem to appear overnight.
"Now is the time to be on the watch for potential pockets of resistant weeds," says Mark Jeschke, Pioneer agronomy research manager.
"Resistant populations tend to grow on a logarithmic scale, growing at a low level then exploding all at once. So it may seem like resistant weeds are a new problem, but they probably have been in the field for a few years as weeds uncontrolled by herbicide treatments. This makes it all the more important to keep an eye out for weeds that aren't being controlled."
Weed resistance to herbicides has been a management challenge for decades. Growers have widely embraced the use of herbicide tolerance technology, not only because of its convenient, effective and economical weed control, but also because many weed species were no longer being controlled by other herbicides.
Unfortunately, the long-term use of any single herbicide mode of action can lead to the development of weed resistance, and now glyphosate-resistant weeds have become an issue as well.
The continued widespread use of glyphosate makes it likely the problem will only get worse.
To date, glyphosate resistance has been confirmed in 21 weed species worldwide, including 12 in North America, with glyphosate-resistant weed populations identified in 25 states, according to the International Survey of Herbicide Resistant Weeds.
To help combat this issue, Pioneer agronomists recommend growers not only scout their fields, but also adopt integrated management practices that can help minimize risk while providing growers with a more consistent, effective weed control program.
This includes rotating their herbicide modes of action, focusing on herbicide efficacy, persistence and frequency of application. In addition, agronomic practices such as crop rotation and tillage can decrease herbicide selection intensity by reducing weed populations, depending on the weed species.
"Another good practice, if you know there are resistant populations in the area, is to clean tillage and harvest equipment when moving between fields," Jeschke says.
"Weed seeds carried by machinery can spread resistance from field to field."
Because weed management can be a localized issue, Pioneer also has equipped its local sales professionals with information on managing resistant species within a specific geographic area, as well as training modules to help build knowledge.
"In cases where resistance already exists, the way to manage the problem really depends on your situation," Jeschke says.
"Growers may need to look to a different herbicide. In corn, there are still some options available. However, soybeans can be more difficult, especially postemergence, as a lot of glyphosate-resistant weed species are resistant to other herbicides as well."
Development of new herbicide-resistant technologies in crops may provide new options for dealing with herbicide-resistant weeds. That said, over-reliance on any new technology in the absence of appropriate weed management practices is likely to create new sources of resistance in weeds.