Resistance Fighters
September 14, 2009 It’s not as if cotton growers in the Mid-South hadn’t been warned. Since the first cases of glyphosate resistance were documented in Georgia in 2005, Extension specialists and weed scientists across the Cotton Belt have been sounding the alarm. Unless every precaution available was taken, resistant Palmer amaranth and marestail would eventually find their way across the Belt.
Sure enough, the epidemic has grown. Resistant pigweed is now the number one weed pest in the state of Arkansas. Growers in western Tennessee and throughout the Mississippi Delta, as far south as Highway 82, are beginning to report outbreaks of glyphosate resistance with greater frequency. Many Extension weed scientists agree that 2008 seemed to be a banner year for resistance.
“The big one that’s getting all the press of course, and I get lots of calls on – we’re all getting calls on – is glyphosate resistant Palmer pigweed. Last year was the first year it really kind of showed up in a lot places,” says Dr. Larry Steckel, Row Crop Weed Specialist with the University of Tennessee’s Department of Plant Sciences.
“In general in Tennessee, we’re finding it up and down the river counties, from Shelby County up to Lake County, that’s where you’re going to find the highest prevalence of glyphosate resistant Palmer pigweed. As you go east in the state it becomes less, but you do find it. It’s likewise over in Arkansas. Up and down the river.”
Speaking at a Union City, TN, Deltapine Field Day, Steckel and Monsanto weed specialist Dr. Bob Montgomery told farmers what many of them were all-too familiar with. As resistance adapts to each new area in which it pops up, different chemistries are becoming powerless against it.
“Just to give you some idea, on the 20th of May this spring, we put 44 ounces of Roundup PowerMax on some pigweed that was 3 inches tall. By the 26th of May, six days later, it had grown almost 5 inches,” Steckel says of some particularly resistant Palmer amaranth he found in a Tennessee soybean field. “I put another 44 ounces on the 26th of May. By the 3rd of June it had grown 7 inches. It grew 12 inches in 13 days with 88 ounces of Roundup PowerMax.
“A lot of it that we’re really having trouble with, it’s not even slowing down with Roundup. And by the time you figure out it’s resistant it’s too big to do anything else with in soybeans,” says Steckel
He recommends a few options if Roundup isn’t able to do the heavy lifting with resistant pigweed in. He calls Blazer, Cobra and Reflex or Flexstar his “picks of the litter” as far as soybean herbicides are concerned.
“And we’re going to need to go out there with the higher rates. The thing is, folks, we’ve got to get on this pigweed up to about three to four inches tall.”
Ugly Strain
A particular strain of resistance that has reared its ugly head in Tennessee and the surrounding states is G/9, which can be found in soybeans and corn, as well as cotton. And while it’s not much comfort to know that other crops are struggling with this formidable foe, that strain’s versatility may well be its downfall.
Montgomery cites a particular field that was having resistance troubles in the Southeast which saw a rotation from soybeans into tobacco for the first time. In the parts of his field where tobacco was planted, the grower was surprised to find 100% resistant palmer amaranth control.
“So don’t think that there aren’t other tools out there,” Montgomery says. In addition to replenishing the soil with nutrients, crop rotation also opens up various combinations of chemistries with which to fight glyphosate resistant pests.
One such tool will be a side-effect of the late start many growers got off to in this growing season. In a crop mix of corn and cotton, where mixing treatments has held the pigweed population in check, a late harvest will be beneficial to growers. Even though growers may lose sleep worrying about an early frost, Montgomery thinks it will ultimately help in the resistance battle.
“The good news is you won’t have all that time for the pigweed to come up and germinate. You’ll be out there harvesting your corn late and a frost is going to come pretty quickly after that and then we won’t have those weed issues,” says Montgomery.
In order to mix chemistries with a corn/cotton rotation, Montgomery offers a few suggestions for corn growers.
“In corn, the products that I would recommend to try to eradicate Palmer amaranth would be three quarts of Degree Xtra. We also have Lasso, but Degree is a step-change above Lasso.
“If you did that you could still add a quart of atrazine pre to corn and that would give you a total of two pounds of atrazine and a decent rate of Degree Xtra and you would still have about a half a pound of atrazine that you could put post of the corn,” Montgomery says.
Whether it be early beans or late corn, Montgomery’s message is clear. The longer any particular patch of ground is under a management program, the less time glyphosate resistant pests have to germinate. An active crop rotation can keep that patch of ground under diverse chemistries that overwhelm resistant weeds.
“I think that in some of these fields we’re going to have to consider not only what (herbicides) we’re going to use, but what cropping system we’re going to use as well,” Montgomery says.
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Dr. Larry Steckel
Dr. Bob Montgomery
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