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Resistant ryegrass populations rise in Mississippi

Glyphosate-resistant Italian ryegrass (Lolium multiflorum) exploded in Mississippi crop fields this spring, infesting between 10,000 acres and 20,000 acres, according to Mississippi State University Extension leaders.

Glyphosate resistance in the state was first confirmed in Italian ryegrass in 2006 by weed scientists at Mississippi State University. At that time, the resistant species was confined to the southern half of Washington County. But wet weather this spring encouraged greater distribution of the weed.

“Fortunately, resistant ryegrass is not distributed geographically like resistant marestail,” said Trey Koger Mississippi Extension soybean specialist. “But where we have it, it’s a serious issue and it’s getting worse. The scary thing is that it’s not crop specific.”

One problem that researchers and farmers face is that Italian ryegrass has always been somewhat difficult to control with glyphosate. “It’s always been a timing issue there,” Koger said. “There is a window in which it becomes very difficult to control. But we have some now that we can’t touch.”

In 2006-07, MSU weed scientist Vijay Nandula reported a three-fold increase glyphosate tolerance in Italian ryegrass in several Washington County fields. In a paper in Weed Science in May 2008, Nandula reported that tolerance to glyphosate in these populations “is partly due to reduced absorption and/or translocation of glyphosate.”

Koger said the weed “has really jeopardized our glyphosate-based burndown programs. We don’t have a lot of spring options. We’ve been looking at some products that have some merit. But for the most part, we need to manage it with fall-applied herbicides.”

Ryegrass will emerge in November and December,” Koger says. “It hangs around during the winter months and doesn’t get real big. Around the first of spring, in February and March, you start to see that clumpy growth habit. It goes to seed in April or May.”

For control, Koger recommends growers look at fall, residual programs before ryegrass emerges. University studies have shown that Dual applied in the fall does a good job on resistant Italian ryegrass, and Mississippi recently received a Section 24-C label for that purpose.

“Command in the fall has also worked well,” Koger said. “Treflan incorporated in the fall does give some activity, although it’s not quite as good as Dual, and since it has to be incorporated, it’s an added expense. To minimize trips across the field, you can use Dual in the fall, then in the spring, use a glyphosate-based burndown to get everything else. But that doesn’t work on any ryegrass that’s out there.”

If ryegrass has emerged in the fall, “you’re not going to be able to depend on those fall residuals like Dual and Command, Koger said. “In that case, the best thing to do is control what’s there with Gramoxone, then use a fall residual to control other weeds until the spring. We’re still working on developing sound practices for managing it.”

Koger said there are some options for control of Italian ryegrass in the spring, but they are more expensive and leave an unwieldy amount of residue to deal with for Mississippi growers who are not heavily into no-till practices.

“We had some fields where it was really a problem,” said Clarkdsale, Miss., farmer Wayne Dulaney. “We put down Valor in the fall to control glyphosate-resistant marestail, but Valor is weak on controlling Italian ryegrass.”

Dulaney still isn’t sure if the uncontrolled ryegrass on his fields is a resistant biotype or if there was a timing issue. “We haven’t really had much of a problem with ryegrass until recently. Earlier, we were hitting it with glyphosate when it was small and taking care of everything. It’s just when it got big on me, I couldn’t control it.”

Ryegrass infestations can reduce yields significantly, studies indicate. “Because there is so much residue there and its growth habit is real clumpy, it can cause thin stands, which contributes to the yield reduction,” Koger said.

Studies also show that tillage isn’t necessarily a good option for control either. “If it doesn’t bury the seed sufficiently, the ryegrass will come right back. We’ve some fields where growers have disked and disked and rowed the ground back up and still can’t kill it.”

Koger says based on research, “fall applications appear to be the most viable control option for glyphosate-resistant Italian ryegrass. Italian ryegrass grows so fast, and even if you can kill it in the spring after it has gotten some size to it, the residue is still there to deal with.”

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