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Roundup-resistant weed new to Idaho

By John O’Connell

University of Idaho

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.”

Some of the waterhemp patches in the Elmore County field were already too well established and probably too large to control with other herbicides, so Adjesiwor instructed the agronomist to bring in hand crews to destroy them before they could go to seed.

Adjesiwor also plans to conduct a survey of some sugar beet fields this fall to investigate the distribution of herbicide-resistant weeds.  

“Our hope is that, if this problem is widespread, we might be able to collect seeds from other sugar beet fields for screening,” Adjesiwor said.  

He is urging farmers who have weeds suspected to be resistant to herbicides to reach out to him or submit samples for screening through the UI weed science page

Adjesiwor has transplanted waterhemp plants from the field supplied by the agronomist and will conduct a broad screening for herbicide resistance.   

He believes the incident should serve as a cautionary tale to alert people in the industry to keep an eye out for the weed and to follow practices to keep it from spreading here. He’s, nonetheless, optimistic that it hasn’t already gotten out of hand.   

“I’m pretty sure we can control this,” Adjesiwor said.    

To avoid future introductions of waterhemp and other weeds into Idaho, Adjesiwor advises farmers to inspect and clean used equipment. He also advises seed mixes used in cover crops, bird feed and restoration may be sources of pigweed seed.

Rights-of-way abutting major highways should be periodically scouted for weeds. Furthermore, farmers who can’t be certain manure planned for use as a fertilizer is free of weed seeds should test it first by planting it on a small plot before applying it to an entire field.