Glyphosate-Resistant Weeds - What’s to Worry About?

As discussed in previous articles and at numerous field days and farmer meetings throughout the state, over the past year or more we have conducted extensive greenhouse research on a potentially glyphosate-resistant common waterhemp population discovered in northwest Missouri. This population was discovered in fields with a continuous soybean "rotation" where glyphosate has been used repeatedly as the sole active ingredient since 1996. The 2006 season was the first year that we were able to conduct research on this population at the actual field site and at this time we feel that we have met all the requirements that are necessary to declare a weed as "officially" resistant. Therefore, this common waterhemp population has now been confirmed as the ninth glyphosate-resistant weed in the world, and the sixth glyphosate-resistant weed that occurs in the U. S.

The eight other confirmed glyphosate-resistant weeds throughout the world include buckhorn plantain, common ragweed, goosegrass, hairy fleabane, horseweed or marestail, Italian ryegrass, palmer amaranth, and rigid ryegrass (for the official lists and information pertaining to glyphosate- and other herbicide-resistant weeds see http://www.weedscience.org).

Although results from our greenhouse research had already led us to believe this population was, in fact, resistant, we wanted to make sure of this in the actual field environment before this confirmation was made. In field trials, most of our glyphosate treatments were made in soybeans more than three weeks ago, and even extremely high rates of glyphosate have provided very poor control of this common waterhemp population (Figure 1).

At this point in our research, we have observed that preemergence soybean treatments like Spartan, Valor, Boundary, Dual II Magnum, and Intrro have provided good control of this glyphosate-resistant waterhemp population. These treatments will continue to be evaluated throughout the summer to determine the length of residual activity on waterhemp that may emerge later in the season. Additionally, we have numerous experiments to evaluate glyphosateresistant waterhemp control with conventional herbicides like Phoenix, Ultra Blazer, Aim, etc. when applied alone or as a postemergence glyphosate tank-mix partner.

In our corn experiments, we have observed a similar trend in that many of our preemergence treatments like Lexar, Lumax, Degree Xtra, Harness Xtra, etc. have provided excellent control of the glyphosate-resistant common waterhemp population. We have also observed good control with postemergence treatments of Distinct and Callisto, but as expected postemergence applications of glyphosate and/or glyphosate-only corn programs have provided poor control of this common waterhemp population (Figure 2).

All of these experiments are ongoing and first-year results will be discussed in more detail as we are able to gather and summarize the data, but at this time one thing seems clear from the field research we have conducted thus far: we can control a glyphosate-resistant waterhemp population in corn with a herbicide program that has alternative modes-of-action (other than glyphosate). While rotating to corn seems like a simple solution to a serious problem, it is much easier said than done in our state.

Why? Because unlike all of the major corn and soybean producing states that surround us, Missouri is a state that plants a considerable amount of continuous soybeans. In 2006, for example, approximately 2.75 million acres of corn and 5.3 million acres of soybean were planted in Missouri. These figures alone show that somewhere, somehow, we have a lot of acreage being planted to continuous soybeans. This concerns us, since glyphosate-only programs in Roundup Ready soybeans are certainly the norm, and since the examples of glyphosate-resistant weeds found to date have been discovered in areas where a Roundup Ready crop, such as soybean or cotton, has been planted continuously without rotation and where repeated applications of glyphosate have been made year after year.

As discussed in a previous article written this year (Glyphosate-Resistant Weeds: Pay now or Pay Later. Which is Better?), we feel that the identification of this glyphosate-resistant waterhemp population should be viewed as a serious wake-up call to growers who are planting continuous soybeans and relying solely on glyphosate for weed control. Common waterhemp is our most troublesome weed in corn and soybeans and has shown its ability to adapt to other herbicide chemistries (ALS-inhibitors, Protox-inhibitors, triazine herbicides) in the past. Also, there is no reason to believe that independent selection events cannot occur throughout the state, meaning that other farmers who have been in a continuous soybean rotation and who have relied solely on glyphosate may also start to see instances of glyphosate-resistant common waterhemp in their fields as well. This appears to be what has happened in the southeastern United States with the glyphosate-resistant palmer amaranth that has surfaced in continuous Roundup Ready cropping systems during the past two years.

We are already receiving calls this year from around that state about inconsistent control of common waterhemp with glyphosate. This is not that unusual as we receive these kinds of calls each year, but there seem to be more this year than ever before. Are there other glyphosateresistant common waterhemp populations in Missouri or surrounding states? It’s difficult to say but given the number of acres that common waterhemp infests, the answer is probably yes.

If you are concerned that you may have a glyphosate-resistant weed in your field, one of the first things to consider is the herbicide use history of the site where you are experiencing a problem. If you have been applying glyphosate as the sole herbicide for the past several years in this location, then the chances are much higher that you may have a resistant weed population that is either already established or becoming established in this location. If so, one of your first courses of action should be to rotate to a herbicide(s) with an alternative mode-of-action next year.

Figure 1. Common waterhemp that has survived an application of Roundup Original Max at 88 fluid ounces per acre in soybean experiments conducted at the northwest Missouri site.

Figure 2. Common waterhemp that has survived 2 applications of Roundup Original Max at 22 fluid ounces per acre in corn experiments conducted at the northwest Missouri site.