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Extension > Agriculture > Crops > Forage Production > Pest management > Using herbicides to establish alfalfa

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Using herbicides to establish alfalfa

Roger Becker

Alfalfa seeded with a companion crop is a traditional practice that has served Minnesota producers well. Many today seed alfalfa with forage grasses to potentially increase in vitro digestible dry matter yield. More intense management of alfalfa often uses direct seeding (sometimes termed "solo" seeding). The use of herbicides is generally most adapted to direct seeded alfalfa. Few herbicide options are available for application to alfalfa seeded with a companion crop or when seeded with forages grasses. Additionally, more selective options are available specific to individual weed species or groups of weed species for direct seeded alfalfa. Conversely, fewer weeds encroach during establishment when alfalfa is seeded with forage grasses, or with companion crops that are harvested for silage. In fields with high weed pressure, the slow initial growth and vigor of some forage grasses can leave an opening for annual grass and broadleaf weeds to invade. If severe competition occurs, no herbicide can selectively remove the annual grasses and not severely injure, or completely kill the forage grass seedlings. Similarly, no herbicides are labeled to remove broadleaf weeds in alfalfa that is seeded with forage grasses. Confusion arises because bromoxynil (Buctril, Moxy) is labeled to apply to CRP acres where alfalfa may be present with forage grasses. Forage grasses in these labeled CRP sites are not cleared for feeding to livestock.

Oat–mulch system

We conducted extensive research to explore the use of grass active herbicides to remove oat companion crops in the 1990s in Minnesota. We termed this an "oat mulch" system where by companion crops could be used to protect alfalfa seedlings from wind and water driven soil erosion and reduce weed encroachment during alfalfa establishment. Oat seeded with alfalfa was removed with sethoxydim (Poast, others) or clethodim products (Select, others) to mimic direct seeding while minimizing soil loss potential. By the mid–1990's, imazethapyr (Pursuit) followed by imazamox (Raptor) were labeled to remove oat, and more recently glyphosate tolerant lines (Roundup, others) are available for oat mulch systems. Using oat mulch systems make it possible to establish alfalfa with an oat companion crop in areas where erosion potential is a concern, yet oat competition is removed before long–term stand or vigor damage to alfalfa occurs. This technique also works well on sandy soil to prevent damage from blowing sand common when alfalfa is direct seeded on these soils. It is recommended that 1 to 1.5 bu/A bin–run oat seed be used to reduce costs. Higher seeding rates than 1.5 bu/A can increase the protective mulch cover and compete more effectively with weeds in severe conditions, but generally are not needed.

Oat should be sprayed, targeting oats that are 4 to 6 inches in height, before considerable competition has occurred. If Raptor or Pursuit are used, move the application timing up to 4–inch oat as these products are slower to control oat, and under dry conditions, may not provide complete control of oat, especially when applied to oat taller than 4 inches. Oat is recommended because other small grains are not as easily controlled, and therefore may be more expensive to remove. Forage yield may be increased the establishment year by seeding a companion crop and harvesting it for silage. However, companion crops can compete with seedling alfalfa and like weeds, may impact alfalfa production before companion crops are harvested and in subsequent years. Direct or oat mulch seeding increases the production of alfalfa forage that is high in quality the establishment year. Although overall annual tonnage may be reduced, yields often are similar to those obtained with companion crops seeded and harvested for silage. Direct seeding can also result in increased alfalfa vigor and population density during establishment, especially when the alfalfa seedlings are stressed such as occurs when droughty during establishment. The rest of this article is a summary of herbicides labeled to establish alfalfa.

DISCLAIMER: The label on the product you are using always supersedes information in this article. Always refer to the product label for specific instructions and restrictions.

Preplant incorporated control of annual grasses and some small–seeded broadleaf weeds

Balan (benefin)

Balan preplant–incorporated gives good control of annual grasses and fair to good control of pigweed and kochia. Wild oats, common ragweed, and wild mustard control is not adequate. Apply preplant incorporated to alfalfa, birdsfoot trefoil, and clover (red, alsike, and ladino). Use 2 lbs dry flowable/A on coarse and medium soils and 2.5 lbs dry flowable on fine–textured soils. It is generally ineffective on peat or muck soils. No preharvest interval (PHI) stated.

