An over-the-top glyphosate application during establishment of a spring-seeded Roundup Ready alfalfa crop will suppress tough perennial weeds, but it won’t kill them, warns Mark Renz, University of Wisconsin Extension weed scientist.

A fall application will do a much better job on weeds such as quackgrass, Canada thistle and dock, says Renz.

“I should emphasize that glyphosate is by far the best option that growers will have to control these weeds,” he says. “But it’s not going to eliminate them in the establishment year. It will control them eventually, but it’s going to take multiple applications, and it’s going to be very expensive.”

If perennial weeds are present in a field that will be spring-seeded to alfalfa, such as in a no-till situation, apply glyphosate the previous fall. “Then you should be in good shape,” says Renz.

The herbicide is registered at up to 1.5 lbs of acid equivalent per acre on Roundup Ready alfalfa. For easy-to-control annual weeds, ¾ lb/acre acid equivalent is typically all that’s needed. If you have harder-to-control annuals or biennials, you might need to apply 1 lb/acre.

“And typically with most perennials, that pound will give you good control,” says Renz. “But if those perennials have been there a long time, like quackgrass that’s been there for five or six years, you might have to go up to that pound and a half.”

Studies have shown that most companies’ glyphosate formulations perform identically if applied at equal acid-equivalent rates. When there are differences, it’s usually because the weeds are stressed or too big to be easily controlled. In those cases, products with surfactants and/or rain-fast ingredients have shown an advantage.

Except for those situations, choose a product based on cost per pound of acid equivalent, but be sure it’s registered for use on Roundup Ready alfalfa, Renz advises.