ALS– RESISTANT COMMON CHICKWEED IN WHEAT

James R. Martin—Extension Professor of Wheat Science

In 2012, consultants with Wheat Tech reported finding common chickweed in several fields in Logan, Simpson, and Warren Counties that was not controlled with ALS inhibitors such as Harmony Extra or Finesse. ALS stands for Acetolactate Synthase which is an enzyme sys-tem that helps make certain amino acids in plants. The following photo is an example where an ALS-inhibitor her-bicide failed to control common chickweed in Kentucky this past season.

In 2008, ALS-resistant common chickweed was discovered in wheat in Virginia and is now believed to be in Delaware, Maryland, North Carolina, and Pennsylvania.

The potential impact of this resistance could be significant to Kentucky wheat growers. ALS-inhibitor herbicides play an important role in managing chickweed, henbit, and wild garlic in wheat in Kentucky. Harmony and Harmony Extra (and several generics of these) are applied on about 75% of the wheat acres in Kentucky.

Since common chickweed is fairly competitive, it is desirable to control plants when they are small. The majority of common chickweed emerges in the fall; therefore, fall applications tend to be preferred over spring applications for controlling this weed in wheat. The activity of ALS inhibitors tends to be slow, particularly during periods of cool temperatures. Consequently, it may be several weeks after fall applications before you know if the treatments were a success or a failure.

ALS-inhibitor herbicides tend to be prone to developing resistant weeds. The fact Kentucky growers have avoided ALS-resistant common chickweed in wheat for more than 25 years of widespread use of Harmony and similar herbicides, is no accident. Our rotation system of three crops over a two year period (i.e. Wheat / Double-Crop Soybean / Corn) has limited the development of ALS-resistant common chickweed. Applying burn-down applications of glyphosate, paraquat, or atrazine prior to corn planting early in the spring can limit production of common chickweed seed, if applications are applied before plants produce viable seed. Keep in mind, ALS-inhibitor products used in burndown appli-cations in corn such as Basis, Basis Blend, Resolve Q, Leadoff, or Crusher may not control populations of common chickweed that are resistant to this chemistry.

The Kentucky Small Grain Growers Association recently funded a the project to confirm this resistance and to evaluate alternative herbicide programs.