If you’re old enough to remember Mr. Peabody and his Wayback Machine from the Rocky and Bullwinkle Show, then you probably remember the days when glyphosate-resistant marestail and Palmer amaranth weren’t even a blip on the radar screen.
So let’s set the Wayback Machine for 1995. “Think back to 1995 or so,” says Mississippi Extension cotton specialist Dr. Darrin Dodds. “What were you doing for weed control? You went out in the field, identified the weed species and how big they were. Then you selected chemistries knowing that that they were going to take out certain weeds and leave certain weeds. That’s why we started coming in with more than one chemistry.”
Fast forward to today. “What we’d like to do is put out 22 ounces of WeatherMAX then go to the house,” he continues. “Most of our weed-control programs are going to be based on glyphosate for the foreseeable future, and it still controls the vast majority of the weeds we have. But in light of glyphosate resistance, there are some things we need to be looking at. In Mississippi, if I talk to a grower about marestail, I consider it resistant because it’s not worth taking a chance.”
Dodds urges growers to reconsider the use of some of the older but still very effective herbicides to extend the life of glyphosate. “We’ve turned around 180 degrees,” says Dodds. “It’s time for us to start using residuals and we recommend it. I tell my guys that if we have Palmer amaranth issues, or even think we do, I would absolutely get a residual in the mix.”
Envoke and Staple continue to be good postemergence herbicides, but if ALS resistance is present, you lose them. Even worse, if you have both ALS- and glyphosate-resistant weeds, Envoke, Staple and glyphosate are out of the equation. “That’s losing three options,” Dodds says. “If you can’t control them when they are growing, you’re going to have to start looking at things like Cotoran pre, or Dual over the top.
Valor at 2 ounces per acre has worked very well at late burndown or early preemegence, but it takes at least an inch of rain to activate. Less than that can cause damage. “Do not take that lightly,” says Dodds.
Other recommendations of residual herbicides that were once standards include the trifluralines, metolachlors, Prowl, Cotoran and Caparol. Post directed materials include Cotoran, Caparol, Cobra, Reflex and Aim.
The Palmer Problem
Palmer amaranth continues be the most economically devastating of the glyphosate-resistant weed species for cotton in confirmed areas. The first confirmation came in North Carolina in 2004, then it moved into Georgia in 2005. Dr. Stanley Culpepper, a Georgia Extension weed scientist says when it appeared in Georgia, it moved with a vengeance from 10 counties and 3,000 to 5,000 acres in 2005, to 40 counties and a minimum of 300,000 acres by 2007.
“I haven’t screened everything yet, but I would not be surprised if we don’t have 20 more counties with resistance,” says Culpepper. “This pest is clearly having the greatest impact on our ability to produce all of our agronomic crops in Georgia, and most importantly cotton.”
The Mid-South got its first dose in 2007 in Arkansas, and it has been seen as far west as Texas. “The point is, if you don’t have it, you will,” adds Culpepper.
Palmer amaranth has male and female plants and some studies suggest that one male plant can produce up to 1 million seed. A conservative estimate is 250,000. Seeds are as small as grains of sand and can move from field to field on a slight breeze. So it’s easy to figure out the rest of the story if seeds from a field of glyphosate-resistant male plants move into field with non-resistant females. The sage advice for a grower who sees even a small patch of Palmer amaranth is to assume it’s glyphosate resistant and do one thing: Go pull it up. That’s another flash back to the days when growers did the same with Johnsongrass.
Before Roundup Ready varieties came along, there were many pre and post residual herbicides with good to excellent weed control. Glyphosate alone gave us something we’d never had before with tank mixes – convenience and control. But resistance has made growers wisely consider going back to some of those herbicides with residual control
“Residual, residual, residual and pray for a timely rain,” says Culpepper.
Maybe it’s not time just yet to pull the hooded sprayers and layby rigs, but think about mowing around them and replace the hoses and nozzles.