Southwest wheat farmers should take herbicide resistance seriously. It costs too much not to, says Gaylon Morgan, Texas AgriLife Extension Service wheat specialist.

Morgan, speaking at the Bayer CropScience Wheat Technology Meeting in Oklahoma City, said ryegrass and wild oats cause significant yield reduction and dockage losses.

Ryegrass in harvested wheat, he says, could mean as much as 80 cents per bushel dockage. Wild oats may knock 71 cents per bushel off grower receipts.

“Applying herbicides two months late also reduces yield because of crop competition,” he says. “These weeds are also hosts for aphids, Hessian fly and curl mites.”

Herbicide resistant ryegrass is becoming a significant factor in parts of Texas wheat country. Morgan says samples taken from the Texas Blacklands and the Northeast corner of the state indicate 70 percent of samples showed resistant ryegrass. “We have to take this issue seriously and we have to take herbicide selection seriously.”

Morgan says 315 herbicide resistant weed biotypes have been identified, including 183 species of grasses and broadleaf weeds. Infestations have occurred in 128,000 fields.

“But we have more herbicide options available in wheat than we did just five years ago.”

“Most have advantages and disadvantages.” He says tank mixes of sulfonylurea herbicides and organophosphate insecticides, for instance, may be “too hot and cause significant crop injury. Crop stress following some herbicide tankmix applications also may encourage injury, as can exceeding the recommended nitrogen rates in a tankmix.

“Italian ryegrass is one of the most troublesome weeds in Texas and the Southeast,” Morgan says. “It has multiple flushes, a narrow window of opportunity for herbicide application (1- to 3-leaf stage), and is resistant to ALS and other herbicides.

“Italian ryegrass is very competitive,” he says, “and may result in 23 percent crop yield loss. Some producers actually want Italian ryegrass in wheat to increase forage in a dual purpose production system.”

He says dual-purpose wheat actually can promote herbicide resistance due to continuous use of herbicides with short grazing restrictions and also may limit control efficacy. “We have fewer herbicide options with dual purpose wheat.”

He says grazing limits competiveness of the wheat and leaves room for ryegrass and other weeds to compete.

Available wheat herbicides include: Amber, Glean, Axiom, Osprey, Olympus Flex, Axial, Prowl H2O, Hoelon, Finesse Grass+Broadleaf, and PowerFlex.

“We see good control with timely applications and appropriate adjuvants,” Morgan says. Amber and Glean suppress ryegrass. “Axiom looks good in our trials and is a different mode-of-action.”

Growers should consider other control options, as well. Morgan recommends:

• Cultivate pre-plant when possible.

• Rotate herbicide modes of action.

• Plant weed-free wheat seed.

• Rotate crops and use different herbicides with different modes of action in these crops.

• If growers suspect resistant ryegrass they should increase management intensity.

Morgan says certified seed is a good option for limiting the spread of resistant ryegrass. In bin-run seed samples from Texas, ryegrass seed ranged from zero to 36 seed per pound. In certified seed, no ryegrass was found.

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