What is in this article?:
- Glyphosate-resistant pigweed in a class all alone among cotton pests
- Must stay ahead of the weed
• Growing at rates as fast as 2 inches a day, the weed has been known to reach a height of 9 feet and a weight of 40 pounds, far exceeding the dimensions of any of its competitors.
• From 2005 until now, this resistant weed has continued to spread throughout the Eastern Seaboard and down into south Alabama.
AUBURN UNIVERSITY EXTENSION weed scientist Mike Patterson explains the continuing battle against glyphosate-resistant pigweed during the recent East-Central Alabama Row Crops Workshop held in Shorter.
Must stay ahead of the weed
“However, since residual herbicides generally don't last more than three weeks even with sufficient rainfall, proactivity — staying ahead of the weed from burn down to layby — is another critical factor in controlling pigweed.”
As an example of what farmer should be doing to control resistant pigweed, Patterson cites Walt Corcoran, who farms north of Eufaula in the southeastern corner of Alabama. He follows a proactive strategy that has set the standard for growers in the rest of the state, he says.
Corcoran puts Gramoxone and Diuron down two weeks ahead of planting, killing emergent pigweed. Then, he plants Roundup Ready cotton.
At planting, he applies Reflex alone or with Prowl and hopes for rain. “Both of these herbicides have soil residual effects against pigweed, but the key is to never let the pigweeds emerge,” says Patterson.
Two to three weeks later, this is followed by an over-the-top application of either Dual or Warrant plus glyphosate (Roundup).
Three to four weeks later, Corcoran runs a hooded sprayer in the middles with Gramoxone, followed with a layby application containing Valor plus MSMA or glyphosate, which provides additional residual effect.
“Layby is critical, and Valor is one of the best possible materials for a resistant pigweed that you haven’t yet killed. But everything depends on activation by rainfall or irrigation,” he says.
Patterson says his goal is to try and scare farmers into being proactive against resistant Palmer amaranth.
“Roundup is a good herbicide, but resistance can put you out of business. Unless you use residuals, you’ll go broke, no matter if you’re using glyphosate or Ignite. Liberty will kill a Palmer at 4 inches tall, but you won’t be able to keep up with it.”
While new products are on the horizon that’ll help with resistant pigweed, some won’t be available for awhile, says Patterson.
“Two of the most promising new technologies include Monsanto's dicamba-tolerant cotton and Dow’s new line of cotton resistant to 2, 4-D applications,” he says.
While these products won’t insure that resistant pigweed is “graveyard dead,” Patterson says they will provide at least 80-percent control when used alone and even better control when used with glyphosate or Liberty (gluphosinate).
“These are not magic bullets, and neither of these varieties will be available until the next three to four years.”
A more immediate option might be Bayer’s GlyTol-LibertyLink varieties that allow the application of glyphosate and Liberty. These should be available this season, he says.
Rotation always is a good plan, says Patterson, but consistent, proactive soil-residual herbicide applications to reduce seed populations will remain the best strategy for most producers.