Agricultural pest management
Integrated Weed Management in Seedling Alfalfa
(Reviewed 3/17, updated 3/17)
Uncontrolled weeds in seedling alfalfa can cause loss of the stand during crop establishment. Weed infestations can weaken young alfalfa plants, retard growth, delay the first cutting, reduce quality, and result in long term damage to crop yield and stand persistence.
Proper establishment and management of an alfalfa stand are essential for weed control. It is not cost-effective to control weeds in a thin or weak stand. Plant alfalfa at the right time (early fall) and depth; alfalfa that germinates and grows rapidly in response to warm temperatures, adequate soil moisture, and shallow planting (0.25 inch) will usually develop into a competitive stand with reduced weed problems throughout the stand life. Adequate soil preparation and soil fertility, especially phosphorus, is also essential in establishing and maintaining a vigorous stand. Plant only alfalfa varieties that are well adapted to your climate and soil type.
For soils with a high salt content or areas where soils lack adequate drainage, such as heavy clay soils, plant alfalfa using broadcast or drilled methods on raised beds or with shallow corrugations which allow water to drain off the field. Possible weed problems associated with this practice include increased weed numbers in the furrows and decreased effectiveness of water-run herbicides, caused by uneven water distribution across the bed. When planted on beds, alfalfa is not as competitive against weeds, because the furrow is not covered completely by the crop canopy.
Start looking for weeds when moisture conditions favor germination and as the crop emerges. Correctly identifying weeds 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 more difficult to identify. However, weed control decisions must be made early, on the basis of identifying weed seedlings. For help in identifying weed seedlings, view photos of common winter annual, summer annual, and perennial weed seedlings. Properly identify weeds; misidentification could reduce or eliminate the efficacy of weed control practices.
Monitor for weeds when they are expected to emerge.
The need for herbicide depends on weed numbers and species, their competitiveness and toxicity to livestock, and the potential market for the alfalfa. Vigor of the alfalfa stand is a complicating factor; weakened stands will require an herbicide application, whereas vigorously growing denser ones may not.
WEED MANAGEMENT BEFORE PLANTING
Prepare fields so drainage is adequate to prevent ponding or uneven irrigation. Avoid planting in fields that have serious perennial weed infestations. One option is to preirrigate and then apply an appropriate herbicide or cultivate emerged weeds before planting.
Time of Seeding
It is important to select planting time carefully; generally, fall (September–October) is the preferred time in the San Joaquin and Sacramento valleys with February and March being the next best window. Late August is the preferred time in the intermountain area for a fall planting and April to mid-May for spring seeding. Fields planted in spring can be seriously infested by summer annual weeds. Alfalfa seed planted too late (November–December) will germinate and grow slowly, allowing winter weeds that grow faster to become established before alfalfa reaches a size that can be safely sprayed with an herbicide.
If a spring seeding is planned, a planting date of February through March in the Central Valley can reduce problems with winter annual weeds.. However, planting too late in spring can require irrigation, causing summer grasses to become an issue. Planting after November usually results in lower yields than planting in September through October.
If fields infested with field bindweed, perennial grasses, or nutsedge must be used, plant in early fall while these weeds are dormant and less competitive, or consider planting Roundup Ready alfalfa and manage with glyphosate. This will ensure that alfalfa is established and vigorous when these perennials start regrowing again in spring.
Depth of Planting
Depending on the soil type and texture, plant seeds shallowly into a firm seedbed (no deeper than 0.5 inch) to provide both proper soil-seed contact and encourage timely alfalfa seed germination. Seeds placed too shallowly may dry out and die, or they may develop poor roots. Seeds planted too deeply may be unable to reach the surface to emerge after germination or may have reduced vigor.
One of the few tools for weed management in organic production is the use of higher rates of seeding for competitiveness against weeds. Plant between 15 to 25 pounds of seed per acre, or slightly higher when seedbed conditions are sub-optimal. The goal is 25 to 70 emerged plants per square foot. However, seeding methods (depth, soil-seed contact, and soil preparation) are more important factors affecting stand vigor and success than seeding rate; high seeding rates cannot make up for suboptimum soil preparation or poor seeding methods.
Interplanting Oats in Seedling Alfalfa
Planting a low density of oats with alfalfa can suppress weeds without the use of herbicides and reduce erosion during stand establishment. It is especially appropriate for organic production systems. The first cutting will have lower alfalfa content but, in combination with the harvested oats, will provide higher yields of forage than pure alfalfa stands. The next two cuttings are usually only slightly affected and there is no effect on later cuttings or stand life, provided the oat seeding rate is not too high (i.e., above 20 lb/acre).
