Italian ryegrass has become a serious problem for Louisiana producers and if left uncontrolled can reduce crop yield. LSU AgCenter weed scientists Donnie Miller and Daniel O. Stephenson have been focused on stopping the spread of ryegrass for years and are making progress.

Miller, who works out of the Northeast Research Station in St. Joseph, said that ryegrass is currently the most troublesome winter species in corn, cotton and soybean.

“If growers can’t control the grass, it becomes a tremendous competitor with crops, especially those planted early, such as corn, which reduces yield for producers,” he said. “The weed may also host insect populations, which is doubly problematic.”

Miller went on to say that often attention is not focused on later-emerging spring populations or earlier misses in control until it’s too late. The size of the ryegrass, along with resistance development to once effective postemergence herbicide options, limits effective control at that time.

Stephenson agreed that glyphosate-resistant ryegrass has become a major problem. He and Miller have completed a three-year project evaluating different programs to control ryegrass.

Initiated in fall of 2022, their research in ryegrass management expanded in the fall of 2023 and 2024. Findings have shown that the use of cover crops and/or soil residual herbicides applied in the fall after harvest can be effective tools in combatting spread.

“Cereal rye at a seeding rate of 80 pounds per acre can effectively compete with emerging ryegrass and limit tiller and seedhead production,” Miller said. “Ryegrass tiller number reduction in both years was good to moderate (43% and 81%) in spring with only cereal rye planted in fall and reduced 50% and 45% with only s-metolachlor applied in the fall at 1.33 pints. The combination of the two, however, resulted in a 95% and 97% reduction by spring.”

Left: Ryegrass population four weeks before soybean planting with no treatment or cover crop used in the fall. Right 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. Photos by Donnie Miller

According to Miller, seedhead production was significantly reduced only by the combination (93%) in spring of 2024 but was reduced by the cereal rye (81%) and s-metolachlor (41%) in 2025 in addition to the combination treatment (97%).

A second part of this approach is identifying optimum timing for herbicide application in relation to cover crop and ryegrass emergence, Miller said.

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

Research initiated in the fall of 2023 and repeated in 2024 has shown that some residual herbicides, such as s-metolachlor or pyroxasulfone, can be applied at the spiking stage (80% or greater cereal rye or black oat plants emerged through soil), while others such as metribuzin and clomazone are much better tolerated when applied two weeks after emergence to one- to two-leaf plants. Plant injury with these two herbicides, however, is greater than with s-metolachor and pyroxasulfone at this timing as well.

“A noninjurious soil residual herbicide applied soon after cover crop emergence in fall offers producers an effective program for managing ryegrass during winter and spring months ahead of planting,” Miller said. “In addition to weed management, agronomic and soil benefits associated with cover crops are realized.”

Miller went on to say that this combination offers a one-two punch approach to ryegrass management, allowing the soil herbicide to eliminate or limit emergence of ryegrass during late fall and early winter months while the cover crops limit spring emergence and competition from plants not controlled in fall due to prolific biomass production in spring.