LSU AgCenter weed experts Daniel O. Stephenson and Donnie Miller have been working this past year to combat herbicide-resistance in johnsongrass and ryegrass, the latter of which has become a big problem — and not just for soybean farmers.

Stephenson said not much has changed from last year when he received complaints from producers in the Macon Ridge and Mississippi River parishes about glyphosate’s ineffectiveness on johnsongrass.

“Glyphosate resistance is similar to antibiotic resistance in bacteria,” he said. “Multiple sub-lethal doses of glyphosate year after year are the cause.”

He said it is the same scenario as many other weedy species that are currently resistant to a herbicide, asserting a weed management plan that has a holistic approach must be adopted to manage the problem or potentially avoid it.

In addition to johnsongrass, Stephenson said that glyphosate-resistant ryegrass has also “exploded.” He and Miller started a project last fall looking at different techniques to control ryegrass.

Miller, who works out of the Northeast Research Station in St. Joseph, said that ryegrass has become the most common and troublesome winter species to all crops — not just soybean.

According to Miller, this is because of increases in population, impacts on early season crop growth and dwindling effective over-the-top herbicide options, including glyphosate, clethodim, many ALS (acetolactate synthase) herbicides and, most recently, paraquat, due to developed resistance.

Miller said 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. The weed may also host insect populations, which is doubly problematic.

“It’s just one of those weeds that can slowly take over. It sits on the edge of the field for a couple of years then, all of a sudden, it’s 10 feet into the field edge,” Miller said. “Then, next year, it’s 30 feet in. And when the farmers conduct tillage operations, they can drag seed from plants on the outer field edges into their production fields.”

Often attention is not focused on later-emerging spring populations or earlier misses in control until it’s too late, Miller said. And the size of the ryegrass limits effective control in populations that aren’t resistant to over-the-top herbicides.

Miller’s research was initiated in fall of 2022 regarding ryegrass management, either through cover crops and/or herbicides applied in the fall after harvest. Initial indications are that cereal rye and black oats can effectively compete with emerging ryegrass and limit populations.

“When coupled with Dual Magnum, a soil residual herbicide applied two weeks after cover crop emergence in fall, producers have 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 due to prolific biomass production in spring.