Using herbicides to establish alfalfa

Alfalfa seeded with a companion crop is a traditional practice that has served Minnesota producers well. Many seed alfalfa with forage grasses to potentially increase in vitro digestible dry matter yield.

More intense management of alfalfa often uses direct-seeding, sometimes called solo seeding. Using herbicides is generally most adapted to direct-seeded alfalfa.

Few herbicide options are available to apply 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 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’s seeded with forage grasses.

Confusion arises because bromoxynil (Buctril, Moxy) is labeled to apply to Conservation Reserve Program (CRP) acres where alfalfa may be present with forage grasses. Forage grasses in these labeled CRP sites are not cleared for feeding to livestock.

The oat mulch system

We conducted extensive research to explore using grass-active herbicides to remove oat companion crops in the 1990s in Minnesota. We called this an “oat mulch” system, in which companion crops could be used to protect alfalfa seedlings from wind- and water-driven soil erosion and reduce weed encroachment during alfalfa establishment.

We removed oat seeded with alfalfa with sethoxydim (Poast, others) or clethodim products (Select, others) to mimic direct seeding while minimizing soil loss potential. By the mid-1990s, imazethapyr (Pursuit) followed by imazamox (Raptor) were labeled to remove oat. More recently, glyphosate-tolerant lines (Roundup, others) became 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, while removing oat competition before long-term stand or vigor damage to alfalfa occurs. This technique also works well on sandy soil to prevent damage from blowing sand, which is common when alfalfa is direct-seeded on these soils.

Seeding and spraying guidelines

It’s recommended to use 1 to 1.5 bushels per acre of bin-run oat seed to reduce costs. Higher seeding rates than 1.5 bushels per acre can increase the protective mulch cover and compete more effectively with weeds in severe conditions. However, it’s generally not needed.

Oat should be sprayed, targeting oats that are 4 to 6 inches tall, before considerable competition has occurred. If using Raptor or Pursuit, move the application timing up to 4-inch oat. These products are slower to control oat and, under dry conditions, they may not completely control oat, especially when applied to oat taller than 4 inches.

Advantages

Oat is recommended because other small grains aren’t as easily controlled, and therefore may cost more to remove. Seeding a companion crop and harvesting it for silage may increase forage yield the establishment year.

However, companion crops can compete with seedling alfalfa. Like weeds, they 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’s 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 increase alfalfa vigor and population density during establishment, especially when the alfalfa seedlings are stressed, such as from droughty conditions during establishment.

Balan preplant incorporated gives good control of annual grasses and fair to good control of pigweed and kochia. Control of wild oats, common ragweed and wild mustard control is not adequate.

Application guidelines

• Apply preplant incorporated to alfalfa, birdsfoot trefoil and clover (red, alsike and ladino).

• Use 2 pounds of dry flowable per acre on coarse and medium soils and 2.5 pounds of dry flowable on fine-textured soils.

• Generally ineffective on peat or muck soils.

• No preharvest interval (PHI) stated.

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).

Application guidelines

• The 3.5 to 4.5 pint rates give fair to good control of pigweed, lambsquarter and velvetleaf.

• Wild oats and wild mustard generally are suppressed.

• You can use the 2.25 pint rate on alfalfa for annual grass control.

• EPTC rates depend on soil type, and are 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.

• It 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 preharvest interval (PHI) stated.

Trifluralin products are more economical and available than Balan and have a similar spectrum of weed control, but have slightly more injury potential than Balan. Most trifluralin formulations aren’t labeled for alfalfa establishment.

Application guidelines

• Supplemental label on AgriSolutions Trust formulation.

• Apply 1 to 1.5 pints per acre preplant incorporated (PPI) to direct-seed alfalfa. Rate depends on soil type.

• Some alfalfa stand reduction and stunting may occur.

Preharvest interval (PHI): 21 days. 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.

Roundup WeatherMAX and Roundup PowerMax provide rate-dependent, excellent control of seedling weeds and suppress many perennial weeds with minimal pre-harvest intervals.

Application guidelines

• See label for species-specific use rates.

• Not all glyphosate formulations are labeled for use on Roundup Ready alfalfa.

• Up to 10 percent non-tolerant seed may be present.

• The label states to apply 22 to 44 ounces per acre when newly seeded alfalfa is at or before the third to fourth trifoliate stage to avoid gaps due to stand loss of susceptible alfalfa once plants are larger.

Second applications

This relatively 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 ounces per acre after the fifth trifoliate stage.

