A popular cover crop has the potential to become a weedy nuisance with economic consequences because of its herbicide resistances, say two Purdue Extension weed science specialists.
Annual ryegrass, also known as Italian ryegrass, has some biotypes that are resistant to herbicides, and has become economically detrimental in the southern and western portions of the U.S.
"The spread of this species from a cover crop to a weed is likely if herbicide burndown programs are not successful in controlling annual ryegrass prior to production crop planting and seed that germinates after crop establishment is repeatedly sprayed with glyphosate or other herbicides," Travis Legleiter said.
"Poor control with burndown treatments occurred in our own program at a Purdue research facility in which previous escapes of annual ryegrass were not controlled with 0.75 pounds acid equivalent glyphosate per acre applied at the end of March on 6 to 8-inch plants."
Legleiter said evaluations two weeks after herbicide application showed the control attempt had failed. Additional applications had to be made.
Current Purdue Extension recommendations are to apply glyphosate to small plants and at an increased rate of 1.5 pounds acid equivalent per acre. Other herbicides to enhance control can be added depending on the intended crop to be grown.
"In a spring like we have experienced this year, annual ryegrass has flourished in the warm temperatures. The likelihood of timely applications to small plants was rare and reduced burndown control," said Bill Johnson. "Producers who have already made termination treatments to their cover crops need to scout fields and confirm complete death of all cover crops and determine further control practices that need to occur."
Another part of the problem, he said, is that many people confuse ryegrass and rye. There are big differences between annual ryegrass and rye, which refers to cereal rye.
Cereal rye has traditionally been grown as a small grains crop that can grow up to 6 feet tall and can be controlled easily with herbicide applications or rolling/crimping operations, Johnson said.
Annual ryegrass, on the other hand, is an aggressive winter annual forage-like grass that can grow to 3 feet tall and is very difficult to control with herbicides.
"If you are considering cover crops for the future, be aware that rye and ryegrass are two different species with different management tactics," he said.