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Farmers struggle to control glyphosate-resistant horseweed

For the last couple of years we have been doing plot work in Crittenden County, Ark., looking at control options for glyphosate-resistant horseweed in soybeans. We have discovered there are no excellent options for postemergence horseweed control in soybeans in the absence of glyphosate. In these situations, success or failure of a weed control program begins at burn down.

Glyphosate herbicide still provides a lot of weed control at burn down. It is cheap, has a broad spectrum and has no plant-back restrictions to speak of. Therefore, it is still economical to use glyphosate even in the presence of a resistant weed or two.

In the case of glyphosate-resistant horseweed, Clarity or dicamba applied at 8 ounces per acre is our best recommendation for horseweed control. Clarity should be applied at least 14 days prior to planting soybeans and 21 days prior to planting cotton. These plant back intervals begin after a 1-inch rain.

If there is not enough time for the plant-back interval, you have only a few options: making time for the plant-back interval, working the field, or using a full rate of Ignite and good water volume for coverage and hoping for the best.

In both cotton and soybeans, there are few options for postemergence glyphosate resistant horseweed control.

For in-season horseweed control in soybeans, the best product that we have looked at so far is FirstRate herbicide at 0.3 ounce per acre tank-mixed with a good crop oil concentrate at about 1.2 percent volume per volume.

The maximum use rate of FirstRate per season is 0.6 ounce per acre and many times you may want to use it as a tank-mix partner with glyphosate twice, under severe infestations.

The smaller the horseweed the better for FirstRate. Even under ideal conditions, however, 70 to 80 percent control may be the best you can hope for. This will at least make a crop and you will not lose the field to horseweed.

A good program approach for soybeans prior to using FirstRate in-season is to use either Valor or Synchrony XP pre-emergence if horseweeds have not emerged at planting. Both products have provided around 30 days of residual on horseweed control in our plots.

Synchrony will stunt the horseweed and provide some postemergent suppression. Valor has only residual activity.

These products can also be used in a burn-down application with Clarity. This will provide both burn-down and residual control.

Several farmers have asked me about the new product Prefix (Dual plus Reflex) from Syngenta, we have looked at this product on pigweeds and other summer annual broadleaf weeds and it looks excellent. I have not, however, had much experience with it on horseweed. It is included in my spring and summer trials this year.

In cotton, it is even more critical to get horseweeds out prior to planting. There are basically no options for glyphosate-resistant horseweed control in-season in cotton. As mentioned above, glyphosate plus Clarity is our first line of defense.

As in soybeans, Valor can be used if the application goes out at least 30 days prior to planting. The same salvage option with Ignite can be tried if the plant-back interval is too short for Clarity, but this will likely not provide 100 percent horseweed control.

Cotoran/Meturon and Direx provide residual control of horseweed that is not emerged and are options pre-emergence.

Once cotton has reached the five-leaf stage you can use a full rate of Envoke to try and “freeze” the horseweed in its place. You will not kill it, but you may injure it to the point that it stops growing and becomes non-competitive. Envoke also provides excellent residual control of horseweed.

Who would have ever thought that a ditch weed like horseweed would become the weed that we have to key many of our weed control decisions around? This weed has now adapted to the point where I even get an occasional call on horseweed in rice (levees and furrow irrigated).

We are now examining horseweed that is resistant to other groups of chemistry in addition to glyphosate, so the weed is not going away anytime soon.

By the way, Regiment works pretty well on it in rice.

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