Heat Stress in Cattle: Signs, THI Chart & Prevention
Quick answer: Cattle begin heat stress at a temperature-humidity index (THI) of 72 — a 78°F day at 65% humidity. Above THI 84, milk drops, gain drops, fertility crashes, and cattle die. The three-pillar prevention plan is shade, water, and airflow. National losses from heat stress top $2 billion per year in dairy alone. This is a manageable problem with known solutions.
What Is Heat Stress?
Heat stress occurs when a cow's ability to shed heat can't keep up with the heat she's producing plus the heat she's absorbing from the environment. Cattle generate enormous internal heat from rumen fermentation, muscle activity, and lactation. Unlike humans and horses, they sweat inefficiently — most cooling comes from panting, blood flow to the skin, and lying on cool ground. When ambient temperature and humidity rise past the thermoneutral zone (40–75°F for most cattle), the cooling system falls behind and body temperature climbs.
The consequences scale rapidly with THI. Mild heat stress reduces feed intake and comfort. Moderate stress cuts milk production and reproductive performance. Severe stress causes measurable production and mortality losses. Emergency-level heat events can kill dozens of cattle in a single afternoon — the classic feedlot disaster is a still, humid summer night after a hot day, when cattle can't shed the day's accumulated heat.
The THI Chart
Temperature-Humidity Index (THI) combines temperature and humidity into a single number that predicts heat-stress risk more accurately than temperature alone. A hot dry day is far more manageable for cattle than a warm humid one.
None
Mild
Moderate
Severe
Emergency
Quick THI reference (approximate)
| Air Temp (°F) | 50% Humidity | 70% Humidity | 90% Humidity |
|---|---|---|---|
| 75 | 70 (comfort) | 72 (mild) | 74 (mild) |
| 80 | 74 (mild) | 77 (mild) | 80 (moderate) |
| 85 | 79 (moderate) | 82 (moderate) | 85 (severe) |
| 90 | 83 (moderate) | 86 (severe) | 89 (emergency) |
| 95 | 87 (severe) | 90 (emergency) | 93 (emergency) |
| 100 | 91 (emergency) | 94 (emergency) | 98 (emergency) |
Cattle need overnight temperatures below THI 72 for at least 6 hours to shed accumulated body heat. When nighttime temperatures stay elevated (typical during summer humidity), cattle enter each morning already carrying a heat debt — and by afternoon of day 2 or 3, welfare and mortality risks are severe even at moderate daytime THI. Sustained heat is more dangerous than a single hot day.
Signs & Respiration Scoring
Respiration rate is the most sensitive early indicator of heat stress and the easiest to observe. Count flank rises for 15 seconds and multiply by 4.
Additional heat stress signs
- Crowding shade or water sources
- Refusing to lie down (lying increases heat load from the ground)
- Standing in water tanks, ponds, or wet mud
- Reduced or stopped feed intake
- Drooling excessively
- Weakness, staggering, collapse (emergency)
Production & Fertility Impact
| Impact | Typical Loss |
|---|---|
| Dairy milk production | 10–25% during heat events; 15–30 lb/cow/day |
| Milk fat and protein | 0.2–0.4 percentage point reduction |
| Conception rate | Down 20–50%; up to 90 days of "summer infertility" |
| Feedlot ADG (average daily gain) | Down 0.3–0.7 lb/day during hot weeks |
| Feed intake (all classes) | Down 10–25% |
| Mortality during severe events | 1–5% (heat waves) |
Heat-stress conception losses often persist for 60–90 days after the actual heat wave, because damaged follicles and embryos affect breeding cycles months later. This is why summer breeding rates lag behind spring rates even when September temperatures cool off. See our Heat Detection Guide for more on summer breeding management.
Emergency Cooling
A cow with respiration score 3+, open-mouth breathing, and staggering is a veterinary emergency. Heat stroke can be fatal within 30–60 minutes of the first collapse.
