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Herbicide resistance a major issue in corn

In corn, as in all crops, herbicide resistance has become a major issue, and it’s one that must be weighed carefully as growers enter a new production year.

“We’ve been spending a lot of time in the last several years talking about glyphosate resistance, and that certainly is one of the most important issues in Georgia with Palmer amaranth pigweed,” says Eric Prostko, University of Georgia Extension weed scientist. “We also have some ALS resistance issues, and for you as a corn grower, that would include herbicides like Accent, Steadfast and others.”

Unfortunately, in 2007, a possible resistance to atrazine was discovered in Macon County, Ga., says Prostko. “Some farmers and dealers knew about it even earlier. But it’s not as big a concern because of the way they’re managing their crops in that area,” he says.

Currently, he says, there are 13 weed species in the world that are resistant to glyphosate. “The big one here is Palmer amaranth pigweed. The original resistant populations were found in Macon County and subsequent testing has been done in surrounding areas where it also has been found,” he says.

Resistance to the ALS herbicides has been confirmed in at least 22 Georgia counties, says Prostko. “Right now, the ALS-resistant pigweed is more of a widespread issue than the glyphosate-resistance issue, but we’re probably just beginning to scratch the surface with the glyphosate problem,” he says.

Screening currently is being conducted, he says, to confirm that there is resistance to triazine herbicides.

“The location where we’ve seen the problem is on a dairy farm where they typically have grown the same crop rotation for the past 15 to 20 years. They’re growing ryegrass, corn and soybeans, and they have been using multiple applications of atrazine in a single year for 15 to 20 years. This has led to a possible resistance problem,” says Prostko.

As corn growers head into a new production year, they’re faced with the choice of being a conventional weed manager, a Roundup Ready system manager or a Liberty Link system manager, he says.

“Before we had herbicide-resistant technology, we were able to control weeds. The technologies have brought some ease with the systems, but we still have some good materials out there for conventional weed control,” says Prostko.

Atrazine, he says, is and always will be a major player in corn weed control, along with Accent and 2, 4-D. Some of the older chemistries, he adds, are still great products. “We can do a lot with a conventional system, although we still have a few gaps that can cause problems,” he says.

Roundup Ready systems are heavily used in both corn and soybean production, says Prostko. “Most soybean acres are now planted to herbicide-resistant technology. In the past couple of years, we’ve seen an increase in the use of Roundup Ready technologies in corn production, currently comprising more than 50 percent of the U.S. crop. There are many advantages to the Roundup Ready system but there are disadvantages as well,” he says.

The price of glyphosate is increasing, he says, going from $11 per gallon for a generic product to $22 per gallon.

“I would still prefer to see atrazine on every acre regardless of the system we’re using. To get the most out of atrazine, we need to use it pre and post. That way, we can put out the maximum allowable rate of atrazine on corn. That would help us with our pigweed management, as well as with morningglories and other things.

“I would love to see a pre-application of at least a quart go out on every acre. But many growers aren’t willing to do that. You may want to go all postemergence, and that’s fine. But if you go only post, the maximum amount of atrazine you can legally put out in a year is 2 quarts.”

If you go only with a postemergence application of atrazine, it will give you some resistance management benefits as well as residual control later in the season, says Prostko.

There are several pre-packaged products, including Sequence, Expert and Halex GT, that contain glyphosate and another ingredient, says Prostko. “Whether or not you need the other ingredient is another issue. If you make an application of one of these, you’re putting several modes of action over the crop and not relying on just one.”

Prostko reminds growers not to forget seed costs when calculating budgets for the coming year. “If you’re planting a Roundup Ready system this year, you could see a $4 to $6 per acre increase in the price of seed. One of the benefits of Roundup is that you’ve been able to wait and still control weeds. I hate to hear that. If you wait, bad things happen. The longer you wait for that first application, the more yield you’ll lose. We have to make herbicide applications on a timely basis — we can’t let the weeds get extremely large.”

He recommends that growers not use Roundup alone in their weed control program. “You need some kind of residual material in your program to help with resistance management. If you’re a strip-till farmer using glyphosate for a burndown, there is an opportunity to switch to something else if you are concerned about resistance management, such as using atrazine or some other mode of action.”

In the areas where resistance to glyphosate has been confirmed, some growers are cultivating to control the troublesome weeds, says Prostko.

“If you do see some weed escapes, it’s not a bad idea to go out and pull up those weeds if it isn’t too large a population. One pigweed can produce 500,000 to 1 million seed.”

The Liberty Link system is another option for corn producers, he says. “If you’re a cotton grower, you’re probably familiar with Ignite, and this is basically the same thing, but used on a different crop. There’s no technology fee associated with the seed, and you can go over-the-top of corn up to 24 inches tall. You can tank-mix with atrazine, and there’s a pre-mix called Liberty ATZ that’s pretty good on weeds like morningglory, Texas panicum and coffee weed.

“It can be effective on pigweed, but the weeds need to be 2 inches tall or smaller. There have been issues with the performance of Liberty Link hybrids, but they have improved over the years, and now they are comparable to some of our better hybrids.”


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