Ohio State University weed scientists have discovered another herbicide-resistant weed, making it the eighth confirmed weed species in the state that has gotten the upper hand on chemicals.

Smooth pigweed, commonly found throughout Ohio, shows resistance to aceto-lactase synthase-inhibitor herbicides, commonly referred to as ALS-inhibitors. This herbicide group kills weeds by preventing the plants from producing essential amino acids that are needed for proper growth and development.

Jeff Stachler, an Ohio State weed scientist, says that smooth pigweed shows resistance to some commonly applied ALS-inhibitors, such as Harmony GT and Python. "Harmony GT and Python applied post-emergence at two times the labeled rate provided only 20% and 4% control of this smooth pigweed population, respectively," says Stachler.

Smooth pigweed joins the ranks of other Ohio ALS-resistant weeds such as shattercane, giant ragweed, common ragweed, marestail, Powell amaranth, common cocklebur and waterhemp.

Despite the growing list, Stachler reports that smooth pigweed might not create headaches for growers if they take the necessary management steps. "Smooth pigweed is really not a big problem for most farmers compared to common ragweed, giant ragweed, or common lambsquarters," he says. "It's those growers who are using ALS-herbicides every year or every other year that will most likely be affected."

While most ALS-resistant weeds exhibit complete cross-resistance (meaning they have developed resistance to products within the same herbicide mode of action), smooth pigweed sets itself apart in that it only exhibits partial cross-resistance. This population is still sensitive to ALS-inhibitors such as Pursuit, Raptor, Lightning and Scepter.

"This is positive news for growers," Stachler says. "At least there may be some options with ALS-inhibitors when controlling smooth pigweed. However, continued use of these ALS-inhibitors will cause problems down the road, so be sure to use non-ALS herbicides as often as possible to effectively control smooth pigweed and other weeds."