(08/07/24) ST. JOSEPH, La. — Many farmers already know cover crops provide a range of benefits. Planted in between cash crop growing seasons when fields would otherwise sit empty, cover crops help reduce erosion, enrich the soil nutrient profile and break up compacted areas.

Now, an LSU AgCenter researcher is exploring another use for these off-season crops: competing with difficult-to-control weeds to keep them at bay.

Donnie Miller, a weed scientist at the AgCenter Northeast Research Station near St. Joseph, has found success in suppressing notoriously unruly Italian ryegrass with a combination of a fall residual herbicide and a planting of cereal rye, a common cover crop. He spoke about his work at a recent field day at the station.

Italian ryegrass has been a growing concern in Louisiana agriculture circles in recent years.

“In 2019 and 2020, we saw a tremendous burst in the population of Italian ryegrass. I don’t think you could ride by a roadside, a highway, a fencerow where you didn’t have Italian ryegrass,” Miller said. “And it seems like every year since then, we’ve increased that population.”

Italian ryegrass spreads quickly, and herbicide options are dwindling. Across the mid-South, the weed has grown resistant to postemergence herbicides that once worked — including glyphosate, clethodim and, more recently, paraquat.

Italian ryegrass tends to creep into fields from the edges. This process can be accelerated when farmers drive equipment through fields, taking the weed seed with them.

Managing field-edge Italian ryegrass is the best way to prevent larger infestations. A boxing fan, Miller used a pugilistic analogy to illustrate the importance of keeping the weed out of fields.

“You never want your opponent to get off the mat,” he said.

For those already contending with Italian ryegrass in their fields, however, there is hope.

“There’s a number of fall-applied soil residual materials out there that do a fantastic job of controlling Italian ryegrass,” Miller said.

His cover crop test plots also have shown promise. Cereal rye appears to effectively compete with emerging Italian ryegrass, limiting its ability to produce tillers and seedheads.

“The ryegrass tiller number in spring of 2024 was reduced 43% with only cereal rye planted in fall at an 80-pounds-per-acre seeding rate and reduced 50% with only S-metolachlor applied in the fall at 1.33 pints,” Miller said. “The combination of the two, however, resulted in a 95% reduction by spring of 2024.”

Seedhead production was significantly reduced only by the combination of cereal rye and herbicide, which eliminated 93% of seedheads.

When used in conjunction with cereal rye, some herbicides need to be applied earlier or later than others. Miller is working to identify the best times to apply them in relation to cover crop and Italian ryegrass emergence.

“Unfortunately, these species often emerge simultaneously,” he said. “So having the herbicide out as early as possible is most beneficial.”

In his trials, Miller planted 80 pounds of cereal rye seed per acre. Farmers typically use only 40 to 60 pounds per acre.

“We think we need to go a little bit higher to help us compete with that Italian ryegrass,” he said.

Italian ryegrass seed remain viable in the soil for about 18 months.

“If you can implement a good program for two solid years, you can really knock back that population,” Miller said.

Plus, farmers who employ this strategy can also reap the soil health benefits of planting a cover crop.

“We’ve found a good marriage that producers can use that also fits into some conservation programs,” Miller said. “It’s good weed science. It’s good for the soil all around.”

Italian ryegrass control four weeks prior to soybean planting in spring with a cereal rye cover crop treated over the top with Dual Magnum two weeks after emergence in the fall. Photo by Donnie Miller/LSU AgCenter

Italian ryegrass population four weeks before soybean planting with no fall herbicide treatment or cover crop. Photo by Donnie Miller/LSU AgCenter