Some growers compromise and apply 44 ounces per acre glyphosate at the third to fifth trifoliate stage. This reduces 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 seven days apart. You may apply:

• Up to 5.3 quarts of 4.5 pounds of acid equivalent (ae) per gallon formulations per year, including any preplant burndown applications.

• Up to 4.1 quarts per year in-crop, up to 44 fluid ounces of product in any single application.

Fees and PHI

This is a transgenic crop. There’s an estimated technology fee of approximately $30 to $40 per acre. Applications to non-tolerant Roundup Ready alfalfa will cause severe crop damage or stand loss.

The preharvest interval (PHI) is five days. Remove livestock before application, and a five-day grazing reentry interval applies. Any in-crop applications must be at least five days before cutting.

Many glyphosate products can be used in clovers, vetches, trefoil and alfalfa not tolerant to glyphosate. It’s applied preharvest only to remove a stand and, in so doing, it can suppress or control perennial weeds.

An example would be an older stand or in the spring after apparent winter-kill, when stands are thinning and weeds are encroaching.

Application guidelines

• Alfalfa: For pre-harvest applications, the maximum rate is 44 ounces per acre (4.5 pounds of ae formulations), with a minimal preharvest interval (PHI) before grazing or harvesting alfalfa of 36 hours (36-hour PHI).

Other legumes: The maximum application rate is 32 ounces per acre and the minimum grazing or harvest interval is three days (three-day PHI). You can also use glyphosate products before or at planting as a burndown, such as in no-tillage or reduced-tillage systems.

Paraquat is a non-selective, contact burn herbicide. Alfalfa foliage present at application will be burned.

It may be useful where late flushes of annual weeds occur, and for weeds where significant growth is below the cutting height. It will desiccate chickweeds, mustards and seedling dandelions.

Application guidelines

• 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 pints per acre to first-year alfalfa within five days after cutting, before alfalfa regrowth reaches 2 inches.

• Because it’s a contact herbicide, weeds must have adequate remaining foliage to be controlled. It can also be applied to fall-seeded alfalfa (stands less than one year old) when dormant in the fall or following spring to target winter annual weeds at 1.0 to 2.0 pints per acre.

• Paraquat can also be used at 2.5 to 4.0 pints per acre (3L formulation) at planting before crop emergence, such as no-till seedings to burn down emerged weeds.

• It can also be applied to clovers; see label for specific use rates.

• Restricted use herbicide.

Preharvest interval (PHI): 30-day PHI for in-crop applications (except for fall applications), 60-day PHI for fall applications and 70-day PHI for preplant burndown applications.

Apply Pursuit postemergence when seedling alfalfa or clovers are greater than the second trifoliolate stage and the majority of the weeds are 1 to 3 inches tall.

It controls many annual grass and most broadleaf weeds in direct-seeded or oat-mulch systems. Plus, it will suppress some perennial weeds.

Application guidelines

• Apply 3 to 6 ounces of Pursuit 2S per acre and vary rate by weed species.

• Use a nonionic surfactant with greater than or equal to 80 percent 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 percent volume/volume volume/volume (v/v), or methylated seed oils (MSO) at 1 percent v/v. Use MSO when there’s moisture stress.

• Always add urea-ammonium nitrate (UAN) at 1.25 to 2.5 gallons per 100 gallons of spray, or 12 to 15 pounds of ammonium sulfate per 100 gallons of spray solution. Apply in 10 or more gallons per acre (gpa) via a ground carrier or 5 or more gallons per acre via 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 or interseeded oat systems.

• Do not apply more than 4 ounces per acre north of Hwy 210 in Minnesota. Do not sequentially apply Pursuit or Raptor within 60 days of one another due to increased potential for alfalfa injury.

• Preharvest interval (PHI): 30-day PHI before graze, feed or harvest forage.

Replanting

If replanting is necessary, do not replant for four 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.

Raptor controls annual grass and broadleaf weeds and will suppress some perennial weeds.

Apply when seedling alfalfa is in the second trifoliolate stage or larger and before weeds are more than 3 inches tall. 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.

Application guidelines

• Apply Raptor postemergence at 4 to 6 fluid ounces per acre.

• Use a nonionic surfactant with greater than or equal to 80 percent 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 to 2 percent volume/volume (v/v), or a high-surfactant oil concentrate (HSOC) at 0.5 percent v/v. Use MSO when there’s moisture stress.

• Always add UAN nitrogen at 1.25 to 2.5 gallons per 100 gallons of spray, or 12 to 15 pounds of ammonium sulfate per 100 gallons of 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 three-month restriction is required before replanting alfalfa back into the stand.

• Do not sequentially apply Pursuit or Raptor within 60 days of one another due to increased potential for alfalfa injury.

