Stack Cover Crops with Residual Herbicides to Conquer Italian Ryegrass

By Amy Sullivan

By the time North Carolina farmers noticed that their Italian ryegrass wasn’t responding to paraquat in October of 2020, it had already taken over. Italian ryegrass’ reign of terror as a herbicide-resistant weed in the U.S. began in the 1990s with ACCase-inhibiting herbicides (Group 1). The recent confirmation of paraquat-resistant ryegrass also led to the discovery of Italian ryegrass biotypes with multiple resistances to as many as four different herbicide groups (Groups 1, 2, 9, and 22). This sprawling web of resistant ryegrass has left many farmers across the United States with just fall-applied residual herbicides and integrated weed management to manage this weed.

“[Farmers] were abandoning fields of corn, just walking away from them because they couldn’t control the ryegrass,” North Carolina State University (NCSU) Extension weed specialist, Dr. Charlie Cahoon explains. 

That bleak reality drove Cahoon and his team to research the best cover crop and residual herbicide combination for farmers to combat Italian ryegrass, also known as annual ryegrass. His two-year study found that combining large-biomass-producing cover crops with residual herbicides they can tolerate, such as pyroxasulfone or S-metolachlor, will suppress Italian ryegrass and its seed production. But farmers will need to avoid using either method alone, as using these two methods together covers all weed-smothering bases. 

Friend Turned Foe

Italian ryegrass hasn’t always been a dreaded sight in fields. It was once (and still is, in some areas) a popular erosion-preventing cover crop for farmers in the United States, according to Cahoon. The soil types and climate of North Carolina’s Southern Piedmont region is particularly well-suited for Italian ryegrass. 

That made the region especially susceptible to a hostile takeover, as multiple herbicide-resistant ryegrass populations have emerged.

“If you ever find yourself in [North Carolina], you’ll see what looks like fields and fields of just Italian ryegrass,” Cahoon shares. 

Cover Crops and Residual Herbicides, A Powerful Duo

It might be tempting to use a fall-applied residual herbicide alone for Italian ryegrass control. After all, managing cover crops can get pricey. But residual herbicides can only squash Italian ryegrass early in the season. Late-season Italian ryegrass control with residual herbicides was nearly nonexistent, Cahoon found.

Similarly, it might sound like a good idea to only use a cover crop and skip the residual herbicide altogether. Cereal rye was just as effective as residual herbicides when it produced over 4,400 pounds per acre of biomass, the NCSU researchers found. But there’s always a chance that your cover crop might underperform. In those cases, a residual herbicide is critical to help cereal rye control weeds.

That’s why Cahoon examined how cereal rye, crimson clover, and a no-cover-crop control plot interacted with the use of four residual herbicides: flumioxazin, metribuzin, pyroxasulfone, and S-metolachlor.