Weed resistance spreads fast in U.S.

Palmer amaranth and glyphosate | Researcher says zero tolerance threshold is required

The seeds from a single glyphosate resistant weed can destroy an entire crop.

University of Arkansas scientists released seeds from a glyphosate resistant Palmer amaranth weed onto test cotton fields in the winter of 2008. By the summer of 2010, it had infested 95 to 100 percent of the test fields.

“(It caused) complete crop loss since it was impossible to harvest the crop,” University of Arkansas weed scientist Jason Norsworthy said in the abstract of a paper published in the spring issue of Weed Science.

“These results indicate that resistance management options such as a zero-tolerance threshold should be used in managing or mitigating the spread of GR palmer amaranth.”

Norsworthy and his colleagues spread 20,000 seeds from a glyphosate resistant Palmer amaranth plant onto a one sq. metre patch of a cotton field. The four test fields had no history of glyphosate resistant Palmer amaranth and were small in size, ranging from 1.3 to 1.9 acres.

By 2009, the second year of the experiment, the weed had spread to the edges of all four fields and infested 20 percent of the total area.

Palmer amaranth covered all four test fields by the third year of the experiment, completely overwhelming the crop.

“In this research, it took only 20,000 seeds initially introduced into one (metre sq. area) to effectively colonize .53 (1.3 acres) to .77 hectare (1.9 acre) fields … which is far fewer than the number of seed produced by most Palmer amaranth females,” Norsworthy said in the paper.

“A single escape is way too many to allow for this species, justifying the need for a zero-tolerance approach in managing this weed.”

Nasir Shaikh, a weed specialist with Manitoba Agriculture, said the Arkansas research on cotton is applicable to grain and oilseed crops in Western Canada.

Glyphosate resistant kochia has been detected in Manitoba, Saskatchewan and Alberta. Shaikh said Canadian growers should consider a zero tolerance policy for glyphosate resistant weeds.

“I would strongly support that position,” he said.

“It looks like a very impractical concept, but that should be the goal.”

Palmer amaranth can produce more than one million seeds per plant, which is why it spreads quickly and rapidly develops resistance to herbicides. Kochia generates fewer seeds, but it is a tumbleweed and is larger than pigweed.

“Kochia can easily grow to six to eight feet under good growing conditions,” Shaikh said.

“It will have a more negative impact on the crop yields compared to the Palmer amaranth.”

Shaikh said zero tolerance for glyphosate resistant weeds includes regular field scouting, applying multiple modes of herbicides and hand weeding.

Shaikh said it might take more than three growing seasons to take over an entire field if the Arkansas experiment was duplicated on the Prairies with glyphosate resistant kochia seeds. Still, like Palmer amaranth, glyphosate resistant kochia would eventually destroy the crop.

“Definitely, it would end up in that same situation, sooner or later.”

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