Eptam (EPTC)

Eptam should be applied preplant and incorporated immediately after application for annual grass and some broadleaf weed control in alfalfa, birdsfoot trefoil, and clovers (red, alsike, and ladino). The 3.5 to 4.5 pt rates give fair to good control of pigweed, lambsquarter, and velvetleaf. Wild oats and wild mustard generally are suppressed. The 2.25 pt rate may be used on alfalfa for annual grass control. EPTC rates are dependent on soil type, generally ineffective on peat or muck soils. Some alfalfa stand reduction and stunting may occur. First emerging leaves may stick together but seedlings usually recover after the first rain or irrigation. May be impregnated or applied in liquid fertilizer. Do not use on alfalfa if any atrazine has been applied the previous 12 months or serious injury may result. No PHI stated.

Trust 4EC (trifluralin)

Supplemental label on AgriSolutions Trust formulation. Apply 1 to 1.5 pts/A preplant–incorporated to direct seed alfalfa. Rate is dependent on soil type. Some alfalfa stand reduction and stunting may occur. Trifluralin products are more economical and available than Balan, have a similar spectrum of weed control, but have slightly more injury potential than Balan. Most trifluralin formulations are not labeled for alfalfa establishment. 21 day PHI. Follow the PHI for applications to established alfalfa on the full label attached to the product container. The supplemental label for Trust does not does not state a PHI.

Postemergence control of annual grasses and broadleaf weeds and oats in oat–mulch systems

Roundup Ready Alfalfa

Roundup WeatherMAX, Roundup PowerMax provide rate–dependent, excellent control of seedling weeds and suppress many perennial weeds with minimal preharvest intervals. See label for species-specific use rates. Not all glyphosate formulations are labeled for use on Roundup Ready alfalfa. Up to 10% non-tolerant seed may be present so label states to apply 22 to 44 oz/A when newly seeded alfalfa is at or before the 3 to 4 trifoliate stage to avoid gaps from stand loss of susceptible alfalfa once plants are larger. This relative early application may require a second application to control weeds that may emerge after Roundup application, but before crop canopy closure. For this later application, apply 44 oz/A after the 5th trifoliate stage. Some growers compromise and apply 44 oz/A glyphosate at the 3 to 5 trifoliate stage, reducing the chance that a second application will be needed to control a new flush of weeds while minimizing the issue created with removing larger susceptible plants. Sequential applications must be at least 7 days apart. May apply up to 5.3 qts of 4.5 lb ae/gal formulations per year including any preplant burndown applications, up to 4.1 qts per year in crop, up to 44 fl oz. product in any single application. This is a transgenic crop. There is a technology fee estimated to be approximately $30 to $40 per acre. Applications to non-tolerant Roundup Ready alfalfa will cause severe crop damage or stand loss. 5 day PHI. Remove livestock before application, and a 5 day grazing reentry interval applies. Any in–crop applications must be at least 5 days before cutting.

Other legumes and non–Roundup Ready alfalfa. Glyphosate (Roundup, others)

Many glyphosate products can be used in clovers, vetches, trefoil and alfalfa NOT tolerant to glyphosate applied preharvest to remove a stand and in so doing, suppress or control perennial weeds. An example would be an older stand or in the spring after winter–kill is apparent, when stands are thinning and weeds are encroaching. For preharvest applications, the maximum rate is 44 oz/A (4.5 lb ae formulations) with a minimal preharvest interval before grazing or harvesting alfalfa of 36 hours (36 hour PHI). For other legumes, the maximum application rate is 32 oz/A and the minimum grazing or harvest interval is three days (3 day PHI). Glyphosate products can also be used before– or at–planting of these legumes as a burndown such as in no–tillage or reduced–tillage systems.

