Glyphosate and a conventional herbicide both were effective at removing weeds from established Roundup Ready alfalfa in a Michigan State University study. But weed removal had little effect on yield, quality or stand persistence, reports Phil Kaatz, Extension forage educator at the university.

Kaatz says the study, designed by Michigan State weed scientist Jim Kells and forage agronomist Rich Leep, compared a Roundup Ready alfalfa variety treated with glyphosate, an untreated stand and a conventional herbicide (Velpar) treatment. In addition, high-intensity (28-day cutting interval) management was compared to moderate-intensity (35-day cutting interval) management.

The alfalfa was seeded in 2003 and the trial concluded in 2010. Kaatz emphasizes that it was specifically designed to compare weed management and harvest frequency after establishment, and didn’t evaluate the effect of weed control during alfalfa establishment.

The findings:

  • High-intensity (four to five cuttings per year) management produced higher yields than the moderate-intensity (three to four cuttings per year) treatment early in the stand life and lower yields later in the stand life.
  • Forage from the high-intensity schedule usually had higher relative forage quality (RFQ) scores, but yield declined faster over time, and there were more weeds later in the stand life.
  • Harvest intensity had no measurable effect on stand persistence.
  • Glyphosate and Velpar were both effective in removing weeds.
  • Weed removal had no consistent effect on alfalfa or total forage yield.
  • Weed removal had no consistent effect on RFQ
  • Weed removal with herbicides had no measurable effect on alfalfa stand persistence.

“The question remains about whether the use of herbicides on an established field will help the alfalfa stand stay in production longer,” says Kaatz. “It’s the classic question of which came first, the weeds to decrease the stand life or the plant stand decreasing, letting more weeds into the field? The implication is that continuous weed removal from established alfalfa will probably not increase the productive life of the stand.”

View a fact sheet on the study