Forage and weed specialists were quizzed by Hay & Forage Grower about which weeds were more worrisome for alfalfa growers in their regions in the U.S. Curly dock took a dubious first place for most prevalent and difficult to control in several Eastern and Midwestern states. Growers are also dealing with weeds such as mustards, lambsquarters and volunteer grains; some anticipate the return of Roundup Ready alfalfa to help fight problem pests.

West

Intermountain California

For fall alfalfa seedings in Intermountain California, mustards such as shepherd's purse and tansy mustard are problems, says Steve Orloff, Siskiyou County farm advisor with the University of California. With spring seedings, lambsquarter is prevalent. All these weeds emerge with alfalfa after planting, are primarily first-cutting problems and can be controlled with Raptor. Dense populations can affect alfalfa stand density. Uncontrolled weeds can outnumber alfalfa and hurt forage quality.

Annual grasses hare barley and downy brome (cheatgrass) can be prevalent in established alfalfa's first cutting, reducing forage quality. Hare barley's long awns can cause mouth lesions; if infestations are high enough, hay must be ground before fed.

Dense, vigorous alfalfa stands, properly irrigated and fertilized, and timely dormant-season chemical applications give good defense. Many growers spray too late in winter, when adequate rain to incorporate soil-active herbicides is less likely, weeds are larger and harder to control, and crop injury can be greater.

Quackgrass and dandelion are hard-to-control perennials in established stands throughout the growing season. Dandelion can cause “slugs” and often turns black in the bale, visually affecting quality.

Idaho

Common lambsquarter may be the most-abundant broadleaf affecting spring-planted alfalfa, says Glenn Shewmaker, extension forage specialist, University of Idaho. It reduces alfalfa stands and slightly affects forage quality, but postemergent herbicides provide reasonable control if applied when soil moisture is adequate.

Tumble mustard and other annual mustards hurt established alfalfa. They are easily controlled with dormant herbicides; don't till for control in spring, Shewmaker says. Mustards are mostly a first-cutting problem, mature rapidly and can significantly reduce forage quality and marketability.

Cheatgrass affects first cuttings, matures very rapidly, lowers protein and raises ADF and NDF. Cheatgrass awns are a problem for livestock; control the weed with dormant herbicides and grass-specific herbicides.

Montana

Volunteer small grains, widespread in seedling alfalfa, reduce first-cut quality and stand establishment, says Dennis Cash, extension forage specialist, Montana State University. They can be controlled with good post-harvest management of the previous year's grain, clipping or early application of a graminicide such as Poast, Poast Plus, Select, Prism or Arrow. When Roundup Ready alfalfa was tested, Montana researchers found better stands where glyphosate was applied at unifoliolate stage than when it or Pursuit (or Raptor) was applied two to six weeks later.

Recent summer broadleaf problems in seeding-year alfalfa are late-germinating pigweed, mallow and kochia. Some areas have ALS-tolerant kochia. Control with good crop rotation before planting alfalfa, plant early and apply herbicides. Weeds take hold in early July in slow-establishing alfalfa, causing reduced stands and subsequent yields. When transgenic alfalfa was tested, “intriguing things about the benefits of eliminating weeds early during establishment, and how this effect carried over later,” were learned, Cash says.

In established alfalfa, Canada thistle is widespread and difficult to control. Cash suggests good crop rotation before planting and spot-treating bad areas with wick applicators the first year. A late-summer problem, it has limited impact on yield or quality, but can cause mold in bales and reduce palatability and alfalfa stand life.