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Early Season Weed Control in Alfalfa

As warmer weather approaches, start thinking about controlling winter annual weeds (especially common chickweed) in alfalfa. Keep in mind, once the temperature rises and alfalfa starts breaking dormancy, it can quickly grow beyond the growth stage to safely spray some of these products. In most cases, 2-inches of alfalfa regrowth is the maximum height listed. Spraying beyond this stage may cause stand and/or yield reductions. Keep in mind all of these herbicides are labeled for use in pure-stand alfalfa. However, metribuzin, Pursuit, Prowl H2O, and Aim can be applied to established alfalfa-grass mixes. Below are a few guidelines about weed control options and available products (generic alternatives are available for some of these products listed):

Your best bet when controlling…

Chickweed — Gramoxone SL, Pursuit, Raptor, Metribuzin (if chickweed is Group2/ALS-resistant, Pursuit and Raptor will not control it.)

Annual bluegrass — Gramoxone SL

Henbit/deadnettle — Gramoxone SL, Metribuzin

Yellow rocket and other mustards — Pursuit, Raptor, Metribuzin, Aim (Butyrac is most effective on wild mustard, pepperweed, pennycress, and shepherdspurse; but not as active on yellow rocket.)

Annual fleabane — Pursuit, Raptor, Metribuzin, Gramoxone SL, Butyrac (however, these will only provide 70-75% suppression, no alfalfa herbicides are very effective on this weed.)

Palmer amaranth/waterhemp — Gramoxone SL+ Warrant, Prowl H2O, or Chateau after 1st or 2nd cutting (pure stand alfalfa only)

Common groundsel — control of groundsel is very difficult once it’s large and starts to flower in early spring; Gramoxone, Velpar, Chateau, and Sharpen have activity on this weed but application timing is critical for both crop safety and weed control; Pursuit only provides suppression even at the highest use rate (6 fl oz/A)

Roughstalk bluegrass — various university studies suggest that after 1st cutting, Select/clethodim provides about 85% control, Raptor and Poast, 80% control, Gramoxone SL is about 75-80% effective, and Pursuit only provides suppression (55-60%) in alfalfa-grass stands. In Roundup Ready alfalfa varieties, effective control (90%) from glyphosate can be expected.

Herbicide Parameters

Gramoxone SL 3L (paraquat, Group 22) — may be applied at 0.7 to 1.33 pt/A to established "dormant" stands before 2 inches of spring regrowth. The weeds must be actively growing at the time of application. Gramoxone is also labeled at 0.7 to 1.33 pt/A for dormant application on new fall seeded stands. 0.7 pt/A can be applied between cuttings but no more than 5 days after harvest and before 2” of regrowth. Gramoxone will desiccate any green tissue including actively growing alfalfa and possibly cause stand and/or yield reductions. However, if chickweed infestations are thick, this may be a necessary option to help suppress it. Be especially cautious with new fall seedings. Do not tank-mix with metribuzin on newly seeded (less than 1 year old) alfalfa. Gramoxone is effective on low to moderate infestations of winter annuals including chickweed, henbit, deadnettle, and mustard species. Do not use on mixed stands.

Pursuit 2AS (imazethapyr, Group 2) — Pursuit may be used for weed control in seedling (2 trifoliates or larger) or established alfalfa. Apply Pursuit at 3 to 6 fl oz/A plus adjuvants to actively growing weeds 1 to 3 inches in height. Pursuit performance improves with warmer temperatures. If spraying during extended cold periods, expect weeds to respond slower or the herbicide can have reduced activity. Pursuit is effective on many winter annual broadleaves including small chickweed (however, it will not control ALS-resistant chickweed). Pursuit may be used on established alfalfa-grass mixtures.

Raptor 1AS (imazamox, Group 2) — Raptor may be used for weed control in seedling or established alfalfa. Apply Raptor at 4 to 6 fl oz/A plus adjuvants to small, actively growing weeds and to established alfalfa in the fall or in the spring. Any application should be made before significant alfalfa growth or regrowth (3 inches) to allow Raptor to reach the target weeds. Like Pursuit, Raptor performance is influenced by temperature. Therefore, try to apply the herbicide on warmer days and when weeds are actively growing. Raptor has a similar spectrum of winter annual weed control as Pursuit. Do not use on mixed stands.

Metribuzin 75DF (Dimetric, Glory, MetriCor, etc. Group 5) — may be used on established alfalfa. Apply 0.5 to 1 lb/A before spring regrowth. Impregnation on dry fertilizer can improve crop safety and allow for slightly later applications (up to 3 inches spring regrowth). Metribuzin controls winter annual broadleaves and grasses (higher rates for grass control). May be used on mixed alfalfa-grass stands. Higher rates can potentially cause a reduction in grass stand.