No preharvest interval (PHI) stated. There are no grazing or haying restrictions with Raptor on any crop.

Buctril controls most broadleaf weeds postemergence in seedling alfalfa. Broadleaf weeds shouldn’t exceed 2 inches in height, the 4-leaf stage or 1 inch in diameter, whichever comes first.

Application guidelines

• Apply 1 to 1.5 parts of the 2L formulation or 0.5 to 0.75 pints of the 4EC formulation per acre to seedling alfalfa in the fall or spring when the majority of the field has at least four trifoliolate leaves.

• Unacceptable injury may occur to alfalfa in the second trifoliolate stage or smaller, typical with uneven stands or under weather conditions favoring leaf burn.

• Using 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 to control redroot pigweed, velvetleaf, ragweed, kochia and wild mustard.

• Tank-mix the low rate of bromoxynil with 1 quart per acre 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 pints of 2L or 0.25 to 0.38 pints of 4EC) can also be tank-mixed with the equivalent of 3 to 6 ounces per acre of 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 degrees Fahrenheit at or three days after application.

• Experiences in Minnesota have shown alfalfa injury associated with abnormally cold or abnormally hot spring temperatures.

• Do not add surfactants or crop oil concentrate.

• Preharvest interval (PHI): 30-day PHI before cutting, feeding or grazing spring applications, and 60-day PHI for fall applications.

Butyrac 200 controls certain annual broadleaf weeds postemergence. Butyrac can be used to establish alfalfa or birdsfoot trefoil.

Application guidelines

• Apply 1 to 3 quarts per acre when weeds are less than 3 inches in height. Does not specify legume height at application.

• Can add 0.25 percent volume/volume (v/v) nonionic surfactant in dry, low-humidity areas in seedling alfalfa.

• Can be tank-mixed with Poast or Buctril.

• Do not apply when alfalfa is drought-stressed, or if temperatures are expected to exceed 90 degrees Fahrenheit 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 this case, treated forage cannot be grazed or harvested if released.

• 60-day preharvest interval (PHI).

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 can only be used on alfalfa.

Application guidelines: By species

• Wild proso millet: Use a half pint of Poast or 12 fluid ounces of Poast Plus per acre.

• Wild oat, foxtails, barnyardgrass, crabgrass and fall panicum: Use 1 pint of Poast or 24 fluid ounces of Poast Plus per acre.

• Sandbur and volunteer small grains (except interseeded oat): Use 1.5 pints of Poast or 36 fluid ounces of Poast Plus per acre.

• Interseeded oat: 0.75 pints or 18 fluid ounces of Sethoxydim per acre will suppress, but not control quackgrass.

• Green and giant foxtail, barnyardgrass and fall panicum: Poast Plus is labeled at a reduced rate (18 fluid ounces), and Poast at 0.75 pints per acre for early application.

Other application guidelines

• Depending on weed species, UAN or ammonium sulfate may also be needed (see label).

• Ensure 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.

• Preharvest interval (PHI): Seven-day PHI before feeding, grazing or cutting for forage (undried), and 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 applied postemergence will control annual grasses in seedling alfalfa, birdsfoot trefoil or sainfoin grown for forage or seed.

Application guidelines

• Apply 6 to 16 fluid ounces per acre.

• 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 percent emulsifier at 1 percent volume/volume (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 ounces per acre.

Preharvest interval (PHI): Label states a 15-day PHI before grazing, feeding or harvesting (cutting) for forage or hay.

Buctril applied postemergence controls most broadleaf weeds in companion-seeded alfalfa.

Broadleaf weeds should not exceed the 4-leaf stage or 2 inches in height or 1 inch in diameter, whichever comes first. Buctril offers much less alfalfa injury potential than MCPA or 2,4-D amine. It’s cleared for wheat, barley, oats, rye and triticale underseeded with alfalfa.

Application guidelines

• Apply after small grain emergence and up to the boot stage; alfalfa should have a minimum of four trifoliolate leaves at application.

• Preharvest interval (PHI): 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 degrees Fahrenheit at or three 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 is not cleared for application to small grains).

MPCA 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, as not all products are labeled. Labels vary on application instructions.

Application guidelines

• MCPA at 0.25 to 0.5 pints per acre of the 4 pound ae formulations can be applied.

• Legumes are more tolerant of MCPA than 2,4-D. However, do not use except for rescue situations to control severe infestations of broadleaf weeds that threaten legume seedling survival. 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.

• Reduce sprayer pressure and lower spray volume to no more than 5 to 10 gallons per acre (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 seven days of slaughter.