Postemergence dormant and between–cuttings control of annual grasses and broadleaf weeds

Gramoxone Max SL (paraquat)

Rates and preharvest interval vary by age of the alfalfa stand, and dormant vs. between cutting applications. See label for specific details. Apply 1.0 pt/A to first year alfalfa within 5 days after cutting before alfalfa regrowth reaches 2 inches. Weeds must have adequate foliage remaining to be controlled since it is a contact herbicide. Can also be applied to fall seeded alfalfa (stands less than 1 year old) when dormant in the fall or following spring targeting winter annual weeds at 1.0 to 2.0 pt/A. Paraquat is a non–selective contact burn herbicide. Alfalfa foliage present at application will be burned. May be useful where late flushes of annual weeds occur, and for weeds where significant growth is below the cutting height. Will desiccate chickweeds, mustards, and seedling dandelions. Paraquat can also be used at 2.5 to 4.0 pts/A 3L at planting before crop emergence such as no–till seedings to burndown emerged weeds. Can also be applied to clovers, see label for specific use rates. Restricted use herbicide. In–crop applications, 30 day PHI except for fall applications, then the PHI is 60 days. Preplant burndown applications, 70 day PHI.

Pursuit (imazethapyr)

Pursuit should be applied postemergence when seedling alfalfa or clovers are greater than the second trifoliolate stage and the majority of the weeds are 1 to 3 inches in height. Controls many annual grass and most broadleaf weeds in direct–seeded or oat–mulch systems. Will suppress some perennial weeds. Apply 3 to 6 oz/ A Pursuit 2S and vary rate by weed species. Use a nonionic surfactant with greater than or equal to 80% active ingredient at the rate of 1 quart per 100 gallons of spray solution. Organo–silicon surfactants can be used instead of nonionic surfactants. Crop oil concentrate (COC) can be used at the rate of 1.25% v/v, or methylated seed oils (MSO) at 1% v/v. Use MSO when moisture stress is present. Always add UAN nitrogen at 1.25 to 2.5 gal/100 gallons of spray, or 12 to 15 lbs ammonium sulfate/100 gallons spray solution. Apply in 10 or more gpa carrier (ground) or 5 or more gpa (air). Imazethapyr can be applied to suppress oat at the 3– to 4–leaf stage to achieve direct seeded alfalfa or clovers in oat–mulch / interseeded oat systems. If replanting is necessary, do not replant for 4 months following Pursuit applications. Imazethapyr can be tank mixed with 2,4–DB, bromoxynil, clethodim, or sethoxydim. See the imazethapyr label and tank mix product labels for specific recommendations. Do not apply more than 4 oz./A in Minnesota north of Hwy 210. Do not apply Pursuit or Raptor sequentially within 60 days of each other due to increased potential for alfalfa injury. 30 day PHI before graze, feed, or harvest forage.

Raptor (imazamox)

Raptor can be applied postemergence at 4 to 6 fl oz/A. Apply when seedling alfalfa is in the second trifoliolate stage or larger and before weeds are greater 3 inches in height. A few sensitive weeds such as eastern black nightshade and redroot pigweed are labeled up to 5 inches in height at higher rates of Raptor. Controls annual grass and broadleaf weeds and will suppress some perennial weeds. Use a nonionic surfactant with greater than or equal to 80% active ingredient at the rate of 1 quart per 100 gallons of spray solution. Organo–silicon surfactants can be used instead of nonionic surfactants. Or, add crop oil concentrate (COC) or methylated seed oils (MSO) at 1–2% v/v, or a high surfactant oil concentrate (HSOC) at 0.5% v/v. Use MSO when moisture stress is present. Always add UAN nitrogen at 1.25 to 2.5 gal/100 gallons of spray, or 12 to 15 lbs ammonium sulfate/100 gallons spray solution. Can be tank mixed with bromoxynil, 2,4–DB, sethoxydim, or clethodim. Interseeded oat or suppression in oat–mulch systems is not mentioned on the Raptor label, but control of volunteer oat up to 3 inches in height or less is labeled. A 3–month restriction is required before replanting alfalfa back into the stand. No PHI. There are no grazing or haying restrictions with Raptor on any crop. Do not apply Pursuit or Raptor sequentially within 60 days of each other due to increased potential for alfalfa injury.

