Roundup Ready™ crops have been producers’ preferred weed management strategy wherever the technology is present. There are estimates that up to a billion crop acres have been treated since RR soybean was released in 1996. Crop producers using this highly effective technology to control weeds have also inadvertently been changing the weed population to weed biotypes that are not as easily controlled as they once were. Weed scientists noticed just a few years after RR soybean were released that some species such as waterhemp were requiring increasing glyphosate rates to control this species. They were reluctant to call these weeds glyphosate resistant because they were still able to control the plant even though it took higher rates of application and terms like ‘rate creep’ were used to describe what they were observing . This has changed. Herbicide resistance is defined as the “inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type” and weed scientists use the term ‘low level resistance’ to describe weeds exhibiting these responses to glyphosate.

Carlyle Holen, Integrated Pest Management specialist at the University of Minnesota posed the following question to farmers: “Is it too late to close the barn door?” It is much easier to be proactive about dealing with glyphosate resistant weeds, than reactive. Your options are limited if you already have glyphosate resistant weeds in your fields, so start utilizing best management practices before you even recognize any problems.

To avoid resistant weeds, Holen advises that weed management strategies should be diversified. “Remember that small plants will still produce seeds” advises Holen. “A two pass strategy will help to eliminate those smaller plants that can still produce seeds. And farmers may want to consider residual herbicides applied early preplant or preemergence as one of the two passes.“ Other best management practices recommended by Holen include mechanical control, rotation of herbicide modes, timely application, and finally, don’t cut glyphosate rates.

Not all weeds that escape control in a field are resistant to glyphosate. Holen advises you double check a few issues before making the determination. “Evaluate the fields with weed escapes and see if all the escaped weeds are of the same species such as common ragweed or common lambsquarters” says Holen. If several weed species have escaped control, it is unlikely your problem is glyphosate resistance. Also check the range of injury to the weeds. Does the injury range from completely dead plants, to stunted, to green and healthy? If you see a single weed species that has escaped control in a field and the plants exhibit a range of symptoms from dead to alive, it is highly possible these plants have some level of glyphosate resistance. Finally, make sure you can rule out application issues such as wind, rain, or the incorrect rate. Application errors do occur, and a second application of glyphosate may be the most cost effective method of correcting the error.

To recognize glyphosate resistance, it is important to get down in the dirt and examine even the smallest plants. The weed in this picture has survived two applications of glyphosate (1.36 lbs ae/acre). This weed is still capable of producing seeds, and causing problems in subsequent years. Although the weed may look dead at first glance, it is important to get down in the dirt and take a closer look to make sure weed resistance is not occurring in your fields.

If the barn door is opened to glyphosate resistance, your options are limited. The same strategies used to avoid resistance can be used to manage your field once resistance is confirmed, but there is no solution to the problem. That is why working to keep the glyphosate resistance door closed needs to begin early, and be a constant management concern each year.