Peanut weed control hampered by drought

Dry weather can create a multitude of weed control problems in peanuts, and Southeastern growers witnessed more than their share during 2000. Dry weather was especially prevalent in May, during which 85 to 90 percent of Georgia's peanut crop was planted, says Eric Prostko, University of Georgia Extension weed specialist.

“Dry weather has a significant impact on peanut weed control, and it can affect soil-applied herbicides in a couple of ways,” says Prostko. “We need moisture to activate the herbicide, and we need water to move the herbicide through the soil profile. If we don't get moisture when it's needed, we basically have a layer of herbicide on the surface which won't do much to control weeds that will develop below that layer.”

Growers sometimes can avoid this problem with irrigation and mechanical incorporation, he adds. “But in a dry year, we can have problems even with mechanical incorporation. In dry soil conditions, a herbicide can become more tightly absorbed in the soil surface, which makes it less available to be taken up by weeds or plants. So, in a dry year, mechanical incorporation isn't a guarantee that you'll have great success,” says Prostko.

Dry weather also affects the performance of postemergence herbicides, he notes. “When the weather is dry, several things are occurring in the plant that affect the ability of the herbicide to get into the weed. And if it doesn't get in, it won't control the weeds.”

The waxy layer that forms on the leaf of the plant becomes thicker in dry weather, he says, making it more difficult for herbicides to move into the plant. In addition, the physiology of the weeds become altered in drought conditions.

“You get a reduction in some of the processes within the plant. It's more difficult for the herbicides to get into the plant, and the movement of the herbicides into the plant is reduced, resulting in poorer weed control,” says Prostko.

There are several reasons, he adds, for failures of over-the-top herbicide products. The primary reason, he says, is that growers allow the weeds to become too large before treating.

“Timing is critical to the success of herbicides. It's much easier to control a small weed than a big weed. In some situations, lower rates can be effective on smaller weeds and we can reduce that early season competition.

“Our research shows that only a one-inch difference in the size of a weed can make a difference in the effectiveness of the herbicide application. The longer you delay an application, the less likely you'll get good control.”

Turning to new herbicide products, Prostko says growers' experiences with Strongarm this past year generally were good. In its first year of availability, Strongarm captured about 11 percent of the peanut market, he says.

“In a poll of Georgia county Extension agents, most of them rated the performance of Strongarm in 2000 in the fair to good range. Sicklepod and nutsedge were the top weeds for escaping control of Strongarm, and that's no surprise to those of us who have worked with the compound over the past few years.

“Does Strongarm work? We saw very good control in tests conducted this past summer. It does work, and it's very effective on certain species. Strongarm would be best utilized in those fields where we have hard-to-control broadleaf weeds — weeds like bristly starbur, eclipta or wild poinsettia.

“We continue to suggest that you tank-mix Strongarm with a DNA herbicide, such as Prowl or Sonalan. If you have irrigation, you might want to water it in. You're less likely to have mechanical incorporation failures if you use irrigation to incorporate the tank-mix. Keep in mind that it's variable on nutsedge, and don't expect sicklepod control.”

If a grower is using Cadre after Strongarm, to catch escaped nutsedge and sicklepod, he should be conscious of rotation restrictions, notes Prostko. “Also, if you continue to use Strongarm and Cadre, think about weed resistance. Both compounds have the same mode of action, and we might be setting ourselves up for potential problems.”

In the past, Georgia peanut producers have used the Starfire formulation of paraquat as an at-cracking herbicide treatment, he says. That will change this year, however, and the available paraquat formulations will be Gramoxone Max and Boa. “The active ingredient per gallon is different in these products, and that will affect the rates you apply. Boa is a two and a half pound gallon and Gramoxone Max is a three-pound gallon. The price per acre is basically the same, at $1.50 to $1.60.”

If your at-cracking herbicide application hasn't been successful in the past, there's several things that can be done to improve it, says Prostko. “Always calibrate your sprayer. And, the smaller the weeds, the better the control. Ideally, we'd like to treat them at two inches or less. In addition, if you're considering re-tooling your spray system, you might consider new nozzles. Some nozzles produce a more uniform droplet than others, and that might work better for at-cracking applications.

“Also, we tend to spray some herbicides at low water volume. If you're making an at-cracking application, consider a higher water volume to insure better coverage. At least 15 gallons per acre would be sufficient.”

Another broadleaf soil-applied herbicide — Valor — may be registered for use this season, says Prostko. “Valor has been one of our most consistent materials for Florida beggarweed, and that's our Number One weed problem in Georgia. We're in a diverse agricultural system, and Valor won't create major problems with rotations. It has a unique mode of action, and that'll help in resistance management.

“Valor also has some weaknesses, including cocklebur, nutsedge and sicklepod. It should be applied as close to planting as possible. The longer you delay the application after planting, the more likely it is that you'll get crop injury.”

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