Pest Management Guidelines
Special Weed Problems in Established Alfalfa
(Reviewed 11/06, updated 7/09)
WEEDS POISONOUS TO LIVESTOCK. Several weed species found in California alfalfa fields are poisonous to livestock. The most dangerous are fiddleneck, common groundsel, poison hemlock, and yellow starthistle. Many other weeds, and even some crop plants, may accumulate nitrates, which are dangerous to cattle and hogs but not to horses or sheep. Some plants (sorghum, sudangrass, curly dock) accumulate nitrates during stress by drought, lack of sulfur or phosphorus, or unusual weather such as low temperatures or warm spring weather followed by a long cold spell. Plants that are injured but not killed by phenoxy herbicides such as 2,4-DB, may also pick up nitrates. Plants that accumulate nitrates during stress will convert nitrates into safe materials after the stress period is over.
SUMMER GRASSES. Summer grasses, like barnyardgrass (watergrass), yellow foxtail (pigeongrass), crabgrass, and cupgrass, are a major problem in many alfalfa stands. To reduce their numbers, keep the alfalfa growing vigorously with proper irrigation and allow enough time between cuttings to maintain crop vigor. Because different species of summer grass weeds may germinate at different times during spring and early summer, a field infested with several grass weeds may require several applications of herbicides to provide adequate control.
Summer grasses can be controlled in established alfalfa with trifluralin (Treflan TR-10 granules). Apply in winter or early spring before grasses germinate. January through mid-February is the best time to apply trifluralin in the Central Valley, but it can be applied up to early April in the Intermountain Region. In the low desert, apply after the February cutting. Trifluralin-treated fields must receive rain or irrigation within 3 days; moisture incorporates the herbicide.
To control some grasses and nutsedge, apply EPTC (Eptam) in irrigation water or use a granular formulation. The liquid formulation requires uniform metering of the herbicide into water during irrigation. Light irrigations on uniform, well-leveled soils are best. Apply before grasses emerge in mid-February in the Central Valley, or after a February or March cutting in the low desert. One application can control grasses for 30 to 45 days. The first application should be 3 pounds active ingredient per acre. Later applications of 2 pounds active ingredient per acre are necessary after the third and fourth cuttings. Read label for precautions concerning disposal of drainage water.
In a field where summer grasses have already germinated, apply sethoxydim (Poast) or clethodim (Select Max) to grass seedlings in early spring after a cutting. Early application, before grasses become large and well-tillered, has proved most effective; May to June is an appropriate time for application in the Central Valley and the low desert and June and early July in the Intermountain Region. The field should be cut, irrigated, and then treated within 2 to 4 days. At this time, grasses are actively growing and alfalfa growth won't interfere with spray coverage. Use a nonherbicidal crop oil adjuvant. Two applications may be needed in a season. In Roundup-ready alfalfa, glyphosate (Roundup) provides effective control.
NUTSEDGE. Yellow and purple nutsedge can seriously affect weak alfalfa stands, especially in sandy soils. When sethoxydim (Poast) or clethodim (Select Max) has been used to remove a thick population of summer grasses, it may leave thin spots in the field; if nutsedge is present, it will take advantage of such open areas. Sethoxydim and clethodim do not control nutsedge.
Roundup Ready alfalfa varieties also provide an excellent management opportunity to control nutsedge. Research trials conducted in the Central Valley have demonstrated that 1 to 2 applications of glyphosate during the growing season adequately controlled and reduced the long-term population of nutsedge.
Halosulfuron (Sandea) is very effective for postemergent control of nutsedge in established alfalfa. It can cause temporary stunting and yellowing of the crop, however, when applied during the growing season in the Sacramento and San Joaquin valleys. Application of this product causes less injury and yield loss in the low desert regions of California.
The use of herbicides in rotational crops such as barley, cotton, corn, and in fallow fields can also be helpful in reducing populations of yellow nutsedge. Several seasons will be necessary in a rotational system to reduce yellow nutsedge to a manageable level. By maintaining selective herbicide pressure in this weed, the population should be adequately reduced by the third year.
JOHNSONGRASS. Johnsongrass, a troublesome perennial weed, is not controlled by most alfalfa herbicides, though constant mowing slows its growth. Sethoxydim (Poast) and clethodim (Select Max), however, should help to control established johnsongrass in many fields. Johnsongrass is very susceptible to sethoxydim or clethodim applied when the grass is shorter than 12 to 18 inches. Multiple (two to three) treatments are required for effective control and/or eradication. Watch for new infestations developing from seeds in the soil. EPTC and benefin (Balan) control germinating johnsongrass from seed but only suppress growth of established plants for several weeks. In Roundup-ready alfalfa, glyphosate (Roundup) provides effective control.
DODDER. Dodder, a yellow-orange, threadlike parasitic weed, can seriously affect alfalfa fields. It has no roots in the soil and derives water and nutrition from the alfalfa. Dodder can be controlled culturally, chemically, and by flaming. A dense, vigorous stand of alfalfa discourages dodder, as it requires sunlight to germinate. A dodder infestation can be suppressed or killed by cutting the alfalfa below the point at which dodder is attached.
Trifluralin in the granular formulation, applied to established alfalfa, effectively controls dodder. Apply this herbicide before dodder emerges (mid-February in most of central and southern California) for adequate control. Twenty pounds of granules per acre (2 lb a.i./acre) provides dodder control into June; extended control requires a second application of 20 pounds after the first or second cutting. (Other herbicides, such as imazethapyr-Pursuit and imazamox-Raptor can be applied during stand establishment to control germinating dodder at early attachment before it becomes embeded in alfalfa stems. These herbicides are more effective in seedling alfalfa than in established alfalfa.) In Roundup-ready alfalfa, glyphosate is effective for controlling dodder. Research studies are currently underway to determine the frequency and rate of glyphosate for best control.
Flaming or nonselective herbicides can control dodder after it is attached to the alfalfa. For effective control, the alfalfa stems and foliage must be killed below the point of dodder attachment. Repeat treatments may be necessary after each cutting, as dodder continues to germinate through most of the growing season. To help prevent reinfestation, treat before dodder begins to seed.
Flail mowing effectively controls attached dodder. It involves cutting the alfalfa at the ground surface after the bales have been removed. This practice is less time-consuming and less injurious to the stand than burning. However, it may be less practical in flood-irrigated fields because of difficulty in mowing over the borders.
Because dodder is especially difficult to manage, the best strategy for preventing widespread infestations from developing is to eliminate isolated patches as they appear. Dodder is not effectively controlled by crop rotation because its seed is longlasting.
FIELD BINDWEED. Bindweed is best controlled before planting alfalfa. It can be controlled in previous crops, such as grains, with broadleaf herbicides. Better yet, control bindweed in fallow, with intensive tillage and/or nonselective (e.g., glyphosate) or phenoxy (e.g., 2,4-D) herbicides. Fields infested with field bindweed should be planted in fall when the field bindweed is dormant or growing very slowly. A good stand of vigorously growing alfalfa competes effectively with field bindweed. In Roundup-ready alfalfa, glyphosate (Roundup) provides effective control.
BERMUDAGRASS. Bermudagrass is a troublesome weed that may crowd out alfalfa stands. In new plantings, seedlings may be controlled by preplant treatments of benefin or EPTC. In established alfalfa, repeated applications of sethoxydim or clethodim will provide effective control. In Roundup-ready alfalfa, glyphosate (Roundup) provides effective control.
UC IPM Pest Management Guidelines: Alfalfa
Weeds in Established Alfalfa
Acknowledgment for contributions to Weeds: