Pest Management Guidelines
Integrated Weed Management in Established Alfalfa
(Reviewed 11/06, updated 11/06, corrected 3/10)
Uncontrolled weeds in alfalfa can seriously reduce yield as well as quality and retail value of alfalfa hay because some weeds are less palatable to livestock and less nutritious than alfalfa. The presence of poisonous weeds in hay, such as common groundsel, fiddleneck, and hemlock further lower the value or make it totally unmarketable. Tough, fibrous weeds may damage equipment during harvesting. Weeds that retain moisture can cause rotting or start fires in stored hay.
The need for treatment depends on weed species, their competitiveness and toxicity to livestock, the potential market for the alfalfa, and time of year. Vigor of the alfalfa stand is an important factor; weakened stands will require treatment when denser ones don't. Weeds are managed with adjustments of cutting schedules, cultivation in winter, grazing, interplantings of other legumes or grasses, and herbicides.
Monitor for weed seedlings just after alfalfa is cut. Most winter annual weeds start to germinate in late September or October and continue to germinate until late January whenever soil moisture and temperature conditions permit. Summer annual weeds, especially grasses, start to germinate in late February and early March (late March and April in the Intermountain Region) and can continue to germinate until midsummer with each irrigation. Survey for weeds in winter, summer, and early fall after alfalfa has been cut. Record observations on a monitoring form .
Identifying weeds correctly is fundamental to planning a weed control program. It is important to know the kind and abundance of weeds present in an alfalfa field. Weeds are easiest to identify when full grown and flowering; seedling weeds can be difficult to identify. Many weed control decisions, however, must be made quickly, on the basis of identifying weed seedlings.
Short cutting intervals (alfalfa reaching only the bud stage between cuts) reduce alfalfa vigor and encourage weed growth. Cutting alfalfa on longer schedules (letting it reach 60% regrowth with buds averaging 0.75 inch long or in the Intermountain Region when 90% of the alfalfa is in the bud stage) keeps the alfalfa vigorous and competitive with weeds. However, high quality hay for dairy cows should be cut before any flowers appear, or when very few crown buds have started to grow. Cutting schedules are thus a compromise between producing high quality hay and maintaining a strong alfalfa stand. Precise cutting schedules are hard to recommend because the growth rate of alfalfa depends on location and time of year.
Delay the first cutting of a fall-planted seedling field at least 2 to 3 weeks past the normal cutting date for established stands. The interval between the first and second cutting of a new alfalfa stand should be about 2 weeks longer than normal. This allows root reserves to build up, keeping the alfalfa vigorous.
Irrigate fields as shortly before harvest as practical. This allows the alfalfa time to regrow after harvest before an irrigation is needed, thus reducing the threat of root rot or scald. In addition, the soil can dry out after harvest, thus minimizing weed germination at the time when the alfalfa canopy is missing. Delaying irrigation following a harvest also helps suppress summer annual grasses by giving the alfalfa time to grow back and shade the ground; this is more difficult to accomplish on sandy soil than on loamy or clay soils.
Flaming vegetation with a propane or diesel burner can provide satisfactory control of dodder. Foliage should be as dry as possible for flaming. Observe all agricultural burn regulations.
Established alfalfa is sometimes cultivated with a spring-toothed harrow during late winter. Such cultivations uproot weed seedlings but may also injure alfalfa crowns; crown injuries can delay the first cutting, reduce yields, and permit invasion of crown diseases.
Grazing off (sheeping-off) fall and winter alfalfa growth can aid chemical weed control by exposing the soil and seedling weeds. A large number of animals should graze a field rapidly. If the grower only allows the animals to graze until the forage removed approximates a cutting, alfalfa vigor can be maintained through winter. However, alfalfa should not be grazed repeatedly or continuously, because overgrazing can deplete root reserves, reduce crop vigor, and lead to a thin, weedy alfalfa stand. Also, avoid grazing too early in fall or when the field is excessively wet.
Oats and other annual or perennial grasses or clovers can be planted into an older, declining alfalfa stand to increase yields and suppress weeds without the use of herbicides. This is done typically in stands with a low alfalfa population. Plant grasses in December or January, during alfalfa dormancy, or after sheeping-off the alfalfa. In the cold, northeastern part of California, plant grasses in March or early April or in early fall. Interplanting with perennial grass can often extend stand live for more than one year.
The best oat-seeding rate is 50 to 60 pounds per acre. Broadcast oats, then disc or use a spring-tooth harrow to cover the seed. Or harrow the ground first, then use a grain drill to plant. To help oat growth, add nitrogen at 30 to 40 pounds per acre. The combination of oat competition and harrowing or disking at planting will suppress most weeds.
A short-season variety, such as 'Montezuma,' produces the most growth by the time of harvesting. Fine-stemmed, leafy oat varieties produce the most valuable forage. Cut oat-alfalfa hay when alfalfa would usually be cut. This cutting will be about 15 to 50% oats, depending on the alfalfa stand density and vigor. Oat plants that are cut before heading are likely to regrow during the next cutting cycle, adding to the next yield.
Overseeding clovers offers several advantages over grasses, including higher crude protein and lower fiber, which makes the harvest suitable for dairy markets. Berseem clover is best adapted for California conditions and significantly increases yield for the first three to four harvests in the Central Valley. Berseem is primarily fall-planted, from September to early November with mid-October being optimal. Berseem is overseeded at 6 to 10 pounds per acre and planted shallow (less than 0.5 inch). Inoculating the seed with Rhizobium trifolii is recommended before planting. Because of vigorous winter growth of Berseem, earlier harvests than usual are necessary and could be problematic if rains occur. The forage is often difficult to cure so alfalfa/berseem mixtures are usually fed as sileage or greenchop.
Overseeding red clover can extend the life of a depleted alfalfa stand for 2 years or more. Seed bed preparation is similar to that used when planting Burseem clover. Red clover should be planted about the same time as alfalfa would be planted, using a 6- to 12-pound per acre seeding rate.
For additional information on interplanting, see Overseeding and Companion Planting in Alfalfa, UC ANR Publication 21594.
Herbicides are used along with proper cultural weed control techniques to obtain effective, economical weed control. If winter annuals need to be controlled, apply preemergent herbicides in established alfalfa in fall, winter, or spring before new growth begins and before weeds become established. Preemergent herbicides must be incorporated by winter rainfall or sprinkler irrigation. Some herbicides with soil activity can cause yellowing of foliage and delay the first cutting when used on alfalfa that has resumed growth. In California's southern desert valleys where nondormant varieties of alfalfa grow year-round, some preemergent herbicides with soil activity cannot be used. To protect water quality, do not use preemergent herbicides during the last year of the stand in areas where surface runoff is a concern.
Some postemergent herbicides, such as paraquat, work on contact, so complete coverage is necessary. Stage of weed growth is also important; young weeds are usually easier to kill.
The use of transgenic alfalfa varieties such as Roundup-ready alfalfa allows glyphosate (Roundup) to be applied to emerged alfalfa at any growth stage without the risk of crop damage. By applying glyphosate according to the size of the weed, and not the crop, weeds can be controlled early in the life of the alfalfa stand before they compete with and damage the crop, a problem that exists with conventional alfalfa herbicide programs. For more information, see TRANSGENIC HERBICIDE-TOLERANT ALFALFA.
For more Information, see Irrigated Alfalfa Management.
UC IPM Pest Management Guidelines: Alfalfa
Weeds in Established Alfalfa
Acknowledgment for contributions to Weeds:
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