Pennsylvania

Common chickweed and other winter annuals reduce quality and yield in established alfalfa. If anticipating chickweed, seed alfalfa in spring and work to ensure a successful seeding and a good stand, says Bill Curran, weed scientist, Pennsylvania State University.

Apply herbicide in late fall or early spring before chickweed growth takes off. It's mostly a problem in late spring before first cutting, has less protein and yield potential than alfalfa and can increase hay-drying time. Identify the weed correctly.

Perennial weeds such as curly and broadleaf dock and dandelion hurt established alfalfa at first cutting (dandelion) or in mid-summer when alfalfa is less vigorous. Difficult to control, they shouldn't be present when establishing new seedings. Maintain good fertility and soil pH and manage cuttings to keep weeds in check.

Several herbicides suppress these perennials, including Pursuit, Raptor and perhaps Velpar or Sencor. The weeds can reduce alfalfa quality and yield and spread if allowed to seed.

If perennial weeds are numerous in alfalfa stands older than three years, rotate back to corn to eliminate them.

Vermont

In seedling alfalfa, summer annuals like lambsquarter, pigweed, ragweed, crabgrass and foxtail are problems, says Sid Bosworth, University of Vermont extension agronomist. Early planted stands seeded at proper rates usually outgrow weeds; after first cutting, the weeds are less of a problem. Some Vermont growers plant oat companion crops to reduce weed pressures.

Most established alfalfa stands are mixed with grasses, which help prevent weed problems until stands start thinning in the fourth or fifth year.