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New weed problems emerge in Arkansas

Most producers in the Delta regions of Arkansas, Missouri, Mississippi and west Tennessee can now assume that if they have horseweed in their fields it is at least to some degree resistant to glyphosate herbicides. This weed has gone from a west Tennessee problem, to a curiosity in some northeast Arkansas fields, to a full blown epidemic in just a matter of five years.

Controlling horseweed in soybeans, cotton and even rice has dominated my phone calls through the first part of the growing season.

In cotton, horseweed must be controlled prior to planting.

It can be suppressed with Envoke if it is small and controlled pre with a Valor applied at lay-by if it has not yet emerged at that time. But there are not any real good over-the-top options.

In soybeans, this weed is tough to kill with conventional herbicides. Like cotton, it is best to clean it up with a good burn-down program.

However, in-season, in soybeans, FirstRate at 0.3 ounce per acre applied early post has done fairly well. Later on big horseweed, FirstRate will control horseweed only around 60 percent.

When a good burn-down program has been utilized and a residual such as Valor or Synchrony XP has been used pre-emergence, following with an in-season application of FirstRate, Classic or Flexstar tank-mixed with Roundup has looked pretty good as a complete program for horseweed. As a standalone treatment, however, FirstRate was the best at around 50 to 60 percent suppression in our studies last year.

In addition to horseweed, yellow nutsedge prior to planting soybeans has been a major issue this year in Arkansas. Nutsedge is being reported in both reduced tillage and conventionally tilled fields. It seems to be just as bad following rice as it is soybeans, corn or cotton.

In our work, glyphosate alone controls yellow nutsedge around 75 percent when applied preplant alone at around 1 quart per acre. Pursuit (at 1.44 ounces per acre) is a good tank-mix partner with glyphosate for nutsedge, but the label requires a 40-month rotation to rice and long rotations to most crops except for Clearfield rice or corn, corn and soybeans.

One of my better programs for nutsedge is to use Dual or Dual plus Sencor applied ppi. Pre-emergence treatments are weaker on nutsedge. Then I go back post with glyphosate plus 0.33 ounce of Classic.

Permit herbicide can now be used postflood in rice.

Do not pass up an opportunity to control nutsedge in rice prior to rotating to soybeans.

We are currently investigating the possibility of a preplant label for Permit in soybeans with Gowan Company, but we have observed some injury in the past and we need more data before we can determine a fool-proof plant-back interval.

Also, this year was the first year that I have received calls on pokesalad, eastern black nightshade and horsenettle surviving burn-down. These are traditionally row crop and pasture weeds. One consultant I spoke with about these weeds surviving burn-down programs said that we were farming pastures, so it is no surprise that we are dealing with pasture weeds.

In the case of eastern black nightshade (in the same family as groundcherry), it was coming back from root-stock in many no-till fields. I got reports on this weed from as far north as Clay County and as far south as Arkansas County, near Stuttgart. This weed is normally an annual in row crops, but behaved more like a perennial this year and came back from root stock.

Glyphosate alone was not effective as a burn-down. I think a tank-mixture with 2,4-D will be required, probably at about a quart per acre rate.

Most of you are aware of the discovery of a glyphosate-tolerant pigweed in Mississippi County, Ark., this past fall. We are still investigating that weed, but it appears that it has similar tolerance to the population that Larry Steckle and others at the University of Tennessee are looking at. It is not nearly as tolerant as the one that others are investigating in Georgia.

This new weed biotype represents a serious threat to agriculture. We have a fact sheet dealing with the control of this weed in cotton and soybean at

In addition, to these weeds, some others seem to keep popping up. Both common and giant ragweed continue to generate a few calls. Both weeds seem to have a certain intrinsic tolerance to glyphosate. We have at least one population of common ragweed in Jackson County that is more tolerant to glyphosate than some other populations tested.

My giant ragweed locations did not have enough weeds for testing this year. We have sampled seed from around the state and will be continuing this next fall with a greenhouse screening. In the meantime, both FirstRate and Flexstar have performed well as tank-mix partners for ragweed control in-season in soybeans.

Most of the emerging weed problems being brought to my attention are ones not being controlled well enough with glyphosate alone. I think my tenure here at the University of Arkansas will likely be spent looking for tank mixtures with glyphosate for controlling weed gaps or dealing with glyphosate-tolerant/resistant weeds in Roundup Ready crops.

Bob Scott is an associate professor and Extension weed specialist with the University of Arkansas. e-mail: [email protected]

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