Roundup-Resistant Weed New to Idaho

By John O’Connell

A University of Idaho Extension weed specialist is investigating the recent discovery in an Elmore County sugar beet field of a weed that had never previously been detected in Idaho and appears to be resistant to the glyphosate herbicide.    

Albert Adjesiwor, who works from the UI Kimberly Research and Extension Center, confirmed in late June that the weed, which covered patches in the sugar beet field, was waterhemp, a member of the pigweed family that causes significant yield reductions in crops.   

The agronomist who notified Adjesiwor of the discovery said the weeds remained healthy even after two applications of glyphosate, which is the active ingredient in Roundup herbicide.

Though waterhemp hadn’t previously been found in Idaho, it’s widespread in the eastern U.S., as well as the South and the Midwest.   

“This is very concerning because glyphosate is the main – and only – herbicide that provides broad-spectrum weed control in sugar beets,” Adjesiwor said. “All our efforts in proactive resistance management would mean nothing if we cannot prevent the introduction of herbicide-resistant weeds from other regions.”   

Idaho farmers typically plant about 170,000 acres of sugar beets each year and the crop ranks as the No. 6 agricultural commodity in the state when it comes to total farm-gate revenue. Idaho sugar beet growers brought in $396 million in farm-gate receipts last year.

Waterhemp can grow to 8 feet tall and resembles Palmer amaranth, a close relative that Adjesiwor also fears may soon arrive in Idaho and bring a suite of resistance challenges.

Waterhemp can be differentiated from Palmer amaranth by its shorter petioles, which attach leaves to the stalk. A single waterhemp plant can produce upwards of 250,000 seeds.   

Adjesiwor suspects the waterhemp was introduced in the 100-acre field by cattle that had been fed rations contaminated with waterhemp seeds.  

“From what the agronomist told me, they had some cows graze that area,” Adjesiwor said. “This is not surprising because pigweeds like waterhemp have hard seed coats and thus can pass undamaged through the cattle’s digestive tract and deposited on the soil via their manure.”