Tennessee has now joined Virginia and Kentucky with confirmed ALS–resistant common chickweed. We now know of a few wheat fields in Middle Tennessee where chickweed cannot be controlled with herbicides like Harmony, Osprey, Powerflex, etc.
It is disappointing to lose the effectiveness of Harmony on chickweed as we have yet another example of relying too long on just one herbicide mode of action for weed control.
Bob Scott, my Arkansas weed science colleague, pointed out that we have not identified any new glyphosate-resistant weeds in recent years because Palmer amaranth has forced everyone to use a more diverse management strategy to manage weeds.
I thought this was a very accurate assessment for cotton, soybeans and corn in the Mid-South. However, in wheat, we clearly need to up our game in weed management.
Weeds in wheat like ryegrass developed resistance to Hoelon a decade ago and, more recently, ALS-inhibitors like Poweflex and Osprey. Now we are in the beginning stages of ALS-resistant chickweed becoming an issue in wheat.
On the bright side, the ALS-resistant chickweed is by no means widespread across the state. It appears to be in a small pocket in Middle Tennessee and is suspected in a few fields in West Tennessee in counties that border Kentucky.
Moreover, though it is in some other Mid-South states, ALS-resistant ryegrass is not widespread in Tennessee, either. We are in the early stages of losing the ALS-inhibiting herbicides in Tennessee on some key wheat weeds. We still have a chance to keep ALS-inhibiting herbicides as effective tools in wheat weed control if we add more diversity to our weed management programs now.
Research conducted on one of the ALS-resistant chickweed-infested wheat fields showed that fall applied treatments to wheat in the 3-leaf stage of 2 ounces/acre of Metribuzin or 0.3 pint/acre Starane provided good chickweed control. Starane is typically thought of as a pasture herbicide but is labeled and very safe on wheat. However, it has a four month plant back to soybeans which eliminates it as a viable option for typical spring applications.
Another good option would be Axiom which is a premix of metribuzin and flufenacet. Axiom is a great resistance management tool not just on chickweed but for ryegrass and poa as well. Other options like 2, 4-D and dicamba only provided about 60 percent control. Cultural control options like rotating fields into corn that have been in wheat and double crop soybeans for many years is another good option to help manage herbicide-resistant weeds developing in wheat.
Herbicide resistance management is not an easy issue and it takes more than occasionally adding another herbicide to the tank. It takes utilizing a couple effective herbicides applied in the fall as well as another applied in the spring along with some common sense cultural practices.
It also takes everyone in the industry working together on this problem. In that vein I must credit DuPont as they have been out front working on the ALS-resistant chickweed issue from the start.