Planting oats with alfalfa is often a two-step process because of the difference in seed sizes. Establish the stand at the normal alfalfa planting time. If possible, preirrigate and cultivate to eliminate weeds before planting. Oat seed is planted first. The ideal seeding rate for oat in California is 8 to 16 pounds per acre with the standard alfalfa seeding rate. Most standard grain drills cannot plant oats at this low rate, so broadcast and incorporate oats with a disc or springtooth harrow. Do not apply nitrogen; it may make the oats too competitive.
A short, midseason oat variety works best with an early fall planting because it matures with the alfalfa and is less likely to lodge and reduce alfalfa growth. In spring plantings, an early maturing variety produces more growth by the time alfalfa is ready to cut.
The first cutting may be mostly oats. Curing time for the first cutting may be several days longer than that for pure alfalfa. For additional information on interplanting oats in alfalfa, see Overseeding and Companion Cropping in Alfalfa, UC ANR Publication 21594.
If a field has a history of weeds, an application of a preplant herbicide may be an option, but most weed control programs currently focus on postemergence herbicides instead. This approach allows better assessment of the weed numbers and species to select the appropriate herbicide or herbicide combination for the weeds that are present. To control most emerged annual and perennial weeds, a postemergence herbicide can be applied either before planting or before crop emergence, depending on the particular herbicide being used. If postemergence herbicides are to be used before planting, preirrigate to germinate the weed seeds.
If a preplant incorporated herbicide is preferred, such as EPTC (Eptam), it should be applied to the soil surface and mechanically mixed into the soil before planting the alfalfa seed. If a disc or a ground-driven tiller is used to incorporate herbicides, work the soil to double the desired depth of incorporation; power tillers incorporate to the set depth. To be effective, preplant herbicides require soil moisture.
The use of 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 HERBICIDE-TOLERANT VARIETIES AS A WEED MANAGEMENT STRATEGY.
WEED MANAGEMENT AFTER PLANTING
Preemergence herbicides for use after alfalfa emergence must be water-incorporated by sprinklers or rainfall. Preemergence herbicides used postplant do not normally persist in the soil longer than 6 weeks under cropping conditions; cool soil in winter can prolong activity by a few weeks. Be sure to check labels for grazing and harvesting restrictions.
By far the most common means of controlling weeds in seedling alfalfa is to apply postemergence herbicides after the crop and weeds have emerged. The economic returns of an appropriate, well-timed postemergence herbicide application are usually realized in the first harvest alone and may continue with additional harvests.Timing application in relation to the crop plant size (see below) and weed size is very important; best results are obtained when weeds are small (cotyledon to 2nd leaf) and growing vigorously. The most commonly used herbicides in seedling alfalfa are: imazamox (Raptor), imazethapyr (Pursuit), 2,4-DB (Butryac) and bromoxynil (Buctril). These herbicides are used alone or in tank-mix combinations depending on the weed species present. The selective grass herbicides clethodim (Select Max) or sethoxydim (Poast) are often used if grassy weeds are present. Use selective grass herbicides alone or in combination with the broadleaf herbicides mentioned above. The risk of crop injury may be increased with some tank mixes when adding an oil adjuvant in high temperatures. See the table below to learn about some of the characteristics of postemergence herbicides, application timing, and phytotoxicity symptoms.
Clipping, Cutting, and Grazing
Avoid early forage removal unless weeds are dense enough to overtop (shade) and smother the alfalfa and cause stand loss. If weeds do become established and shade out the young alfalfa stand, the weedy alfalfa mix can be swathed early to remove the weeds and promote sunlight penetration to the young alfalfa. Clipping the top two-thirds of the canopy under weedy conditions may assist newly established alfalfa seedlings because cutting inhibits growth of most broadleaf weeds, which allows alfalfa regrowth to compete more successfully. Use caution when removing the forage early so that swaths do not damage young plants and wheel traffic does not compact wet soil.
In many areas, sheep grazing is used to remove winter annual weeds from new alfalfa plantings. Grazing can be worthwhile if these requirements are met:
For more information on weed management in alfalfa, see Irrigated Alfalfa Management for Mediterranean and Desert Zones, UC ANR Publication 3512.
UC IPM Pest Management Guidelines: Alfalfa
Weeds in Seedling Alfalfa
W. M. Canevari, UC Cooperative Extension, San Joaquin County (Emeritus)
Acknowledgment for contributions to Weeds:C. E. Bell, UC Cooperative Extension, San Diego County (Emeritus)
W. T. Lanini, Weed Science and Plant Sciences, UC Davis (Emeritus)
R. F. Norris, Vegetable Crops and Weed Science, UC Davis (Emeritus)
J. L. Schmierer, UC Cooperative Extension, Colusa County (Emeritus)
R. N. Vargas, UC Cooperative Extension, Madera County (Emeritus)
R. G. Wilson, Intermountain Research & Extension Center
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