Postemergence control of annual grasses and some small–seeded broadleaf weeds

Prowl H2O (pendimethalin)

Prowl H2O can be applied postemergence at 1.1 to 2.1 pts/A to seedling alfalfa grown for hay/forage. Seedling alfalfa is defined on the label as planted in the spring or fall and has not gone through a first cutting/mowing. Higher rates can be used on established alfalfa. Will control or suppress many annual grasses, and some small–seeded annual broadleaf weeds. Apply after alfalfa has reached the 2nd trifoliate stage to avoid injury but before alfalfa reaches 6 inches in height to avoid reduced weed control. Some stunting and chlorosis of alfalfa may occur. Postemergence applications require considerably more rainfall or irrigation after application for activation and are more susceptible to photodegradation and volatilization losses compared to preplant incorporated use in other crops and as such, control will be less complete than where Prowl can be incorporated. 28 day PHI for 2.1 pt/A or less for forage or hay.

Postemergence control of annual broadleaf weeds

Buctril 2L, Buctril 4 EC, others (bromoxynil)

Buctril controls most broadleaf weeds postemergence in seedling alfalfa. Apply 1 to 1.5 pts 2L or 0.5 to 0.75 pt 4EC formulations/A to seedling alfalfa in the fall or spring when the majority of the field has a minimum of 4 trifoliolate leaves. Unacceptable injury may occur to alfalfa in the 2 trifoliolate stage or smaller, typical with uneven stands or under weather conditions favoring leaf burn. Broadleaf weeds should not exceed 2 inches in height, the 4–leaf stage or 1 inch in diameter, which ever comes first. The use of EPTC prior to bromoxynil applications may enhance alfalfa leaf burn. Use the low rate to control eastern black nightshade, cocklebur, lambsquarter, shepherd's purse, pennycress, smartweeds and wild buckwheat. Use the high rate of bromoxynil to control redroot pigweed, velvetleaf, ragweed, kochia, and wild mustard. Tank mix the low rate of bromoxynil with 1 qt/A 2,4–DB amine to improve kochia and pigweed control, but the more restrictive 2,4–DB 60 day grazing restrictions apply. Reduced rates of bromoxynil (0.5 to 0.75 pt 2L/0.25 to 0.38 pt 4 EC) can also be tank mixed with the equivalent of 3 to 6 oz/A Pursuit 2S for broad spectrum broadleaf weed control. Do not apply to alfalfa under moisture, temperature, insect or disease stress or that has been stressed by other pesticide carryover or injury. Injury may occur if air temperature exceeds 80°F at or 3 days following application. Experiences in Minnesota have shown alfalfa injury associated with spring temperatures that are abnormally cold, or abnormally hot. Do not add surfactants or crop oil concentrate. 30 day PHI before cutting, feeding, or grazing spring applications, 60 day PHI for fall applications.

Butyrac 200 (2,4–DB)

Butyrac 200 control certain annual broadleaf weeds postemergence. Butyrac can be used to establish alfalfa or birdsfoot trefoil. Does not specify legume height at application, Apply 1 to 3 qt/A when weeds are less than 3 inches in height. Can add 0.25% v/v nonionic surfactant in dry, low humidity areas in seedling alfalfa. Can be tank mixes with Poast or Buctril. Do not apply when alfalfa is drought stressed, or if temperatures are expected to exceed 90°F within three days after treatment. Do not add any wetting agents or detergents unless directed on the label. Do not apply to forage grass/legume mixtures, except on Conservation Reserve Program government acres in which case, treated forage cannot be grazed or harvested if released. 60 day PHI.

Postemergence control of annual grasses and oats in oat–mulch systems

Poast, Poast Plus (sethoxydim)

