Scientists say to overlap herbicide applications to control Palmer amaranth in cotton fields.
Invest in strategies that prevent weeds in cotton
The handful of small weeds that Bowen Flowers saw sprinkled across his Mississippi Delta cotton fields in early spring 2010 seemed to pose little threat to his crop. After all, he reasoned, he had applied a burndown product on the fields that had eliminated at least 80% of the weeds present. The small Palmer amaranth plants, or pigweed, that remained seemed unlikely to cause problems.
Then the spring rains started, and the small, inconspicuous weeds began to grow—rapidly.
"It was pretty much a nightmare. We ended up having to chop some of those pigweeds twice," says Flowers, whose family grows roughly 10,000 acres of cotton near Clarksdale.
"We actually had to disk up 100 acres of cotton. We had to go back to how we were farming in the ’80s," he adds.
No mercy. Ken Smith, a weed scientist at the University of Arkansas, has seen similar scenarios play out repeatedly in cotton fields across the Delta. He recommends that farmers have zero tolerance for Palmer amaranth.
"There is probably no economic threshold for pigweed today," he says.
Even if you have 99% control of 50 female plants and their seed on a per acre basis, approximately 3,750 plants will survive to contribute seed for weed development the following year, he says.
Those escapes must be controlled with additional herbicide applications, chopping or a combination of both.
"Pulling pigweed up by the roots is the best way to go, but sometimes that’s not practical. Whatever you do, get them out of the ground and don’t leave any stalk because they’ll just come back," Flowers says.
Eliminating escapes reduces seed production, which will curtail future populations. This is important, as much of the Palmer amaranth in Southern fields is resistant to many herbicide active ingredients on the market.
"Our data show that pigweed seed survives in the soil for only four years on average. That’s about its only weakness," Smith says.
Overlap herbicides. Those who start the season with clean fields and make timely herbicide applications stand the best chance of preventing a pigweed outbreak. Overlapping residual herbicide applications is a fairly new recommendation from weed scientists, but it has much support.
"We have to apply each herbicide in the field before the one ahead of it gives out," Smith explains.
That process keeps a constant protective barrier in the field and prevents weeds from gaining a foothold.
"Once that weed-control barrier is in place, don’t make any tillage passes in the field which will only damage or destroy that barrier," Smith says.
- March 2012