Temperature-Humidity Index

What is Temperature-Humidity Index?

Temperature-Humidity Index (THI) is the gold standard metric for measuring heat stress in dairy cattle. It combines ambient temperature and relative humidity into a single number that indicates the thermal load on the animal. THI was originally developed for livestock by Thom in 1959 and has since become the industry standard for managing heat stress in dairy operations worldwide.

THI is calculated using the formula: THI = (1.8 × T + 32) – (0.55 – 0.0055 × RH) × (1.8 × T – 26), where T is temperature in °C and RH is relative humidity in %. For practical use, most farmers reference THI charts or use online calculators rather than computing the formula manually.

The critical THI thresholds for dairy cows are: THI <68 = no heat stress (thermoneutral zone), THI 68–72 = mild heat stress (10–15% DMI reduction), THI 72–79 = moderate heat stress (15–25% DMI reduction, 10–20% milk loss), THI 80–89 = severe heat stress (25–35% DMI reduction, 20–35% milk loss), THI ≥90 = emergency — potential death risk for high-producing cows.

Heat stress costs the US dairy industry an estimated $1.5–2.0 billion annually in lost production, reduced fertility, and increased health costs. A 200-cow herd experiencing 30 days of severe heat stress (THI 85) can lose $15,000–25,000 in milk revenue alone.

THI Thresholds by Production Level

Heat stress thresholds vary by production level because high-producing cows generate more metabolic heat. A cow producing 100 lbs/day of milk has a maintenance energy requirement 30–40% higher than a cow producing 50 lbs/day — meaning she generates significantly more internal heat. Research from the University of Florida shows that high-producing Holsteins (>90 lbs milk/day) begin showing heat stress signs at THI 65–68, while low producers (<50 lbs) don't show stress until THI 72–75. Dry cows are the most heat-tolerant, with stress signs appearing at THI 75–78. This is why grouping cows by production level and providing targeted cooling to high producers delivers the highest ROI — cooling a 100-lb cow from THI 80 to THI 70 recovers 15–20 lbs of milk/day, while cooling a 50-lb cow recovers only 5–8 lbs.

Cooling Strategies by THI Level

Mild heat stress (THI 68–72): Ensure fans are running 24/7 in holding pens and loafing areas. Increase water availability — cows drink 20–30% more during heat stress. Feed during cooler hours (early morning, evening). Moderate heat stress (THI 72–79): Activate sprinkler systems in addition to fans — wet the cow's back for 1 minute, then allow 5 minutes of fan drying. This cycle drops body temperature 1–2°F. Increase ration energy density (more concentrate, less forage) to compensate for reduced DMI. Add buffers (sodium bicarbonate 0.75–1%) to maintain rumen function. Severe heat stress (THI 80+): Run all cooling systems continuously. Consider feeding a total mixed ration (TMR) that is more energy-dense and lower in fiber to encourage intake. Provide shade structures for outdoor areas — shade reduces solar heat load by 30–50%. In extreme cases (THI >90), consider emergency measures: ice in water tanks, wet towels on cows, and moving cows to cooled facilities.

THI and Reproductive Performance

Heat stress dramatically impacts reproductive performance. High THI reduces estrus expression — cows show shorter, less intense heats, making detection harder. Conception rate drops 20–40% during summer heat waves due to impaired oocyte quality, reduced embryo survival, and altered hormone profiles. The economic impact is severe: a cow that fails to conceive during summer extends her calving interval by 21–42 days, costing $60–200 in lost production. Embryo mortality increases 2–3x when THI exceeds 75 during the first 14 days after breeding. Timed AI protocols can partially compensate by eliminating the need for heat detection, but conception rates still drop 10–20% during severe heat stress. The best strategy is aggressive cooling during the breeding window — cows bred during cool weather (THI <70) have 15–20% higher conception rates than those bred during heat stress.