Assessing Frost and Flood Damage in Alfalfa Hay Fields

 
Frost Damage in Alfalfa
Unlike grasses, which keep growing points at or below the soil surface, legumes like alfalfa elevate growing points as they mature, making them more susceptible to frost injury. However, crowns and root reserves provide resilience, allowing regrowth if crowns remain healthy.
  • Risk Factors: Air temperatures of -5°C for more than 4 hours can damage crowns, depending on slope, snow cover, soil temperature, and moisture.
  • Cold conditions after snowmelt combined with ice encasement can possibly affect alfalfa viability.
  • Assessment Timing: Inspect stands 3–5 days after frost.
What to Look For:
  • Minor Leaf Burn: Common and not a major concern.
  • Stem Damage:
    • If <30% of stems show wilting/browning, do nothing; first cut may be slightly reduced, but recovery is expected.
    • If most stems are damaged and stand is <10" tall, do nothing; new buds will form at lower leaf junctures.
    • If stand is >12" tall, harvest and allow regrowth.
  • Severe Damage: If all stems are frozen to the ground and <5 plants per sq. ft. remain, rotate to another crop and replant alfalfa elsewhere.
 
Seedling Fields
  • Tolerance: Newly emerged seedlings are frost-tolerant, but risk increases at the 3rd–4th trifoliate stage.
  • Guidelines:
    • Seedlings with one trifoliate remaining will regrow.
    • If all trifoliates are dead, plants will not recover.
  • Stand Density:
    • >20 plants/sq. ft.: No reseeding required.
    • <15 plants/sq. ft.: Consider reseeding or broadcasting additional seed.
 
Pure Alfalfa Stem Density and Yield Potential
Stem Density (per sq. ft.)
Impact on Yield
55+ stems
No limitation to yield
40–55 stems
Some yield reduction expected
<39 stems
Stand may need replacement
Rule of Thumb:
  • Aim for 40–45 productive stems per square foot for optimal yield.
  • Below 30–35 stems, yield potential is significantly compromised.
  • Autotoxicity is not an issue for stands under two years old.
 
Pastures
Most pastures are grass-based, so frost injury is minimal. Added moisture from frost or snowmelt can improve productivity, provided fertility and grazing management are adequate.
 
Flood Damage
Excess moisture reduces oxygen in the soil, harming roots and reducing productivity.
Flooding tolerance of common forage crops:
  • Alfalfa: 2 weeks
  • Alsike Clover: 2–3 weeks
  • Smooth Bromegrass: 2–3 weeks
  • Timothy: 5–7 weeks
  • Meadow Foxtail: 5–6 weeks
Recommendation: Monitor low areas for plant death and reseed using broadcast or sod-seeding with a double disk press drill if necessary.