Poast applied postemergence will control annual and suppress perennial grasses in direct–seeded or oat–mulch systems. Sedges and broadleaf weeds will not be controlled. Poast can be used on alfalfa, clovers, birdsfoot trefoil, and sainfoin, Poast Plus only on alfalfa. Use the following pts/A of Poast or fl oz/A Poast Plus; 1/2 pt or 12 fl oz for wild proso millet, 1 pt or 24 oz for wild oat, foxtails, barnyardgrass, crabgrass, and fall panicum, and 1.5 pts or 36 fl oz for sandbur and volunteer small grains except interseeded oat, which requires only 0.75 pt or 18 fl oz. Sethoxydim will suppress, but not control quackgrass. Poast Plus is labeled at a reduced rate, 18 fl oz, and Poast at 0.75 pt/A for early application to control green and giant foxtail, barnyardgrass, and fall panicum. Always use with crop oil concentrate or Dash HC at 1 pt/A. Depending on weed species, UAN or ammonium sulfate may also be needed (see label). Insure good alfalfa canopy spray penetration and grass coverage. Applications before first cutting generally give the best results. Do not use flood, whirl chamber, or CDA nozzles (poor coverage or canopy penetration) or selective application equipment. Do not add UAN or ammonium sulfate to 2,4–DB tank mixtures. 7 day PHI before feeding, grazing or cutting for forage (undried), or 14 day PHI before cutting for hay (dried). May be tank mixed with 2,4–DB if the more restrictive 60 day feeding, grazing and harvesting restrictions of 2,4–DB are followed.

Select/Arrow 2 EC (clethodim)

Select/Arrow 2 EC applied postemergence will control annual grasses in seedling alfalfa, birdsfoot trefoil, or sainfoin grown for forage or seed. Apply 6 to 16 fl oz/A. Use the higher rates when annual grass pressure is heavy or at the maximum height, or when perennial grasses are present. Always add crop oil concentrate containing at least 15 % emulsifier at 1 % v/v. UAN or ammonium sulfate may be added to improve control. Clethodim has no activity on sedges or broadleaf weeds. May be tank mixed with 2,4–DB products, or with Pursuit W, 70 DG, or 70W to control broadleaf weeds. With tank mixes, the most restrictive harvest interval must be followed. Crop oil concentrate must still be added for clethodim performance which will increase the risk of crop injury if tank mixed with 2,4–DB. When tank mixed with Pursuit, the minimum rate of clethodim is higher; then use Select/Arrow at 8 to 16 oz/A. 15 day PHI before grazing, feeding, or harvesting (cutting) for forage or hay.

Companion crop seedings

Buctril 2L or 4EC, others (bromoxynil)

Buctril applied postemergence controls most broadleaf weeds in companion seeded alfalfa. Apply after small grain emergence up to boot stage, alfalfa should have a minimum of 4 trifoliolate leaves at application. Broadleaf weeds should not exceed the 4–leaf stage or 2 inches in height or 1 inch in diameter, whichever comes first. Offers much less alfalfa injury potential than MCPA or 2,4–D amine. Cleared for wheat, barley, oats, rye, and triticale underseeded with alfalfa. 30 day PHI before cutting for feed or grazing spring treated underseeded alfalfa; 60 day PHI for fall treated underseeded alfalfa. Do not apply when underseeded alfalfa is under moisture, temperature, insect or disease stress or has been stressed by other pesticide carryover or application. Do not add a surfactant or crop oil when underseeded with alfalfa. Do not apply if temperatures are expected to exceed 80°F at or 3 days following application or unacceptable alfalfa injury may occur. Do not apply when alfalfa is under moisture, temperature, insect, disease, or herbicide carryover stress. Do not apply the tank mixture with 2,4–DB when underseeded to small grains (2,4–DB not cleared for application to small grains).

MPCA

MCPA at 0.25 to 0.5 pt/A of the 4 lb ae formulations can be applied to seedling stands of alfalfa and clover (red, alsike or ladino) established with small grain companion crops for emergency control of mustards and other broadleaf weeds (check individual herbicide labels, not all products are labeled, labels vary on application instructions). Legumes are more tolerant of MCPA than 2,4–D, but do not use except for rescue situations to control severe infestations of broadleaf weeds that threaten legume seeding survival as serious legume injury can occur. Apply after the cereal crop is well tillered and 4 to 8 inches tall. The nurse crop and weeds should provide a canopy that will shield legumes from herbicide spray to reduce legume injury. Reduced sprayer pressure and lower spray volume to no more than 5 to 10 gpa to help minimize injury. Do not use ester formulations. Do not use on vetch or sweet clover, which are very sensitive to MCPA. Do not graze or forage meat animals on treated acres within 7 days of slaughter.

DISCLAIMER: The label on the product you are using always supersedes information in this article. Always refer to the product label for specific instructions and restrictions.

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