Herbicide-resistant Italian ryegrass is becoming “a serious problem” in some areas of Mississippi and is now in 18 counties in the state, says Jason Bond, associate Extension and research professor at the Delta Research and Extension Center at Stoneville.

It has also been documented in eight Arkansas counties and at northeast Louisiana locations, he said at the annual meeting of the Mississippi Agricultural Consultants Association. “We added Lee County, Miss., to our list this year — which was significant because it was the first county not in the Delta.

“It’s critical to get control of Italian ryegrass in corn, because its competition can be devastating to the crop. There can also be a big competitive impact in rice. In some work last year, where we had rice competing with ryegrass stubble that had been burned down prior to planting, yield was reduced 15 percent. More significant, there was a delay in maturity of up to 12 days from competition with the ryegrass stubble.”

Control options for Italian ryegrass are somewhat limited by the number of labeled herbicides — only seven — that are active on the weed,” Bond says. “We’re promoting a system of a residual herbicide application in the fall, usually Dual Magnum or Parallel PCS, and then a Select Max application in January or Gramoxone in February to clean up escapes.

“If you do tillage only in the fall to control the first flush of ryegrass, you’re then committed to using a two-pass herbicide program of Select Max and Gramoxone in January and February.”

Ryegrass infestations seem “somewhat worse this year,” Bond says, “and I don’t have a good explanation. Since the middle of January, it has really jumped up in a lot of places.

“Last year and in 2010, we had flushes in emergence around the first of November. In the first part of November 2010, there was a big rain pretty much Delta-wide, and I think that may be part of the explanation.

“We’ve had several sites this year with a lot of ryegrass emerging at different times, but we never got a general rain across the entire Delta during the period of emergence.

“In trying to get an idea of what stimulates ryegrass to emerge,” Bond says, “we’ve looked at average maximum/minimum air and soil temperatures, but have found no real correlation between temperature and emergence that would account for the big spikes we’ve seen. I think the more likely correlation is a combination of temperature and rainfall.

“We’re not really sure what causes the fall flush and then a turnoff for a couple of months. In some areas, we get a significant spring flush, in some areas we don’t, and I think that may be related to variation in biotypes.”