With three-fourths of Alabama's cotton producers now using some form of reduced-tillage, the herbicide burndown application prior to planting is more important than ever, says Mike Patterson, Auburn University Extension weed scientist.
“We're no longer killing weeds with a plow — were depending on herbicides,” says Patterson.
Growers have several options available to them for a burndown treatment, he adds. “
“Clarity is the first cousin to 2, 4-D, and is used at a rate of 8 ounces with a restriction that it must be applied 21 days prior to planting with 1 inch of rain. Caparol is a good burndown material if you mix it with Gramoxone. The restriction is that you can't exceed 1.5 pints for 14 days,” says Patterson.
Valor herbicide also is good for a burndown treatment if mixed with glyphosate, he says. “You can use 1 ounce at 14 days and go to 1.5 ounces or higher is you apply it 21 days before planting. If you mix Clarity with it, you'll have to observe that 21-day restriction,” says Patterson.
While 2, 4-D isn't in the official recommendations, there are some formulations of it that will work as a burndown treatment, says Patterson. “You can go with 1-pint rates and plant after 31 days. If you have cutleaf evening primrose, you'll have a tough time killing it with just glyphosate. Valor, Clarity and 2,4-D all can be mixed with glyphosate or Ignite in a burndown treatment,” he says.
Looking at in-season cotton weed control options for 2006, Patterson says Liberty Link cotton that tolerates Ignite herbicide has been modified. The new Ignite 280 formulation is more concentrated, he says. In addition, Ignite requires more water than glyphosate, and it has a different mode of action.
A state label now allows growers in Alabama to make up to three applications of Ignite, he says, while the previous label allowed only two applications. The Ignite label requires no applications beyond eight leaves, says Patterson.
“Ignite probably is a little weaker than gylphosate on perennial weeds, but it is very effective on volunteer peanuts,” he says.
Envoke herbicide has a lot of broadleaf weeds on its label, but one of its big weaknesses is small-flower morningglory, especially in the South, says Patterson. “In the past two years, Syngenta hasn't wanted any mixing with Staple, and Staple has good activity on small-flower morningglory. But this past year, we received a label in Alabama, Georgia and the Carolinas that will allow us to mix Staple with Envoke to help us pick up that troublesome weed. We need to remember that cotton has to be at the six-leaf stage before we apply Envoke,” he says.
In another change, Staple will be marketed this year as a liquid form — Staple LX. Two ounces of the liquid will offer the same or greater control as 1 ounce of the 85-percent soluble powder, he says.
Roundup Ready Flex is the newest generation of the Roundup Ready technology, says Patterson. “With the last generation, you had to stop spraying Roundup over-the-top at the fifth-leaf stage. Roundup Ready Flex extends glyphosate tolerance to all growth stages. It can be used from emergence to 60-percent boll opening. After that, you can use it as a harvest aide. On those rates, you can use up to 170 fluid ounces per acre per year of Roundup Weather Max. That's a 5.5 pounds-per-gallon formulation, which means you can use about 10 pounds if your pocketbook can stand it,” he says.
As a harvest aide, a grower can use up to 44 ounces of glyphosate if there is grass and other vegetation that needs to be killed before picking. As a preplant treatment, growers can go up to 105 fluid ounces of the concentrated formulation, he adds.
“With the work we've done as far as applying glyphosate in the new Roundup Ready Flex cotton, we've basically used Roundup as you would an adjuvant — it's that tolerant. If we apply an insecticide, we add a little Roundup. We also add a little Roundup if we apply a growth regulator. But Monsanto doesn't want it going out with dimethoate insecticides or with fertilizers containing iron.”
Resistance continues to be a concern in cotton weed management, says Patterson. “We have seen marestail that's resistant to glyphosate. And this past season, they documented palmer amaranth resistance to glyphosate in south Georgia. Palmer amaranth is a large pigweed that can grow 8 to 9 feet tall. There are male and female plants, and each plant can produce up to half a million seed. We don't have much of it in Alabama at this time, and that's a good thing. If you buy used equipment in Georgia, make sure you steam-clean it before you bring it into Alabama.”
Other resistance issues include cocklebur resistant to MSMA in west Alabama and goose grass resistant to Prowl and Treflan in Cherokee County, Ala., and probably in other areas, says Patterson.
“If you look around the country, several weeds have been identified that are resistant to the sulfonylurea herbicides, including Staple, Envoke and Classic in soybeans. We also have resistance to Cadre. Unfortunately, there is palmer amaranth in Georgia and the Carolinas that is resistant to Staple and to Cadre.
“Fortunately, the yellow herbicides still have activity on this weed, but you need to incorporate those materials. We're heavy into conservation-tillage, and incorporation is a dirty word for many of our growers. It remains to be seen whether or not we'll have to go back to incorporation to manage resistance.”
Alabama growers haven't been hit hard by resistance yet, says Patterson, but they'll have to change how they do certain things whenever it does happen.
“If you don't have a resistant weed in your field, make sure you don't bring it in. Always start with a clean seedbed, using whatever it takes to kill those weeds. Crop rotation also helps because we rotate chemistries and use materials with different modes of action, materials like Diuron, Caparol and MSMA — contact killers. Rotation with the Liberty Link system also might be an option.”
Patterson also advises growers not to skimp on labeled rates when applying herbicides.
“Another problem we need to be aware of is tropical spiderwort. This resistant weed has been found in more than 18 counties in south Georgia. It produces a lot of seed underground. In Georgia, they're adding Dual into the mixture with the first over-the-top Roundup treatment. But that pint of Dual costs an extra $10 per acre. For Palmer pigweed, they've added Staple in with the glyphosate for control.”