What is Temperature-Humidity Index?
Temperature-Humidity Index (THI) is the gold standard for measuring heat stress risk in dairy cattle. It combines ambient air temperature and relative humidity into a single number that represents the combined thermal load on the animal. As humidity rises, it becomes harder for cows to cool themselves through panting and sweating, meaning heat stress can occur at lower temperatures than you might expect.
The standard THI formula used in dairy science is: THI = (1.8 × Ta + 32) − [(0.55 − 0.0055 × RH) × (1.8 × Ta − 26)], where Ta is air temperature in °C and RH is relative humidity in %. This formula was developed by the NRC (1971) and remains the industry standard.
THI thresholds vary by breed and production level. Modern high-producing Holsteins (producing >80 lbs/day) begin experiencing mild heat stress at THI 68, while Jerseys typically tolerate up to THI 72 before showing stress signs. Each THI point above the threshold reduces milk yield by approximately 0.2–0.4 lbs/day per cow.
The economic impact is significant: a 100-cow Holstein herd experiencing THI 78 for 30 days loses approximately $4,500–$8,700 in milk revenue alone, not including reduced fertility, increased health costs, and culling losses. Proactive monitoring using THI allows farmers to activate cooling strategies before production losses occur.
THI Thresholds by Breed
No Stress: THI <68 (Holstein), <72 (Jersey). Mild Stress: THI 68–72 (Holstein), 72–79 (Jersey). Moderate Stress: THI 72–79 (Holstein), 79–84 (Jersey). Severe Stress: THI 79–89 (Holstein), 84–89 (Jersey). Fatal: THI >89 for all breeds.
How to Monitor THI
Use a weather station or smartphone app to measure temperature and humidity. Calculate THI manually using the formula, or use our free THI Heat Stress Calculator at dairyfarmmanager.com/tools/thi-heat-stress-calculator. Monitor THI daily during summer months (May–September in the Northern Hemisphere). Activate cooling when THI reaches 68 for Holsteins or 72 for Jerseys.
Why Temperature-Humidity Index Matters
THI is the single most important environmental metric for dairy farms. Every THI point above 68 reduces milk yield by 0.2–0.4 lbs/day. A 100-cow herd at THI 78 for 30 days loses $4,500–$8,700 in milk revenue.
Related Calculators
Frequently Asked Questions
What THI do cows start heat stress?
How do I calculate THI?
How does THI affect milk production?
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