Scouting for Glyphosate Resistance

Jeff: Hello. My name is Jeff Stachler. I have a joint appointment with the University of Minnesota and North Dakota State University as the Weed Science Extension Specialist for sugar beets. Roundup Ready crops are grown on the majority of US crop acreage. The exclusive use of glyphosate to control weeds in Roundup Ready crops allows for the selection of resistant weeds. Glyphosate resistant weeds continue to increase in Minnesota, North Dakota, and the United States. Common ragweed, giant ragweed, and water hemp have been confirmed glyphosate resistant in Minnesota in multiple counties. In North Dakota, common ragweed has also been confirmed glyphosate resistant in multiple counties. Six other weed species have been confirmed glyphosate resistant across the United States.

One reason glyphosate resistance continues to increase is the lack of early detection. Growers and the agricultural industry blame other factors for the presence of late season weeds in fields, instead of suspecting the presence of glyphosate resistant plants. Glyphosate resistance is very different compared to previous cases of herbicide resistance. The initial surviving plants with resistance to ALS or ACC ace inhibiting herbicides demonstrate minimal injury from ALS or ACC ace inhibiting herbicides. However, with glyphosate, nearly every surviving plant shows some amount of injury with each application, and factors such as glyphosate rate and timing and environmental conditions influence the response.

I want to demonstrate to you when you need to be concerned about glyphosate resistant, the pattern within a field, and the response of multiple and individual plants. Here we are in a field of Roundup Ready soybeans late in the growing season. In addition to the soybeans are some common ragweed plants. Two applications of glyphosate have been applied to this field, and there should not be any weeds present. Therefore, we suspect these common ragweed plants as being resistant to glyphosate. Glyphosate resistance will begin with just a single plant within a field. If gone unchecked, the number of those plants over the years will continue to increase, especially if only glyphosate is applied.

You need to look in areas of the field that may be the most stressful and/or have the highest weed population, because that is normally where the greatest genetic diversity is, and allows for the resistant plants to be present. When we apply the glyphosate, we select for those plants. Capturing that early, when there’s just a few plants, maybe only 6 plants within the field, is very, very critical. If not caught early enough, you will have to spend more money to control the problem in future years, and potentially if it gets to a really high density, which we also have in this field, you may have yield loss, which will be a loss of profit.

One you have identified plants surviving glyphosate in a field, the next most important thing to understand is the response of individual plants within that field. So go to an area of the field where you have some surviving plants, and look closely down at the soil. You should be able to see dead plants, as indicated by this plant right here. Hopefully, you’re catching this early enough that most of the plants in the field are actually dead. This is very important to understand and look for. Because if you have dead plants, and other species are dead also, you know that the glyphosate did work.

Near the dead plants, you should be able to locate plants that appear normal in growth, but are stunted compared to an untreated plant. These normal-appearing plants are injured because glyphosate almost always produces some level of activity in plants, even though they are resistant. In addition to observing dead and normal-appearing plants, a continuum of responses from nearly dead, as indicated by this plant, with just a single area of growth, to decreasing levels of activity should be visible. This response is very similar when identifying resistance to growth regular and PPO inhibiting herbicides. The greater the injury, the more likely the level of resistance is low. Understanding and observing this continuum of responses is critical to determining if glyphosate resistant is present in a field.

One other response that’s possible is a plant such as this one, where the glyphosate has killed the main marrow stem of the plant, and all the nodes below that, the axillary nodes, begin to recover and grow and produce branches that appear normal. This is likely to be a lower level of resistance because of that main marrow stem being killed, compared to a plant such as this one, in which the main marrow stem has continued to grow normally. Here’s a normal-looking common ragweed leaf. Notice how wide the area is between the leaf margins on the lobes. But when we go to a plant that is injured from glyphosate and trying to survive, notice how that distance between the leaf margin on the lobe is quite narrow. Then you might have colors of yellow and white intermingled with this, and that is showing that the glyphosate is still present in the plant and having activity.

We have discussed what to look for across the field and individual plant responses. It is important to understand that glyphosate resistant is different compared to previous types of herbicide resistance. When you observe dead plants next to plants that appear normal, but are stunted, and observe a continuum of injured responses between dead and near normal, glyphosate resistant is likely present in the population. This is especially true if observed in just a single species in a field. If glyphosate resistant is suspected, the best way to confirm resistance is to collect weed seeds, grow plants from the seeds in the greenhouse, and spray them with glyphosate.

Identifying glyphosate resistant when a few plants are present in a field should maximize future profit. Changing weed management practices rapidly by utilizing all possible management strategies should ensure effective weed control for the future. I thank you for watching this video.