A couple of new herbicide label challenges and the ominous challenge of weed resistance face Georgia peanut producers as they enter the 2006 growing season, says Eric Prostko, University of Georgia Extension weed specialist.

“There are a couple of herbicide label changes for 2006,” said Prostko, speaking at the recent Georgia Peanut Farm Show held in Albany. “There will be a new formulation of Gramoxone made and sold by Syngenta called Gramoxone Inteon. In the past few years, you've been using Gramoxone Max. There probably will still be some Gramoxone Max in the pipeline, but it eventually will be replaced with Gramoxone Inteon. It's a 2-pound active ingredient per gallon formulation while Gramoxone Max is a 3-pound active ingredient per gallon formulation, so that will affect the rates we typically use for our at-cracking spray.”

Growers will need to use more of the product to get the equivalent treatment, he says. “They added things to this treatment to make it safer to use. Apparently, in other countries, there have been deaths by ingestion of paraquat. All of the things added are designed to help prevent death if you were to ingest the product,” says Prostko.

The normal Gramoxone Max use rate on peanuts is 5.5 to 8 ounces up until 28 days after cracking. With Inteon, growers will use 8 to 12 ounces, and that will be the equivalent active ingredient of Gramoxone Max. The two formulations have shown no difference in performance in field trials, he says.

Also for 2006, BASF is planning to phase out the old formulation of Cadre (70DG) and replace it with a liquid formulation (2AS). The labeled use rate for the liquid formulation will be 4 ounces per acre.

Farmers who plant strip-till peanuts will have a new option for their fields this year, says Prostko. “We don't have as many options for burndown weed control in peanuts as we do in cotton. But FMC has a label for Aim as a burndown in peanuts. Around planting time, I get a lot of calls from strip-till farmers who have morningglories, and they want to do something about it. Roundup isn't good for that, and Gramoxone isn't good. But Roundup and Aim would do a good job of controlling morningglories at planting time in that strip-till situation, and it costs about $5 per acre,” he says.

In 2005, herbicide-resistant weeds became more of an issue in Georgia with the discovery of ALS-resistant pigweed in Georgia, says Prostko. “There are a lot of reasons herbicides aren't working. There are several things that can go wrong in a field, and we need to rule out these before we can say we have resistance.

“Once we think we have resistance, we have to do some testing on the weed to prove that it is resistant. Resistance occurs over time, whether it's glyphosate or a certain class of products that have the same mode of action. We've selected for those plants, and they're already in your fields. Typically, they're not there in large populations. Over time, as you've sprayed, you might have one plant there that is resistant. You may have sprayed a product and killed all the plants that are susceptible while leaving the resistant plant. This plant produces seed, and it makes things worse for next year.”

If growers spray again with a product that has the same mode of action, it kills susceptible plants, he adds. “If you do this over time, you've killed the susceptible plants, but now you have only the resistant plants, and they're out there making seed. They didn't fall from the sky — they already were there, we just selected for them over time.

ALS herbicides are all different products, says Prostko, and they have different activations, but they all have the same mode of action, or they kill weeds in the same way.

“It doesn't matter if you rotate these chemistries. In Georgia, we rotate cotton and peanuts, and we use Staple and Invoke on cotton and Cadre and Strongarm on peanuts. They're all different products, but they kill weeds in the same way.”

If a grower has a two or three-year history of using Staple and Envoke on cotton, and they follow that with Strongarm or Cadre, they've multiple applications of products with the same mode of action, he explains. “These products cover a lot of crops, so we have the potential to put a lot of pressure on those weeds.”

Researchers have found a general increase in the distribution of ALS-resistant pigweed in Georgia, says Prostko. “We knew about ALS resistance a few years ago, but it has raised its ugly head here in the last couple of years. We've collected 61 seed samples from 21 counties, and we're currently screening those for ALS resistance. It could be a more severe problem than we think.”

There are things growers can do to reduce the chances of ALS resistance, he says. “First, we continue to recommend that you use Prowl or Sonolan on every acre of peanuts. That will help us with resistance management because it's a different mode of action. There is pigweed in South Carolina that is resistant to Prowl and Sonolan. We don't know that we have those in Georgia, but we need to be careful.

“We need to think about what's going on in some of our fields and decide when these materials might help us and when they might hurt us. You probably want to think about adding something to your arsenal. With the current farm economy, we need to think about cutting costs, but these resistance issues probably will make us spend more money.”

Growers might want to use products like Valor, Dual Magnum and Outlook in their programs. “We use a lot of Gramoxone in Georgia, but many growers don't like using it. There's nothing to be afraid of. Most likely, we might want to start mixing either Blazer or Cobra in with Cadre to help pick up some those weeds.

“I hear farmers say they don't really want to spray Gramoxone — they'd like to spray Cadre and nothing else. In field trials, we used Gramoxone and followed it with Cadre, and we used Cadre alone. There's a big difference where we used Gramoxone and where we used just Cadre. If we had come in and cleaned up with Classic, we probably would have gotten even better yields. Whenever we used Gramoxone before Cadre, we generally got better weed control and better yields, but it can cause some injury.”

Peanut producers don't use a lot of glyphosate in their production systems — other than for burning down — but they need to be conscious of the resistance issue, says Prostko.

“A lot of us are growing cotton, and some are growing soybeans, and if we ever start growing Roundup Ready corn, we need to be aware of this issue. It will affect us in the long-term in the management of weeds, not only in peanuts but in the other crops we grow.”

There are four weed species in the United States that have been found to be resistant to glyphosate, he says. Last year, resistant Palmer Amaranth or pigweed was found in four counties in central Georgia.

“Basically, we have to be smarter about the choices we make when we're making applications. With Roundup Ready Flex, we have an option to go over a wider stage of growth. There are things we need to think about with glyphosate. Resistant pigweed was found mostly in central Georgia, and the original thought was that it was confined to a few farms, but it might be more widespread than we think.”

In field trials, resistant pigweed was treated with three times the recommended rate of Roundup and it did not die, says Prostko. “Resistance is real, and you need to be aware of it. There's not much in the pipeline in the way of new herbicides, so we have to be careful.”

Another weed management issue of concern for Georgia peanut producers is the increasing threat of tropical spiderwort, he says. “We've discovered that tropical spiderwort is a great host for Southern rootknot nematodes. If you're rotating to another crop to control Southern rootknot nematodes, and you have tropical spiderwort, you'll negate the effect of the rotational crop. It's also a reservoir for white mold. Tropical spiderwort now has been confirmed in 33 Georgia counties.

“One of the controls that might help us the most with tropical spiderwort is tillage, and we're getting more and more away from that to save money. Twin rows also will help. Effective tropical spiderwort control programs have been developed using Dual Magnum as the foundation herbicide.”

e-mail: phollis@farmpress.com