Weed control in wheat: Resistant ryegrass creates new problems

Ryegrass, the most common pasture grass in Northeast Texas, may claim another, more dubious, distinction as the No. 1 weed problem for wheat growers.

In spite of the valuable contributions ryegrass makes as a forage crop, “it is a bad weed in wheat,” says David Kee, a Texas Extension agronomist at Texas A&M-Commerce. Kee and fellow agronomist Don Reid discussed the emerging weed problem during the recent Ag Technology Conference on the Commerce campus.

The characteristics that make ryegrass a good forage crop also make it a threat to wheat, Kee said. “It tillers well, even in poor soil. Improved ryegrasses appear to be harder to control in wheat crops, and as they become naturalized they become weed problems.”

Adding to the dilemma, ryegrass is a prolific seed producer and adapts quickly to a number of stimuli.

As a cool-season plant, ryegrass thrives during the same time as the North Texas winter wheat crop.

“It is temperature sensitive,” Reid said. “Below 43 degrees Fahrenheit, it doesn't grow and above 65 degrees it begins to taper off. It is sensitive to air temperature and, since the crown is above the soil surface, responds more to air than to soil temperature. Ryegrass infestations may reduce grain yield by 30 percent.”

The ubiquitous nature of ryegrass accounts for one aspect of a multi-faceted problem. “We see it everywhere,” Reid said. “Though it's not a native grass, it has escaped from specific locations and naturalized.”

The characteristic that concerns Kee, Reid and farmers is the plant's ability to develop tolerance to the herbicides most commonly used in wheat.

“We're seeing evidence of herbicide resistance in naturalized ryegrass,” Reid said.

Until 1955, few ryegrass varieties were available and none were particularly good forage producers. In 1958, Texas A&M released Gulf Ryegrass. In the early 1990s, A&M released Ribeye and followed with Pine, Jumbo and Big Daddy.

“We now have 35 or more ryegrass varieties available, with 20 of those being separate entities,” Kee said. “We find a lot of variations among those varieties.

Those differences include varying levels of herbicide resistance.

“Amber was a good option for ryegrass control in wheat, but we're beginning to see problems,” Kee said. “Now, ryegrass is not on the label. Some ryegrass also shows resistance to Glean, Hoelon and even Roundup.”

Kee and Reid encountered so much variation among ryegrass varieties they conducted trials to determine which displayed the most resistance to certain herbicides. They tested five ryegrass varieties — Gulf, Local (a naturalized Gulf), Marshall, Tam-90 and Ribeye — with four herbicides — Amber, Hoelon, Axiom, Everest — and a check plot.

“We had ryegrass everywhere in the untreated check,” Reid said.

Results included:

  • Axiom followed by Everest provided good control.

  • Amber efficacy depended on the ryegrass variety and time of application. Control was poor in January but excellent in February.

  • Ribeye could build additional tolerance to herbicides, especially to Hoelon. “If Ribeye becomes naturalized and becomes more tolerant, Hoelon may not be an effective tool,” Kee said.

  • Best wheat yields came from Hoelon treatments, except for Ribeye plots.

    Kee and Reid offer the following ryegrass management options for North Texas wheat farmers.

  • Plant clean seed. Don't plant your own weeds.

  • Watch equipment. Make certain combines and other machines are clean.

  • Know what your neighbors plant and adjust plans accordingly. If nearby farmers plant Ribeye ryegrass, wheat producers may need to alter weed control strategies, for example.

  • Practice integrated pest management. Rotate crops and herbicides. Roundup Ready crops should be good rotation alternatives.

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