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Study Underscores Need for 'Zero Tolerance' Palmer Amaranth Control

Study Underscores Need for 'Zero Tolerance' Palmer Amaranth Control

Study in journal Weed Science shows need for zero-tolerance threshold in managing Palmer amaranth

Farmers across the country are battling the expansion of herbicide-resistant weeds that have the potential to inflict complete crop failure if control measures are not introduced, a study in the journal Weed Science said this week.

The study presents research conducted over four years in Arkansas cotton fields where 20,000 seeds of glyphosate-resistant Palmer amaranth—which may represent only 2% of seed from one plant—were introduced into a 1-square-mile area.

The weeds that resulted were not managed, but allowed to "escape," the study finds, showing the need for a zero tolerance threshold in the management of glyphosate-resistant Palmer amaranth.

Comparison of growth of waterhemp and Palmer amaranth at 4 days and 16 days after emergence. Photo courtesy of University of Illinois.

Difficult to control
Seeds can be dispersed by wind and water, and moved by animals, humans, and machinery, according to the study. Weeds such as Palmer amaranth, which can produce a large amount of small seeds capable of floating in water, can spread rapidly throughout a production field.

Palmer amaranth has prolific seed production, rapid dispersal, and high competitiveness with crops, making herbicide-resistant strains difficult to control.

Related: Prepare to Control Palmer Amaranth This Season

In the current study, glyphosate herbicide was the only weed management used. In the first growing season, a separate patch of Palmer amaranth emerged 375 feet from the original location.

In the second year, resistant plants expanded to reach field boundaries and infested 20% of the field area resulting in decreased yield and significant problems with cotton harvest.

By the third growing season, glyphosate-resistant Palmer amaranth had completely colonized the fields, making the cotton crop impossible to harvest.


The expansion of resistant weeds seen in this research helps to explain the rapid takeover of many farms by glyphosate-resistant Palmer amaranth, particularly when glyphosate was the only means of weed control.

Related: Palmer Amaranth Is For Real

The study also demonstrates the need for a zero tolerance threshold of weed management—keeping all resistant-prone weeds from escaping control—to prevent loss of an herbicide or technology.

"Weed control based on an economic threshold (dollars spent vs. dollars returned) does not adequately consider the soil seedbank and the risk for herbicide resistance," the journal reported.

Growth patterns
A recent study comparing the growth patterns of waterhemp and Palmer amaranth also demonstrated that control efforts must be timely and complete.

Aaron Hager, a University of Illinois weed scientist, said the growth patterns show foliar-applied herbicides must be applied before Palmer amaranth plants are taller than 4 inches.

"Most weed management practitioners are very familiar with the growth rates of waterhemp and many other weed species, but perhaps they are less familiar with the growth rate of Palmer amaranth," Hager explained.

Related: How To Identify Palmer Amaranth

During the project, both weed species emerged on the same day, and the plants were photographed every other day for a month. Photographs show that Palmer amaranth plants reached a 4 inch height less than 10 days after emergence.

"The growth rate comparison illustrated in the photographs was conducted under greenhouse conditions, but experience suggests that a similar growth rate of Palmer amaranth should be expected under field conditions," Hager explained. "Timely applications of foliar-applied herbicides require vigilant scouting throughout a large portion of the growing season."

View the complete Palmer amaranth control study in the journal Weed Science, and the University of Illinois' recommendations for Palmer amaranth scouting and control.

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