Arkansas growers have mainly avoided problems with herbicide-resistant ryegrass in recent years. But that changed in 2021 when treatable populations of the ryegrass species spread into the central and north Delta parts of the state.

“When I look back at the 2021 growing season, it was one of the worst I can remember for issues with Italian ryegrass,” said Dr. Tom Barber, Extension weed scientist with the University of Arkansas System Division of Agriculture.

Barber is no stranger to Italian ryegrass, having worked on it in wheat as a graduate student. That was when Arkansas had more than a million acres of wheat annually, and Italian ryegrass competed with the crop for moisture and nutrients. (Arkansas growers planted 155,000 acres of wheat that was expected to be harvested for grain in 2021.)

“The last several years down in Tillar (Arkansas) where I am today, it’s been a big problem,” said Barber, who spoke during the University’s virtual Corn and Cotton Field Day in late October. “That changed in 2021 when it spread to much of the area north of here.

“Now is the time of year that we need to be thinking about Italian ryegrass management,” said Barber, noting that in the case of corn, which is an early crop, it’s likely Italian ryegrass will be competing with the corn as it comes out of the ground.

“When we look at data from Mississippi State University and other areas where resistant Italian ryegrass has been more prevalent, that's not something that we want. We can lose yield with that competition. So now’s the time to think about management of ryegrass.”

Screening program

Barber said University of Arkansas researchers and Extension specialists are conducting a screening program to determine if the Italian ryegrass in Arkansas is resistant to other herbicides beside glyphosate.

“Some of it may be resistant to clethodim or Select,” he noted. “We’re trying to gauge what products we have that will control it if it is up out of the ground. But, before we deal with it in the spring, it's much easier to deal with in the fall.”

Based on his research on his master’s degree, the best method for controlling ryegrass was to kill the first flush coming out of the ground in the fall.

“If we could kill that first flush, it might take care of 80 percent of the ryegrass we would have to deal with in the spring,” he noted. “So killing that first flush is critical. We can use tillage; we can use non-selective products like Paraquat or Gramoxone.

“When I was doing the research, we used Roundup, but Roundup’s not going to work on most of these populations. Clethodim or Select is an option if the field in question doesn't have Select resistance. Hopefully, with our screening program, we’re going to figure it out. Regardless, we need to kill that first flush or prevent it from coming up. And then we need to use residual herbicides.”

Activated at the right time, residuals can prevent that first flush from emerging. Ryegrass will germinate based on temperature fluctuations.

“It's hard to nail down when those residual applications need to go out,” he said. “Many times it may be easier to tank mix Paraquat with the residual to kill what’s up. Then we have a residual herbicide barrier down to prevent more germination through the fall and winter.”

The residuals, in the Group 15 class of chemistry, include metolachlor, specifically any product containing S-metolachlor; acetachlor or Warrant; and pyroxasulfone or Zidua.

“Those three will be our main products that we use from a residual standpoint, especially if we’re talking about planting crops like corn, cotton, or soybean in the spring,” he said. “If the field is going to rice, we need to be careful because they have restrictions on planting back following those applications.”

University of Arkansas specialists are seeking a Section 24c permit for the application of clomazone or Command specifically for use in the fall, giving growers an increased total rate that can be used in a season.