Resistant waterhemp sprayed 10 consecutive years with 2,4-D

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Waterhemp, which is in the pigweed family, is a major weed in the United States so it’s no surprise the discovery of a 2,4-D-resistant population in Nebraska is making headlines.

The resistant waterhemp were found in a field of native grass seed where 2,4-D had been applied for more than 10 consecutive years, researchers wrote in the July-September journal of Weed Science.

“Similarly, use of 2,4-D for 10 years in New Zealand pastures resulted in selection of a 2,4-D-resistant musk thistle population,” the article states.

The article says the resistant waterhemp “demonstrated at least tenfold resistance to 2,4-D relative to a susceptible population in greenhouse bioassays.”

The highest doses of 2,4-D that were used in an on-site field study were insufficient to control 50 per cent of the waterhemp population.

Researchers gathered waterhemp seeds from this field and performed greenhouse testing against a susceptible waterhemp variety. Twenty-eight days after treatment with the herbicide, visual observation and dry weight values showed a tenfold resistance in the affected sample.

The researchers also found the 2,4-D-resistant waterhemp was threefold less sensitive to dicamba, another herbicide in the same Group 4 as 2,4-D.

The farmers first reported problems controlling waterhemp in 2009.

Although 2,4-D has been used widely worldwide, only 17 weeds have evolved resistance to this herbicide, the article says. They are: wild carrot, Canada thistle, musk thistle, Italian thistle, tall buttercup, Indian hedge mustard, wild mustard, wild radish, field bindweed, kochia, corn poppy, scentless chamomile, prickly lettuce, dayflower, Sawah flowering rush, marshweed and globe fringebrush.

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