Effects of Forage Freeze on Cattle – Vet Guide 2025 🌾❄️

Dr Duncan Houston BVSc reveals how fall frosts can create deadly prussic acid in sorghum grasses, and offers expert prevention strategies to keep your herd safe.

1. What Is Prussic Acid Poisoning? ⚠️

Prussic acid, also known as hydrogen cyanide, forms in certain forages—especially sorghum hybrids, Sudan grass, and Johnson grass—during a frost or freeze. This cyanide toxin disrupts oxygen use at the cellular level, often causing cattle to collapse and die within minutes. Treatment is usually ineffective due to how quickly symptoms develop.

2. How Frost and Freeze Create Danger ❄️

• Prussic acid levels can spike within hours after a frost.
• New tillers or young regrowth are especially dangerous after frost exposure.
• Hay made from frosted forage can still be toxic if not properly dried before baling.

Dr. Steve Barnhart of Iowa State recommends avoiding grazing on nights when frost is likely, as toxic levels can develop overnight.

3. High-Risk Scenarios 🧯

• Frosted sorghum, Sudan, or Johnson grass
• Young regrowth after frost—wait until it’s at least 2 feet tall before grazing
• Fresh hay baled within 6 days of a frost

Tip: Frosted forage is only safe as hay after it's fully dried and properly baled.

4. Symptoms of Prussic Acid Poisoning 🆘

• Sudden collapse or death with no warning
• Bright red mucous membranes
• Foaming at the mouth
• Muscle tremors, breathing difficulty

Because symptoms progress quickly, prevention is your only reliable strategy.

5. Prevention Strategies ✅

🌦️ Weather-Aware Grazing Plans

• Do not graze during or immediately after frost conditions.
• Remove cattle from pastures after a freeze and wait at least 6 days before reintroducing.
• Feed hay or low-risk forage during frost events to avoid temptation grazing.

🌱 Young Regrowth

Do not graze new shoots or tillers after a frost. Wait until Sudan-type grasses are at least 2 feet tall.

🐄 Turnout Caution

• Only turn in a couple of cows for the first 6 hours after a suspected risk period.
• Observe them carefully for signs of poisoning before releasing the full herd.

🪵 Hay Use

Frozen forage baled while still wet may retain prussic acid. Ensure hay is fully cured and dried before baling and feeding.

6. Forage Testing Guidelines 🧪

• Collect samples early in the morning after frost.
• Cool and deliver to a lab promptly or use field assay kits.
• Prussic acid levels under 200 ppm are considered safe; above that, delay grazing.

Testing is helpful, but results can change quickly based on environmental conditions. Use testing in combination with weather and forage maturity observation.

7. Case Studies 🧾

📌 Case 1: Immediate Frost Exposure

A herd of cows grazed Sudan grass after a light frost. Seven animals died overnight before treatment could begin. Lesson: Avoid grazing until the grass has thoroughly dried and regrown.

📌 Case 2: Hay Baled Too Soon

Hay was baled one day post-freeze and fed to lactating cows. Cyanide levels remained high—lab tested at 320 ppm. Lesson: Wait at least 6 days and test hay before feeding.

8. Ask A Vet Tools 🛠️

• 🌾 Ration balancing for forage dilution
• 📊 Forage test result interpretation
• 📅 Seasonal pasture scheduling
• 📍 Geo-mapped frost alerts with Shopify feed planning

9. Download: Grazing Management Guide 📥

Stay organized this fall with our PDF checklist of risk periods, forage prep, testing tips, and turnout plans.

10. Conclusion 🧠

Prussic acid poisoning is silent but deadly. With proactive grazing schedules, accurate forage testing, and the right safety gear, you can prevent fatalities in your herd this fall. Ask A Vet is here to help at every step—from pasture strategy to testing support.

Let’s build a safer, smarter fall forage plan for your ranch in 2025. 🌱