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

Welcome! I’m Dr Duncan Houston BVSc. In this 2025 veterinarian’s guide, we explore prussic acid (cyanide) poisoning risks in cattle after freezing conditions—especially from sorghum hybrids, Sudan grass, and Johnson grass. I'll walk you through prevention, pasture management, testing, and safe grazing strategies, with Ask A Vet providing expert backup.

1. What Is Prussic Acid Poisoning? ⚠️

Prussic acid (hydrocyanic acid, cyanide) is produced when cyanogenic glycosides in forage are released after frost or freeze. These toxins block oxygen delivery to tissues, resulting in rapid collapse and often death in cattle.

2. When Risks Are Highest

Exposure windows open fast—within hours of frost—particularly in:

• Sorghum hybrids
• Sudan grass
• Johnson grass

Young tillers and recent regrowth produce the most prussic acid. Drought following freeze also raises risk—it concentrates toxins within dying plants.

3. Signs & Speed of Onset

Onset is very rapid—often within 30 minutes of grazing frosted forage:

• Salivation, trembling
• Staggering & rapid breathing
• Collapse, seizures, death

Survival chances are slim once signs appear. Prevention is critical.

4. Grazing Safety Tips After Frost

1. Avoid grazing at night when frost is likely.
2. Hold stock off until forage is dry—usually 4–7 days after freeze.
3. Delay grazing on new shoots until Sudan grass is ≥2 ft tall.
4. Bale into hay after freeze; dry hay is safe once cured correctly.

5. Prussic Acid Testing Procedures

• Send forage samples to labs that test for HCN levels.
• Sample collection methods matter—contact Ask A Vet or your vet to ensure correct protocols.
• Results change quickly—test each pasture/frost event separately.

6. Safe Grazing Protocols

Even with test results, monitor carefully:

• Flood cows’ rumens with hay before turnout.
• Turn in a small number first (sentinel cows)—observe for 6–8 hours.
• Only break out the full herd if no signs are seen.
• Stay alert for salivation or weakness—remove animals at first sign.

7. Treatment—It's Often Too Late

By the time symptoms appear, cattle are usually beyond help. Treatments such as sodium nitrite and sodium thiosulfate exist, but are rarely effective in field situations. Prevention is the best medicine.

8. Environmental & Pasture Management

• Test and identify cyanogenic forage varieties on your farm.
• Use fencing to limit post-frost grazing.
• Harvest high-risk forages as hay early and dry properly.
• Utilize alternative forages during freeze-prone seasons.
• Record weather alerts and pasture use to track risk history.

9. Role of Ask A Vet 📱

Ask A Vet can assist you by:

• Offering guidance on forage testing and sample handling.
• Providing real-time decisions after frost events.
• Helping draft grazing protocols and sentinel systems.
• Providing emergency tele-vet support in case of suspected poisoning.

10. 2025 Key Take‑Home Essentials

• Frost triggers toxicity fast—in hours, with no visible signs beforehand.
• Hold off grazing until forages dry thoroughly (usually 4–7 days).
• Small test groups reduce herd risk—don’t turn all animals in at once.
• Sample and test before high-risk grazing, especially after freeze.
• Hay is safer—proper curing removes or degrades toxins.
• Ask A Vet is ready with support for prevention and emergency steps.

Conclusion

Prussic acid poisoning can strike suddenly after freeze events, posing a grave risk to cattle. The key to safety lies in prevention—timely pasture withholding, hay curing, forage testing, and carefully planned grazing. Ask A Vet stands by to help you navigate risk events and protect your herd in autumn 2025 and beyond. Stay vigilant, and keep your cattle safe! 🐮❤️