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.
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.
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.
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.
In new alfalfa plantings, volunteer grain is a spring or late-summer problem while nightshade can hit in mid- to late summer, says Mylen Bohle, extension agronomist, Oregon State University. Apply glyphosate before planting or a grass or broadleaf herbicide during establishment. Volunteer grain can decrease alfalfa quality and yield, although it may increase total first-cutting yield if it doesn't reduce the stand. Nightshade toxicity could totally devalue a field.
In established alfalfa, bluegrass, quackgrass and dandelion are pests throughout the growing season. Apply herbicide (Velpar + Gramoxone) in fall. Depending upon stand, price of hay, etc., Kerb can be applied for grass control. The grasses dramatically reduce quality and stand life; dandelion reduces yield but not lab quality for the most part.
Foxtail grass species and several broadleaves — jimsonweed, cocklebur, pigweed and black nightshade — affect seedling alfalfa stands in first cuttings, says Keith Johnson, forage specialist, Purdue University.
Control options include: 1) seeding pure alfalfa for more herbicide choices, 2) scouting for all these weeds' presence as alfalfa and weed seeds germinate and become seedlings, 3) treating when weeds' growth can be stopped or 4) using companion crops like spring oats.
Foxtail reduces alfalfa composition and quality. With the broadleaf weeds, if not controlled, there's concern as to whether crops should be harvested because of possible toxicity. Thankfully, are all annuals.
Control curly dock in other crops rotated with alfalfa. In Roundup Ready alfalfa, glyphosate can reduce this weed's first-cutting impact. Curly dock's high-moisture, thick stems can mold within bales. Unfortunately, its seed can live in soil for many years.
Cressleaf groundsel has increased in Indiana as more row-crop fields are no-tilled. This winter-annual broadleaf can flourish in this environment.
Summer annuals such as crabgrass, foxtail and pigweed affect spring-seeded alfalfa, and winter annuals such as chickweed, downy brome (cheatgrass) and henbit hurt fall-seeded alfalfa. Some herbicides control these species, but yields will be lower once weeds are eliminated, says Kevin Bradley, plant scientist, University of Missouri.
Curly dock affects established alfalfa throughout the season but mostly at first cutting. Few herbicides control it, but spot spraying with glyphosate where feasible is effective. No current herbicides will remove established curly dock from an alfalfa stand, he believes. Roundup Ready alfalfa would help if it were available, Bradley says.
Perennial bluegrass is a year-round pest in irrigated alfalfa, lowering quality and reducing stands, says Bruce Anderson, University of Nebraska forage specialist. To control it, manage irrigation well and spray.
Tall waterhemp, a late-summer competitor reducing quality and stands, can be controlled with vigorous alfalfa stands and herbicide application.
Winter-annual mustards decrease quality and yield of first-cut alfalfa and can be controlled if herbicides are timed properly.
Winter-annual broadleaf weeds, especially wild turnip and wild radish, are difficult to control in fall-established alfalfa. So say Mark Sulc and Mark Loux, Ohio State University crop scientist and weed specialist, respectively, and Ron Hammond, entomologist at the Ohio Research and Development Center. They suggest applying broadleaf herbicide (for example, Pursuit) in fall for best control. The weeds slow drying time, reduce stand density and yield and usually lower forage quality.
Annual broadleaf and grass weeds compete with spring-seeded alfalfa, but postemergent herbicides, applied when weeds are small, can control most of them. If not controlled, they can harm stand establishment, yield and forage quality.
Perennial weeds, mostly dandelion, curly dock and Canada thistle, are problems in older stands, primarily in spring. Control before seeding alfalfa; then maintain thick, vigorous stands with good fertility and proper harvest schedules. Herbicide control is problematic; Roundup Ready alfalfa would help. Velpar and Sencor dormant applications provide some dandelion control. The weeds reduce crop vigor and yield and will fill areas where stands are not thick and vigorous.
In establishing alfalfa, pigweed and lambsquarter can be significant problems, says Dan Undersander, University of Wisconsin extension forage specialist. Use Pursuit or Raptor after seeding. The weeds are problems only within 60 days of seeding, can reduce alfalfa stand and yield and increase fiber of harvested forage.
For fall-seeded alfalfa, winter-annual weeds such as common chickweed and purple deadnettle are trouble, generally in late winter and early spring and only at first cutting. Common chickweed challenges first-crop cutting and curing, reducing quality. Winter-annual weeds are less of a problem in well-established, thick stands.
Summer-annual crabgrass and common ragweed can hurt spring-seeded alfalfa. Herbicide options are often limited; much of Kentucky's alfalfa is seeded with grasses such as orchardgrass. For new, pure alfalfa seedings, use Pursuit or Raptor, says J.D. Green, extension weed scientist, University of Kentucky. For weedy grasses, selective postemergent herbicides include Select, Poast or other products.
Curly dock is problematic in Kentucky; no herbicides can effectively control it, says Green. It persists throughout the growing season, but is more evident during first cutting, since it produces seed heads in spring. A perennial dicot, curly dock can reduce alfalfa hay quality and yield. Don't plant alfalfa in fields with curly dock present; control the weed in previous crops.
Crabgrass decreases yield and quality in spring seedings. Control with Select or Poast Plus in late April and into May, says Gary Bates, forage specialist, University of Tennessee.
In established alfalfa, Tennessee growers fight pigweed, johnsongrass and curly dock, which drop yield and quality in mid- to late season. Johnsongrass can be handled with Select; Pigweed with Pursuit or Butyrac. Make sure curly dock is out of fields before planting alfalfa.
Most weed prevention is done before seeding, because growers here plant few pure alfalfa stands, says Richard Meinert, cooperative extension educator, University of Connecticut.
Several Roundup applications kill or reduce weed seeds; then alfalfa and grass are seeded. For pure stands, small grain cover crops are used the first year to substitute for grass and suppress weeds.
Most alfalfa here is fall-established, so winter annuals such as henbit, chickweed, shepherdspurse, speedwell species, mustard species, etc., are problems, says Quintin Johnson, crop specialist with the University of Delaware. These weeds are most competitive in the fall and early spring and can affect first-cutting forage quality. Dependent upon species, dormant applications of Gramoxone, Pursuit or Raptor or postemergent sprays of 2,4-DB, Pursuit or Raptor are good control options.
In established conventional alfalfa, horsenettle is a big problem followed by biennial thistles and Canada thistle; all more so after first cutting. Horsenettle and Canada thistle can be controlled with good cultural practices and glyphosate spot-sprays. Biennial thistles are controlled with 2,4-DB or suppressed with dormant or post-cutting Raptor applications. Forage palatability and hay quality are reduced and severe infestations affect yields. Horsenettle berries can also be poisonous to livestock.
With late-winter alfalfa seedings, problem weeds tend to be lambsquarter and pigweed early and crabgrass, foxtail and fall panicum in mid- and late summer. With late-summer seedings, winter annuals chickweed and henbit are problems, says Les Vough, University of Maryland extension forage specialist emeritus. Spray depending on the severity of the problem and amount of competition. The weeds generally reduce quality and can reduce alfalfa stands.
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.
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.