Prowl H2O 3.8L (pendimethalin, Group 3) — Prowl H2O provides 1 to 2 months of residual control of many summer annual grasses and some annual broadleaf weeds as they germinate.  It is typically not as effective on winter annual weeds such as annual bluegrass, chickweed, and mustards. It will not control any weeds that have already emerged at the time of application. Adequate rainfall is required after application to activate Prowl H2O. Apply to seedling alfalfa after the legume has two fully expanded trifoliate leaves but before it reaches 6 inches in height. Established alfalfa is defined by the label as alfalfa planted in the fall or spring which has gone through a first cutting/mowing. Apply to established alfalfa before weed emergence. Applications can be made in the fall after the last cutting, during winter dormancy, in the spring, or between cuttings. Applications should be made prior to the alfalfa reaching 6 inches in regrowth. Some stunting and yellowing of the alfalfa may occur with post applications. (Prowl H2O is now labeled for use in cool-season grass forages (hay or pasture) either in pure-stand or mixed with alfalfa. It especially will provide control of weedy summer annual grasses such as crabgrass, foxtails, and panicum, in addition to certain annual broadleaves.)

Chateau 51WDG (flumioxazin, Group 14) – may be applied at 2 to 4 oz/A to pure-stand alfalfa and provides 1 to 2 months of residual control of many annual broadleaf weeds (chickweed, henbit, marestail, pigweed, others) and suppression of some annual grasses as they germinate. It will not control weeds that have already emerged at the time of application. Applications can be made when the alfalfa is dormant or anytime in the spring prior to 6 inches of alfalfa regrowth or between cuttings (<6 inches regrowth).

Sharpen 2.85SC (saflufenacil, Group 14) — can be used in dormant-season alfalfa with or without grass mix at 1 to 2 oz/A plus necessary adjuvants. Sharpen is effective on marestail and mustards, but will suppress chickweed, henbit, dandelion, and some other broadleaf weeds. The label states it must be applied 28 days before harvest.

Aim 2EC (carfentrazone, Group 14) – can be used in forages (grass pastures/hay and alfalfa /clover ± grass mix) for post control of certain broadleaves such as mustards, lambsquarters, velvetleaf, pigweed, and Star-of-Bethlehem; but is weak on chickweed, marestail, thistles and has no grass activity. The typical use rate is 1 -2 fl oz/A plus necessary adjuvant. Apply Aim during dormancy or to established stands in spring or summer after first cutting or before 2 inches of new crop growth. Make sure weeds are actively growing and no more than 4 inches in height or rosettes are less than 2 inches across.

Warrant 3CS (acetochlor; group 15) can be used in seedling (≥4 alfalfa trifoliates) and established alfalfa (during dormancy, after spring green-up, or between cuttings) to provide residual control of annual grasses and broadleaf weeds at a rate of 1.25 to 2 quarts/A. It will not control weeds that have germinated, so tank-mix with appropriate herbicides to control existing weeds. Allow 20 days between application and forage cutting. It should not be used in alfalfa-grass mixes.

Butyrac, Maestro, and MCPA can be used in seedling alfalfa for control of several broadleaf weeds.

  • Butyrac (2,4-DB) can also be used in established alfalfa, but it does not have a label for use in alfalfa-grass mixes.
  • Maestro can be used in alfalfa-small grain companion seedings but is not labeled for use in established stands.
  • MCPA (certain products) can be applied in seedling alfalfa (or clover, trefoil) that contains a companion seedling of small grains or forage grasses. The use rate is 0.25-0.5 pints/acre. Treat after the grain is tillered and legume is 2-3 inches tall. MCPA is not labeled for application in established alfalfa stands.

On a side note: be on the lookout for weeds like poison hemlock and Japanese stiltgrass especially in grass pastures and along field edges.

  • Poison hemlock: Now is a good time to start controlling them. If poison hemlock rosettes are evident now, herbicide applications are most effective when they are sprayed in the rosette stage and before it bolts in the spring. Effective herbicides include 2,4-D + dicamba, Crossbow (2,4-D+triclopyr), or glyphosate as a spot treatment.
  • Japanese stiltgrass: It germinates in early spring. Currently, pendimethalin (Prowl H2O and Satellite HydroCap) is an option to consider for controlling certain annual grasses like Japanese stiltgrass and broadleaf weeds in cool and warm season forage grass settings. These herbicides must be applied before weed germination in spring, or in-season between cuttings, otherwise weeds will not be controlled.
Source : psu.edu

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