How to Manage Pests

UC Pest Management Guidelines

Uncontrolled weeds in seedling alfalfa can cause loss of the stand during crop establishment. Weed infestations can weaken young alfalfa plants, retard growth, and delay the first cutting.

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. Alfalfa that germinates and grows rapidly in response to warm temperatures, adequate soil moisture, and shallow planting will develop into a competitive and relatively weed-free stand. Adequate soil fertility, especially phosphorus, is also essential in establishing and maintaining a vigorous stand. Plant alfalfa varieties that are well adapted and grow vigorously in your climate and soil.

Planting alfalfa in rows on beds or on shallow corrugations is commonly practiced in areas where soils lack good drainage, such as heavy clay soils, or on those with a high salt content. Possible weed problems associated with this practice include (1) increased weed populations in the furrows and (2) 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 ground is not covered completely by the crop canopy.

MONITORING
Start looking for weeds when the crop germinates. 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 difficult to identify. However, ordinarily weed control decisions must be made quickly, 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.

Monitor for weeds when they are expected to emerge. In the Central Valley 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 and can continue to germinate until midsummer with each irrigation. Record observations on a monitoring form (100KB, PDF).

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 a complicating factor; weakened stands will require treatment when denser ones don't.

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. Preirrigate and then cultivate the germinating weeds before planting. This procedure may be repeated several times when topsoil is heavily infested with weed seeds.

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, whereas late August is the preferred time in the intermountain area for a fall planting and April for spring seeding. Fields planted in summer can be seriously infested by weeds that germinate in summer. Alfalfa planted too late (December) will germinate and grow slowly, allowing winter weeds to become well established before alfalfa can be safely treated with an herbicide.

In areas where soil type allows, choosing a late winter/early spring planting date (February-March) in the Central Valley can reduce problems with winter annual weeds because the weeds can be removed before the seedbed is established. Planting too late in spring, however, can allow summer grasses to become established in seedling alfalfa if preplant herbicides are not used.

If fields infested with field bindweed, perennial grasses, or nutsedge must be used, plant in early fall. This will ensure that alfalfa is established and vigorous when these perennials start growing in spring.

Depth of Planting. Depending on the soil type, plant seeds into a firm seedbed about 0.25 inch in depth to provide for both proper soil-seed contact and 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.

Seeding Rate. One of the few tools for weed management in organic production is the use of higher rates of seeding for competitiveness against weeds.

Interplanting Oats in Seedling Alfalfa. Planting oats with alfalfa can suppress weeds without the use of herbicides and reduce erosion during stand establishment. 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 affected only slightly and there is no affect on later cuttings or stand life, providing 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 such as Montezuma produces more growth by the time alfalfa is ready to cut.

The first cutting will be mostly oats with a small amount of alfalfa. 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 Planting in Alfalfa, UC ANR Publication 21594.

Herbicides. If a field has a history of weed populations, consider making a treatment with a preemergent herbicide. Apply a preplant, incorporated herbicide such as benefin (Balan) or EPTC (Eptam) to the soil surface and mechanically mix into the soil before planting the alfalfa seed. If a disk 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. To control most emerged annual and perennial weeds, a postemergent herbicide can be applied either before planting or before crop emergence, depending on the particular herbicide being used. If postemergent herbicides are to be used, preirrigate to germinate the weed seeds. (view photos of weeds not controlled by herbicides in conventional alfalfa.)

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.

WEED MANAGEMENT AFTER PLANTING

Preemergent herbicides for use after alfalfa emergence must be water-incorporated by sprinklers or rainfall. Preemergent 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.

The most common means of controlling weeds in seedling alfalfa is to apply postemergent herbicides after the crop and weeds have emerged. The economic returns of an appropriate, well-timed postemergent herbicide application can often be realized in the first harvest alone and may continue with additional harvests.Timing application in relation to the crop size (Fig. 1) and weed size is very important; best results are obtained when weeds are small (cotyledon to 2nd leaf) and growing vigorously. The table below summarizes some of the characteristics of postemergent herbicides, application timing, and phytotoxicity symptoms.

Figure 1. Seedling Alfalfa Treatment Stage.

¹ Adapted from Canevari, W. M. et. al. 2002. Postemergence Weed Control in Seedling Alfalfa and Phytotoxicity Symptoms, UC ANR Publication 21615.

Cutting and Grazing. If weeds do become established, a young alfalfa stand can be cut close to the ground and the forage and weeds removed. Cutting inhibits growth of most weeds, especially broadleaves, allowing alfalfa regrowth to compete more successfully. Use caution when removing the forage early so that wheel traffic does not damage young plants and compact wet soil. Young alfalfa plants should have at least three to four stems per crown and be at least in early flower before the first cutting because younger plants have low root reserves. Early forage removal is not generally recommended unless weeds are dense enough to overtop the alfalfa and cause stand loss.

In the low desert, sheep grazing is used to remove winter annual weeds from new alfalfa plantings. Grazing can be worthwhile if these requirements are met: (1) the alfalfa is large enough (three to four stems) to prevent sheep from pulling the crop from the ground; (2) the soil is dry and a corral is available in case of rain; and (3) the sheep are left long enough to eat all the weeds.

PUBLICATION

UC IPM Pest Management Guidelines: Alfalfa
UC ANR Publication 3430
Weeds in Seedling Alfalfa
W. M. Canevari, UC Cooperative Extension, San Joaquin County

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