Agricultural pest management
Special Weed Problems in Established Alfalfa
(Reviewed 3/17, updated 3/17)
WEEDS POISONOUS TO LIVESTOCK
Several weed species found in California alfalfa fields are poisonous to livestock. The most common are fiddleneck, common groundsel, poison hemlock, and yellow starthistle. Many other weeds, and even some crop plants, may accumulate high levels of nitrates from nitrogen sources, which are dangerous to cattle and hogs but not to horses or sheep. Some plants (sorghum, sudangrass, nightshade, pigweed, 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-type herbicides such as 2,4-DB may also accumulate nitrates. Plants that accumulate nitrates during stress will convert nitrates into safe compounds after the stress period is over.
Summer grasses, like barnyardgrass, yellow and green foxtail, crabgrass, and southwest 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. Invading grasses often occur on tail-ends of fields due to ponding of water and death of alfalfa stands. Because different species of summer grass weeds may germinate at different times during spring and summer, a field infested with several grass weeds may require repeated applications of herbicides to provide adequate control.
Summer grasses can be controlled before emergence in established alfalfa with trifluralin (Treflan) or pendimethalin (Prowl H2O). Apply from December to February before grasses begin to germinate (March until July). February is the typical cutoff to apply trifluralin or pendimethalin in the Central Valley, but they can be applied up to early April in the Intermountain Region. In the Low Desert, apply after the February cutting. Fields where trifluralin was applied must receive rain or irrigation within 3 days; moisture incorporates the herbicide. The allowable interval between application and incorporation is considerably longer with pendimethalin than trifluralin.
A less common alternative to control some grasses and nutsedge is EPTC (Eptam). It can be applied as a liquid into irrigation water or applied prior to irrigation using the dry 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 germinate in mid-February for 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 small grass seedlings in spring after the first or second 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 sprayed within 2 to 4 days. At this time, grasses are actively growing and alfalfa growth won't interfere with spray coverage. Use a crop oil or methylated seed oil adjuvant. Two applications may be needed in a season. In Roundup Ready alfalfa, glyphosate (Roundup) provides effective control.
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 also provides an excellent management opportunity to control nutsedge. Research trials conducted in the Central Valley have demonstrated that 1 to 2 applications of glyphosate (early and midseason) during the growing season adequately controlled and reduced the long-term population of nutsedge.
Apply EPTC (Eptam) in irrigation water or use a granular formulation to control nutsedge. The liquid formulation requires uniform metering of the herbicide into water during irrigation.
Halosulfuron (Sandea) can be used for postemergence control of nutsedge in established alfalfa. However, it can cause temporary stunting and yellowing of the crop 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. To reduce yellow nutsedge to a manageable level, it will be necessary use selective herbicides (e.g., glyphosate or halosulfuron) in a crop rotational system for several seasons.
Johnsongrass, a troublesome perennial weed, is not controlled by most alfalfa herbicides. Sethoxydim (Poast) and clethodim (Select Max) will control seedlings and should help to control established Johnsongrass. Johnsongrass is very susceptible to sethoxydim or clethodim applied when the grass is shorter than 12 to 18 inches. Multiple (two to three) herbicide applications are required for long-term control and eradication. Watch for new infestations developing from seeds in the soil. EPTC controls Johnsongrass emerging from seed but only suppress growth of established plants for several weeks. In Roundup Ready alfalfa, two applications of glyphosate (Roundup) provides effective control.
Dodder, a yellow-orange, threadlike parasitic weed, can seriously affect alfalfa fields. It has no roots and derives water and nutrition directly from the alfalfa plant. 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 but a normal swather cutting height is too high to eliminate dodder.
In established alfalfa, trifluralin in the granular formulation or pendimethalin liquid formulation, will effectively control dodder when applied prior to its emergence. In the Central Valley, dodder germinates from mid-February through June, so preemergence herbicides need to be applied, and water incorporated, prior to dodder emergence. Two 20 pound applications (2 lb a.i.) of granular trifluralin per acre (February and April) or 3 to 4 lbs a.i. of pendimethalin in February provide dodder control into June. Other herbicides, such as imazethapyr (Pursuit) and imazamox (Raptor) can be applied in seedling alfalfa to control dodder once it emerges and before too much twining and attachment around the alfalfa plant occurs. In Roundup Ready alfalfa, glyphosate is very effective at controlling dodder once it germinates and before the dodder becomes too thick for complete spray coverage and penetration into the crop canopy. Chateau and Shark are herbicides that burn foliage on contact; they may offer some control of dodder but have not been fully tested. For best results with postemergence herbicides, apply to emerging dodder plants early before large mats of dodder are formed.
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 herbicide applications may be necessary after each cutting, as dodder continues to germinate through most of the growing season (through August). To help prevent reinfestation and reduce the seedbank, apply an herbicide before dodder begins to produce seed. A burning permit is required in certain areas of the state.
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 generally not completely controlled by crop rotation because its seed is long-lasting; however, it does help reduce dodder numbers.
Bindweed is best controlled before planting alfalfa. It can be controlled in previous crops, such as grains, with selective broadleaf herbicides. Alternatively, control bindweed in fallow fields with intensive tillage and nonselective (e.g., glyphosate) or phenoxy (e.g., 2,4-D, dicamba) 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, especially when coupled with the frequent mowing that occurs in an established alfalfa field. In Roundup Ready alfalfa, glyphosate (Roundup) provides effective control.
Bermudagrass is a troublesome perennial weed that may crowd out alfalfa stands. In seedling and 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
W. M. Canevari, UC Cooperative Extension, San Joaquin County
Acknowledgment for contributions to Weeds:C. E. Bell, UC Cooperative Extension, San Diego County
R. F. Norris, Vegetable Crops/Weed Science, UC Davis
J. L. Schmierer, UC Cooperative Extension, Colusa County