Agricultural pest management
Integrated Weed Management in Established Alfalfa
(Reviewed 3/17, updated 3/17)
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 an herbicide application 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 are less competitive with weeds and will require an herbicide application when denser stands occasionally do not. Weeds are managed with adjustments of cutting schedules, cultivation in winter, grazing, irrigation, 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 early February (late March or 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 .
Knowing which weeds are present is fundamental to planning an effective weed control program. 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 early, on the basis of identifying weed seedlings. Recording weed history in your field will assist in identifying weeds each year.
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 alfalfa is in the late bud to early bloom 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 competitive alfalfa stand. Precise cutting schedules are hard to recommend because the growth rate of alfalfa depends on location and time of year. Often, a staggered approach to harvest schedules is recommended so that early-cut alfalfa is followed by a longer cutting period. This allows adequate recovery by the roots and a more vigorous stand.
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.
CULTURAL CONTROLS IN WINTER
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. Winter cultivation is a practice that is usually most appropriate for organic alfalfa.
Grazing ("sheeping-off") or green-chopping 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. Sheep that have grazed dodder-infested fields or fields with sclerotinia can bring in seed or sclerotia, respectively. Therefore, they should be taken to an alternative area for a few days before grazing another alfalfa field.
CULTURAL CONTROLS IN SUMMER
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.
INTERPLANTING GRASSES OR LEGUMES
Oats and other annual or perennial grasses or clovers can be planted into older, declining alfalfa stands to increase yields and suppress weeds without the use of herbicides. This is done typically in stands with sparse alfalfa populations. 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 life for more than one year, but results in a mix between grasses and alfalfa, changing the marketed product.
The best oat-seeding rate is approximately 50 to 60 pounds per acre depending on the alfalfa density. 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. An awnless wheat or barley variety can also be used instead of oats.
Fine-stemmed, leafy small grain varieties produce the most valuable forage. Cut small grain-alfalfa forage when the alfalfa would usually be cut. This cutting will be about 15 to 50% small grain, depending on the alfalfa stand density and vigor. Small grain plants that are cut before heading are likely to regrow during the next cutting cycle, adding to the next yield, which may or may not be desirable depending on the intended market.
Orchardgrass is the most common perennial grass seeded into alfalfa. Orchardgrass and alfalfa mixtures are often in high demand by the horse industry. However, as a cool-season grass, orchardgrass is not well adapted to the hotter areas of California. Tall fescue and ryegrass can also be interseeded into thin alfalfa stands, but they are often too competitive and difficult to maintain the desired alfalfa and grass mixture.
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 and berseem mixtures are usually fed as silage or greenchop.
Overseeding perennial red clover can extend the life of a depleted alfalfa stand for 2 years or more and maintain the integrity of legume hay. Seedbed preparation is similar to that used when planting Berseem 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 preemergence herbicides in established alfalfa in fall or winter before new growth begins and before weeds become established. Preemergence 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 preemergence herbicides with soil activity cannot be used.
Some postemergence 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 significant crop damage. For more information, see Roundup Ready Alfalfa: An Emerging Technology, UC ANR Publication 8153.
For more information on alfalfa herbicides, see Irrigated Alfalfa Management for Mediterranean and Desert Zones, UC ANR Publication 3512.
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
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