The Growing Challenge of Glyphosate-Resistance

By Mark Jeschke | July 22, 2011

The expansion of the ethanol industry has accompanied tremendous growth in corn production that has been based on agricultural technology advances. One of the first genetic engineering advances, the development of herbicide-resistant corn varieties, is now entering a new phase.

With glyphosate-resistant weeds already an issue in many Corn Belt states and throughout the South this growing season, we’re urging growers to scout regularly and carefully for resistant weeds that can seem to appear overnight. Resistant populations tend to grow on a logarithmic scale, growing at a low level and 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.

Crop producers and advisors know that weed resistance to herbicides has been a management challenge for decades. In the past decade and a half, 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, 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 growers know there are resistant populations in the area, is to clean tillage and harvest equipment when moving between fields.  Weed seeds carried by machinery can spread resistance from field to field.

Because weed management can be a localized issue, we at Pioneer also have equipped our 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 the situation. 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, overreliance on any new technology in the absence of appropriate weed management practices is likely to create new sources of resistance in weeds. It’s often said that, given time, pests will eventually find their way around man’s ability to control them and that’s certainly the case here. However, the good news is that good scouting and the right management techniques should help growers maintain effective weed control to help them maximize yields on their acres. 

Author: Mark Jeschke
Agronomy Research Manager,
Pioneer Hi-Bred, a DuPont business
(515) 535-3698