Italian ryegrass shown after a fall, residual herbicide application (Clomazone). Photo by Donnie Miller

Louisiana soybean producers always have a full plate during growing season with factors such as weather, insects and weeds coming into play. This year one of the biggest issues they face is glyphosate-resistant johnsongrass and ryegrass.

LSU AgCenter weed expert Daniel O. Stephenson said he’s been getting a lot of calls from producers about johnsongrass that seems to have built a resistance to glyphosate.

“We had an unusually warm December this year,” Stephenson said. “I remember seeing johnsongrass back then, and it never really got cold enough to knock it back, so it’s been a monster year for it.”

Stephenson went on to say the LSU AgCenter confirmed glyphosate resistance in johnsongrass in 2010 in Rapides and West Baton Rouge parishes. In the years since, glyphosate-resistant johnsongrass has spread throughout the central and southern areas of Louisiana.

Stephenson said that he has received complaints from producers in the Macon Ridge and Mississippi River parishes over the past two years 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 is the cause. This is the same scenario as many other weedy species that are currently resistant to a herbicide. 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 his AgCenter colleague weed scientist Donnie Miller have a project starting in the fall looking at different techniques to control ryegrass.

Miller, who works out of the Northeast Research Station in St. Joseph, said that ryegrass is quickly becoming the most common and troublesome weed to all crops — not just soybeans — due to increases in population and development of resistance to most over-the-top herbicide options, including paraquat, which was confirmed this year.

“In 2020, we saw probably the biggest population of ryegrass I’ve ever seen in northeast Louisiana,” Miller said. “In fields, on borders of fields, in roadside medians, it was just everywhere.”

Miller said if growers can’t control the grass, it becomes a tremendous competitor with crops, especially those that are planted early like 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, then next year 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,” he said. “I like to say it’s like being stalked by a turtle — it closes in little by little until it’s got you.”

Much of the work Miller and his researchers are focused on began last year and will ramp up this year regarding ryegrass management, either through cover crops and/or herbicides applied in the fall after harvest.

“If we apply the herbicides at earlier stages of growth of a cereal rye cover crop in the fall, it can still do a decent job of knocking it back,” Miller said. “So, we’re going to look at some combinations of that with different cover crops, which would be another tool in our toolbox to help producers.” V. Todd Miller