In an article several months ago in the Delta Farm Press I discussed the discovery of a “suspected” glyphosatetolerant population of common ragweed in Arkansas. We had just completed several field observations that left us suspicious that it might be resistant.

In our field studies, common ragweed plants survived multiple applications of 1 to 2 pounds (or more) per acre of glyphosate herbicide. However, at that time we could not confirm the presence of a resistant biotype. Leaving doubts in our minds were factors such as some of the plants already were chlorotic (yellow) and partially controlled by previous applications of glyphosate, and the ragweed was too large in size. We needed clarification over the winter in greenhouse studies or next spring in the field.

After publication of that first article on glyphosate-resistant ragweed, seeds and young plants were harvested from the field and taken to a greenhouse for observation. The plants and seed were grown and sprayed with different rates and timings of glyphosate.

In those studies, varying levels of control was observed, which, we believe, indicates a segregating resistant population — by that I mean both susceptible and varying degrees of resistant plants from the same sources. That is to be expected in early detection of herbicide-resistant weed populations.

Control ratings ranged from 0 to 100 percent with rates as high as 1 pound of active ingredient per acre. The ragweed plants sprayed ranged from three to six nodes (4 to 8 inches). That rate normally provides 80 to 100 percent control of susceptible common ragweed.

So, we now believe we can confirm the presence of a glyphosate-resistant population of common ragweed in Jackson County, Ark.

The graduate student responsible for the work is Chad Brewer. He has decided to work on the weed for his Ph.D. dissertation, under the direction of Dick Oliver in Fayetteville, Ark.

While the discovery of this population is interesting, there are alternative control measures available for common ragweed.

The circumstances in which this population developed are not very common. The field has been farmed dryland, no-till, and kept in a soybean/grain sorghum rotation for at least the past six to seven years. It has received annual burndown applications of glyphosate in all crops. So the selection pressure has been intense.

This population of common ragweed poses only a small threat to soybean production in Arkansas at this time. However, the implication of finding another weed (in addition to horseweed) with tolerance to glyphosate has been a cause for some alarm in weed science circles. Most are concerned about which weed will be next.

If, for example, the next weed discovered were Palmer amaranth, common cocklebur or a grass weed, there could be some serious impacts on soybean farming. Our weed group is motivated to propose and plan studies aimed at controlling some of the more difficult weeds (especially Palmer amaranth) without the use of glyphosate.

Our research on common ragweed from this point forward will focus on alternative control measures, including both burndown and in-season options. We will evaluate conventional herbicides alone and in combination with glyphosate. More studies will be conducted to determine which glyphosate rate (if any) will control these ragweed plants and the mechanism of resistance.

It is of concern that of the weeds known to be resistant to glyphosate, not all develop resistance by the same physiological method. Our initial observations indicate that the plants have a tolerance to glyphosate similar to that of the horseweed that has been confirmed resistant is several counties in Arkansas and in several other states.

Efforts will be made to contain this population and keep it from spreading to other areas, but there are some early indications it may already be in one or two adjacent fields.


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