Emergency cooling steps
- Move to shade immediately — even shifting the cow 20 feet into building shadow helps
- Wet the body with cold water — hose, buckets, or sprinklers on the flanks, back, and legs (not the head or face)
- Set up airflow — fans, open barn doors, or move the cow into breeze
- Provide cold drinking water — do NOT force-drink; make it easy to reach
- Reduce all handling — do not walk or move the animal further than necessary
- Call your veterinarian — heat stroke may require IV fluids and electrolytes
Spraying cold water on a heat-stressed cow's head can trigger circulatory shock. Cool the body — flanks, back, legs, belly — where evaporation is efficient. The head stays out of the cold spray until the body has cooled first.
Prevention: Shade, Water, Airflow
The three-pillar prevention plan is universal across dairy, feedlot, and cow-calf operations. The specifics scale with your situation — a Nebraska feedlot needs sprinklers, a Wyoming range cow needs a tree — but the pillars don't change.
Shade
- Natural shade — trees are excellent; avoid single trees that cause crowding and death from trampling
- Shade cloth structures — 40–60 sq ft per cow, oriented for maximum midday shade coverage
- Barn or shed access — sufficient roof height and airflow to prevent barn from becoming an oven
- Portable shade — for rotational grazing systems
Water
- Volume: Cattle drink 2–3× normal in heat — provide 30–50 gallons/head/day for lactating dairy, 20–30 for feedlot cattle
- Flow rate: Trough refill must keep pace with peak demand at the hottest hour
- Access: 3–4 linear inches of water access per cow; multiple watering points reduce crowding
- Temperature: Cool water is drunk more; consider shaded troughs
- Cleanliness: Algae, manure, and organic matter reduce intake; scrub troughs weekly
Airflow
- Natural ventilation: Orient barns and structures to catch prevailing summer winds
- Fans: Panel or drop fans in freestall barns; feedlot fans over the bunk line
- Sprinklers with fans: Combined cooling (wet coat + moving air = evaporation) is the highest-impact intervention in feedlots and dry lots
- Windbreak removal in summer: Winter windbreaks that block summer breeze cost you
See Feedlot Facilities and Pasture Water Systems for detailed infrastructure guides.
Hot-Weather Handling
- Work cattle before 10 AM or after 8 PM — never in the heat of the day
- Keep handling sessions under 30 minutes
- Never run cattle in hot weather — walking pace only
- Provide water immediately after processing
- Postpone non-essential work during heat advisories
- Delay shipping during 5+ day heat waves
- Time weaning, vaccination, and processing to cool weather when possible
Frequently Asked Questions
At what temperature do cattle get heat stressed?
Cattle begin to show mild heat stress at a THI of 72, moderate at 79, severe at 84, and emergency above 89. On a 90°F day with 60% humidity the THI is 85 — already severe. Cattle are more heat-sensitive than humans because their body mass and rumen fermentation generate massive internal heat.
How do you tell if a cow is heat stressed?
Watch respiration rate — normal is 20–40 breaths/min; heat-stressed cattle exceed 80 and severely-stressed cattle over 120. Other signs are open-mouth breathing with tongue out, drooling, crowding water, avoiding lying down, refusing feed, and eventually collapse.
How much production do you lose to heat stress?
Dairy cows lose 10–25% of milk production during heat stress, plus 20–50% reductions in conception rate. Feedlot cattle lose 0.3–0.7 pounds of daily gain during hot spells. National losses in dairy alone are $1.5–$2.3 billion per year.
How do you cool cattle quickly?
Emergency cooling: move to shade, spray cold water on the body (not the head), provide cold drinking water, and increase airflow with fans. For herd cooling: sprinklers or misters with fans, shade, cold water, and reduced handling during hot hours.
When is the best time to work cattle in hot weather?
Handle cattle before 10 AM or after 8 PM during hot weather. Cattle carry residual heat from the day. If you must work during the day, keep it under 30 minutes, avoid running, and provide water immediately after processing.
This guide is part of the full Cattle Health & Disease pillar — covering every major disease, vaccination schedules, parasites, and emergency response.
Return to Cattle Health Hub →This article is educational and does not replace veterinary care. Heat stroke cases should be seen by a licensed veterinarian for supportive fluid therapy and follow-up assessment. Regional recommendations vary — consult local extension services for specific infrastructure guidance.