Researchers in Arkansas have spent several seasons monitoring the spread of weeds—specifically one type of weed, glyphosate-resistant Palmer amaranth, which plagues farmers by rapidly colonizing fields. The goal was to see how quickly and how far the weed spreads, as well as to study how best to control herbicide-resistant weeds.
The researchers chose four cotton fields that had not previously been infested with Palmer amaranth. In 2008, they released 20,000 seeds, which is about the number produced in a season by a single female Palmer amaranth plant. The small seeds are easily spread by runoff, as well as spread by wind and tracked by animals and farm equipment.
Glyphosate herbicide was the only form of weed management used during the study. In the first season after the seeds were released, Palmer amaranth appeared 375 feet from the release location. In the second, it had taken over about 20% of the fields, reducing cotton yields. By the third season, it had completely colonized the fields and no cotton could be harvested. The researchers' report recommends a zero-tolerance threshold for mitigating the spread of the weed and cautions about the danger of herbicide resistance. “For a weed species that exhibits prolific seed production, high competitiveness with the crop, and rapid dispersal, all seed production must be prevented, especially in a situation where resistance has evolved. Thus, a zero-tolerance threshold should serve as a threshold appropriate for such weed species,” they write.
You can find the complete article, published in the current issue of the journal Weed Science, here.