In my soybean weed control and burndown plots, we regularly get excellent weed control from one or two properly timed applications of glyphosate. This includes weeds like morningglories, nutsedge, sicklepod, pigweeds, prickly sida and several grass species.

Some notable weeds missing from our location, however, are horseweed, common and giant ragweed and lambsquarters. There has been much written over the past few years about horseweed that has developed resistance to glyphosate.

We have also been investigating common and giant ragweed populations that seem to be more tolerant to glyphosate than they should be. We also are getting calls on common lambsquarters, ryegrass and pigweeds.

I have in no way investigated all these weeds, and I am not reporting here today that we have confirmed resistance in any of them. However, tank-mixtures with glyphosate to control specific weed problems are becoming the norm.

Whether a population of weeds is resistant, tolerant, less susceptible or normal is something that causes much debate in my field. However, my main concern is killing weeds and making recommendations to county agents, producers and consultants. What to call a population of weeds is less important to me than knowing if I need to tank-mix something with glyphosate or not, and if I do, which herbicide and rate do I use?

In the case of horseweed, populations have now been tested and resistance has been confirmed. You can see stark differences between individual horseweed plants in the field following glyphosate applications.

In cotton, horseweed control must begin at burndown. This usually involves the addition of 2,4-D or Clarity to control plants glyphosate does not kill. In-season options are limited in cotton.

In soybeans, the same can be said for starting as clean as possible with a burndown treatment for horseweed. However, in a recent on-farm study conducted by the Extension staff in Crittenden County, Ark., some other herbicide options for horseweed control in soybeans looked promising. A preplant application of Synchrony XP or Python performed well. In-season, an application of FirstRate at 0.3 ounce of product per acre tank-mixed with glyphosate also improved horseweed control.

Using a burndown program with either Clarity, 2,4-D, Python, or Synchrony XP followed by glyphosate tank-mixed with FirstRate looks like a pretty good program for horseweed. It should be mentioned that a program like this also helps curb the development of more glyphosate-resistant weeds.

Glyphosate alone is not controlling a few other weeds in some fields, either in-season or as a burndown. Common and giant ragweed both have been difficult to control with glyphosate alone in at least two fields in Arkansas this year.

In our research, 22 ounces of Roundup WeatherMax applied alone controlled giant ragweed around 70 percent in one of these fields. We looked at several tank-mix partners for this weed. Both FirstRate and Flexstar were effective tank-mix partners. Several herbicides, including Scepter, Classic, Blazer, Basagran, Storm, and Resource, were evaluated but did not control giant ragweed. These treatments were sprayed on 6-inch-tall giant ragweed plants.

FirstRate and Flexstar were good tank-mix partners on common ragweed, which we evaluated at another research site this year.

Earlier this year our common ragweed population was confirmed to be glyphosate-resistant. I do not know at this point if our populations of giant ragweed are resistant or not. This also is true for the rumored fields of pigweed, ryegrass and lambsquarters that I hear about, but have not seen or confirmed.

It may simply be that the 1X rate of glyphosate is just too low to control these weeds and that they are not actually resistant. However, if they are not being controlled, it may be enough for those of us in the field making recommendations to simply be aware that attention should be given to these weeds and a tank-mixture will likely be needed before the problem gets worse.

My soybean research program is funded in part by the Arkansas Soybean Promotion Board. Its support is greatly appreciated. I also thank county agents Randy Chlapecka, Mike Hamilton, Jason Osborne and John McFarland and producers John and Todd Allen for the research and research sites mentioned in this article.


Bob Scott is the University of Arkansas Extension weed specialist. e-mail: bscott@uaex.edu