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.”

Control options

In research last year with combinations of Select Max at 12 oz. and 16 oz. and different growth regulator-type herbicides, Bond says, ryegrass control was reduced at the lower rate of Select Max, with Clarity, Distinct, or Latigo.

“It wasn’t a big reduction, only 6 percent to 8 percent. But when we’re getting only 80 percent control with the higher rate alone, you really need all you can get out of the Select application. We’ve seen in the past that if we use the full rate of Select Max, we don’t get any reduction in control by adding the other materials.

“In 2010-11, we looked at incorporating some fall residual treatments to see if we could improve control. It’s reasonably certain that we’ll get a rain in the fall to activate the herbicide. We did get improved control with some herbicides following mechanical incorporation.”

Prowl H20 controlled more ryegrass when it was mechanically incorporated, Bond says. With Canopy, there was no difference in control between incorporation and surface application.

“Our take-home conclusion was that there wasn’t a consistent benefit from mechanically incorporating these herbicides, and that if you make surface applications late enough to reliably get rainfall, results are just as good.”

In a separate experiment, where nothing was done in the fall, and sequential applications of a full rate of Select Max followed by Gramoxone were applied in the spring, Bond says, “We still got only 83 percent to 84 percent control.

“But with double-disking in the fall, followed by the two shots postemergence, we can get above 90 percent control, which is equivalent to using Dual Magnum in the fall and then either Select Max or Gramoxone in the spring.

“We’ve pretty much settled on the two-pass program, with results depending on when those passes occur — one in the fall and one in the spring, or two in the spring.

“Our best treatment has been where we used the Dual Magnum residual in the fall; there were only a few escapes to clean up in the spring, which could probably be done with a spot spray.

“Dual Magnum is also very good on henbit, and the application we made the latter part of November was still holding the henbit this spring. The one caveat about applying Dual Magnum in the fall is primrose — it’s almost an absolute miss on that weed.”

In a 2011 test looking at several different herbicides applied in the fall, Bond says, “Command was the best treatment. Harness has been consistently good the last three years, as has Outlook. Unfortunately, neither of these products is labeled for fall application.’

“Boundary, with a 24C label in the fall prior to planting soybeans, was an additional tool. The metribuzin component of Boundary brings a lot to the table for ryegrass control.

“You also need to keep in mind the big picture for the entire year and the interaction of weed species. The old ‘misery loves company’ adage applies to ryegrass and pigweeds. In soybeans, ryegrass can compete with emerging soybeans and at the same time provide a safe site for emerging Palmer amaranth.”

The cost for controlling Italian ryegrass, based on Mississippi State University budgets, is “pretty significant,” Bond says.

“A question producers ask is, ‘How long will we have to do this?’ A Palmer amaranth weed can make half a million seeds in a year, but an Italian ryegrass plant produces only 40,000 to 45,000 seed.

“We feel pretty confident that this weed is one for which we can invest some money now to get it under control, and then we’ll be able in future years to back off some on the money we’re spending on it.

“We’re evaluating everything we’ve done the past few years, from fall options to postemergence options, to try and develop a fully integrated program for this weed.”