Cotton weed control systems compared

As producers in central Alabama have continued or begun to grow conventional cotton, specialists with the state’s Extension system have conducted trials to compare these systems with the Roundup Ready system.

“We’ll fight certain weeds whether we’re using the Roundup Ready or conventional cotton,” says Mike Patterson, Auburn University Extension weed specialist. “These include sicklepod, some morningglory species — about 10 of them — and pigweed.”

Most growers are aware of the glyphosate resistance problem with Palmer amaranth pigweed, says Patterson. “This is a real problem when 90 percent of your weed control is based on Roundup Ready. Most studies show that if you depend just on straight Roundup Ready technology with no other herbicides — regardless of the crop you’re growing — within five or six years you’ll develop a population resistant to glyphosate,” he says.

Growers also might have problems with annual grasses, crabgrass and goosegrass regardless of the cotton production system they’re using, says Patterson.

In Alabama, 75 percent of the cotton acres are planted in reduced-tillage systems, with more than 90 percent of the cotton being planted in Roundup Ready varieties, he says. “When we look at herbicides, machinery and time costs, the Roundup Ready, reduced-tillage system is probably the least expensive system and most growers are aware of this,” says Patterson.

In a minimum-till system, a grower might run a disk over a field, he adds. “The advantage of that over a strict no-till system is that you can incorporate the yellow herbicides whenever you run a disk. Products like Prowl and Treflan are very effective on pigweed and grasses, but they work much better when you incorporate them — you have to incorporate Treflan,” he says.

When the Roundup Ready cotton system first became commercially available, growers had to stop over-the-top herbicide applications at the four-leaf stage or risk yield loss, says Patterson. “Now we have Roundup Ready Flex. LibertyLink cotton also is good technology using Ignite herbicide. It’s similar to Roundup technology in that it has no residual and LibertyLink materials have excellent tolerance to Ignite,” he says.

Patterson has looked at potential problems and potential benefits of both conventional and Roundup Ready cotton production systems.

“There’s no residual with Roundup Ready herbicide — what you see is what you get. In a rainy year, you’ll get a lot of weed germination and have to make repeated applications. But the Roundup system also allows for delayed spraying. There’s also the risk of developing weed resistance with this system, and the primary weed would be Palmer amaranth pigweed. There are also seeding rate issues. Growers pay a lot for this technology and some will skimp on the seeding rate,” he says.

Disadvantages of the conventional cotton production system include poor activation of pre-emergence herbicides in dryland fields, says Patterson. “If we go without herbicide technology in cotton, we’ll have to use the old residual materials. If you’re in a dryland situation, and you don’t get a rain to activate those, then they basically won’t work. Long-time cotton growers remember those days when cotton and weeds came up at the same time.

“Most older herbicides also need more water than Roundup — 15 gallons per acre opposed to 8 to 10 gallons with glyphosate. And we do risk herbicide injury when we use Cotoran and some of these other residuals on sandy soils — you can get significant crop injury. You need to be very timely with post treatments in a conventional system, especially on those weeds that grow fast. Also, there’s a limited choice of varieties when you’re growing conventional cotton,” he says.

The benefits of a Roundup Ready system include the fact that Roundup is a broad spectrum herbicide, says Patterson.

“You can treat after the weeds come up and you can save time and labor, in my opinion. You can spray Roundup with a highboy, cover 18 rows at 10 miles per hour and 8 gallons per acre, and it’ll work. If it’s a rainy year and you put Cotoran or Cotoran/Caparol down behind the planter, it’ll hold until you get back in there again. Monsanto will give incentives to help you use some of these old residuals in a Roundup Ready program because they realize the danger of resistant weeds with this technology.”

In comparing the economics of a Roundup Ready versus conventional system, Patterson says a lot of assumptions were made. “We’re comparing Flex — the new Roundup Ready generation — with a conventional herbicide program, and we’re doing a no-till or strip-till system. We’re not doing the big tillage with a moldboard plow.”

The burndown materials are the same for both systems, he says, so the costs were the same. “If we throw Valor into the system as a residual, that’ll add about $5 to $10 more. We came up with ranges for both systems, but these are herbicide costs only, with no application costs added. If I go with the low range of the Roundup Ready system, we’re talking about three to four shots of Roundup at $10 each plus Temik for a total cost of about $46. If you throw residuals into the system, you can drive that up to $98 or so for herbicide costs. The technology fee for Flex, just for the herbicide gene, is $37.”

Monsanto is offering incentives for using certain residuals in the Flex system, says Patterson, but if a grower doesn’t use any of these and uses about four shots of Roundup, the cost with the technology fee would be about $83 per acre.

In a conventional system, the burndown cost will be the same as in the Flex program, unless you use Valor, he says. “In my opinion, you have to use as many residual herbicides as possible in a conventional cotton field to fight these weeds. That would include using Prowl, probably adding Cotoran, Caparol or Staple behind the planter, and praying for a rain within seven to 10 days. If you have sicklepod in the field and young morningglories, the only early option you really have is Staple and MSMA. If sicklepod is over an inch tall, Staple by itself won’t do it.”

Once a grower gets cotton up to the seven leaf stage, he can put on Envoke and surfactant for morningglories, very small pigweed and sicklepod, use broadleaf herbicides, or post-direct on seven to eight-leaf cotton with prometyrn or Diuron and MSMA. “You’ve got to use a layby, and you’ve got to use a residual like Valor, Diuron or prometyrn, with or without MSMA. If you don’t have any emerged weeds, or if they’re very small weeds, you can leave out the MSMA.

“If you have grassy fields, you may have to use a post-grass herbicide with a surfactant, and that’ll run about $12 to $15 per acre. If you leave out some of this in a conventional system, you might get by at about $62, but you could drive it up to $91 per acre if you have to use the entire arsenal. This system will require more time, more water and more labor.”

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