Weed control challenge in no-till wheat

Winter annuals grow rapidly in fall; can cause major losses Farmers are not likely to encounter many exotic weed species in no-till wheat compared to what they might find in no-till cotton or soybeans (cutleaf evening primrose, Carolina geranium, swinecress and marestail come to mind).

But, the weeds they are accustomed to seeing in conventionally planted wheat may be more difficult to control in no-till, says Neil Rhodes, Extension weed scientist with the University of Tennessee.

Speaking at the Milan No-Till Field Day, Rhodes told farmers that winter annual and perennial weeds could present greater challenges in no-till. "Wild garlic and the winter annuals such as chickweed, henbit and annual ryegrass grow rapidly in the fall and are capable of causing substantial losses due to dockage or reduced yield," he noted.

No-till wheat is not a new phenomenon in the Mid-South. For years, Delta growers flew wheat seed into standing cotton stalks in the fall to help the wheat get a head start on the fall rains.

It wasn't until they began trying to improve yields with intensive wheat management that many of those growers started planting into conventionally prepared seedbeds. Rhodes and other UT agronomists are trying to make sure farmers who want to go back to no-till planting can do it successfully.

"No-till wheat is no different from no-till corn, soybeans or cotton," he said. "If weeds are present at planting, a burndown herbicide will be needed. Unfortunately, some producers skip the burndown when they plant wheat no-till."

Rhodes said he understands growers wanting to cut expenses. "But, omitting the burndown herbicide can cost you more than you save."

Many winter annuals are just pushing through the soil line in late October to early November. "If you hit them with a burndown herbicide before the wheat comes up, it can put you way ahead."

Herbicides such as Gramoxone Extra can be an effective, inexpensive option to control chickweed, henbit, ryegrass and other winter annuals present at planting, he said. Roundup Ultra and Touchdown 5 would provide better control of perennial weeds, such as johnsongrass and some vines, especially if applied in late fall.

"There is more need for fall scouting and making good treatment decisions in no-till wheat," he said. "It's especially important that you use the burndown to get a head start on ryegrass."

Annual ryegrass, also called Italian ryegrass, is becoming an increasing problem in wheat, alfalfa and no-till corn in the South. A winter annual that can be difficult to control in no-till and conventional wheat, it is a prolific seed producer and a strong competitor for nitrogen.

Rhodes recommends laying out heavily infested fields to keep ryegrass from spreading and to reduce the chances of it developing resistance to herbicides. Growers should burn down those fields with Gramoxone Extra in November or December to reduce the ryegrass seed bank

"Weed scientists in several states have observed bio-types of ryegrass that are not controlled by high rates of Hoelon," he said. "This is a serious problem in Arkansas and other states. We have documented it at Milan, and suspect it at other locations."

If laying out the field is not an option, growers should avoid planting heavily infested fields first, says Rhodes. "Allow the ryegrass to germinate; then kill it with tillage or with Gramoxone Extra prior to planting the wheat."

If resistance is not suspected, Hoelon can be applied pre-emergence in a tank mix with Gramoxone Extra. But, a minimum of two pints of Hoelon per acre will be required for pre-emergence control.

"For the money, the best way to use Hoelon is early postemergence at 1.3 pints per acre," says Rhodes. "This should be targeted about one month after planting when the wheat and ryegrass have emerged. You will be spraying smaller ryegrass at lower rates, and we generally have better spraying weather and better herbicide performance."

Achieve, a relatively new herbicide that was registered for wheat in 1998, should also be applied in the fall for optimum control of ryegrass, he said. University of Tennessee tests show 98 percent control of ryegrass with 0.44 pound of Achieve when applied on Dec. 15 compared to 90 percent control when applied at 0.6 pound on Feb. 10.

"Because Achieve has the same mode of action as Hoelon, it will not control Hoelon-resistant bio-types," he said.

"Fall applications of Harmony Extra have also proven particularly useful for control of curly dock, henbit, chickweed and wild garlic," Rhodes said. "A fall application has usually eliminated the need for a spring application."

Similar applications of Sencor on Sencor-tolerant varieties can provide effective, economical control of winter annual broadleaf weeds, little barley and cheat. "The label has been revised to allow the application of reduced rates of 1 to 3 ounces of the 75 DF formulation on wheat in the two-leaf to three-tiller stage," he noted. "For cheat, apply 3 ounces to the one-leaf stage."

What about Harmony Extra on vetch, a weed that is becoming more commonplace in Tennessee wheat fields?

"Harmony Extra is only partially effective," says Rhodes. "The addition of 6 to 12 ounces of 2,4-D ester will greatly improve vetch control, and it will help where musk thistle and cutleaf evening primrose are troublesome. This tank mix application should be delayed until the wheat is well-tillered, but not jointing."

Several new products or new uses of old products may have potential for weed control in wheat, he says. None are currently labeled for use on wheat.

"Axiom is a new corn and soybean herbicide that appears to be promising on ryegrass and winter annual broadleaves," he said. "Prowl as a delayed pre-emergence application has shown good broadleaf activity, but its control of ryegrass has been variable.

"We are beginning to hear reports about Clearfield wheat and Roundup Ready wheat. Most of the testing on the Clearfield wheat has involved Raptor, which is now labeled in soybeans, and it appears to offer good ryegrass and broadleaf control. The Roundup Ready wheat has not been tested."

Neil Rhodes offers these tips to farmers contemplating planting wheat no-till this fall:

1. Don't cut corners on the burndown herbicide.

2. Weeds develop quickly in no-till. Scout and be ready to make treatment decisions in the fall, especially for weeds such as ryegrass, curly dock, cheat and little barley and chickweed and henbit.

3. Monitor the performance of Hoelon. Rotate heavily infested fields to other crops, if possible.

4. Add 2,4-D to Harmony Extra only if needed to control weeds such as vetch, musk thistle, cutleaf eveningprimrose and cornflower.

5. Try to avoid the need for a harvest aid in wheat.

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