When Heat and Humidity Team Up: How THI Impacts Your Cows' Health From the Inside Out.

No time to read, listen to the podcast episode. - When Heat and Humidity Team Up

On hot summer days, it’s not just the heat that puts stress on your cows. It’s the combination of heat and humidity that can push them past their comfort zone, silently raising their core body temperature and causing serious damage to reproduction, health, and performance.

This blog explains how the Temperature-Humidity Index (THI) works, how it impacts core body temperature (CBT), and why controlling it is key to preventing heat stress on your dairy.

What Is THI? (Temperature-Humidity Index)
The THI is a simple number that combines temperature and humidity to show how hot it feels to a cow.

Think of it like this:
A day that’s 28°C (82°F) with 50% humidity may feel hotter to your cows than a day that’s 32°C (90°F) with dry air.

That’s because cows don’t sweat like humans. When humidity is high, it’s more difficult for cows to get rid of internal heat. 

The higher the THI, the harder it is for cows to stay cool, and the more likely they are to suffer from heat stress.

• THI under 68 (20°C/68°F): No stress
• THI 68–72 (20–22°C): Mild stress
• THI 73–77 (22–25°C): Moderate stress
• THI 78+ (26°C+/79°F+): Severe stress
• THI 99+ Lethal 

Why THI Affects Core Body Temperature (CBT)
Cows are large animals with high metabolic heat production. They can’t sweat efficiently, so they rely on breathing, standing, and panting to release heat. They throw their heat off to the environment around them. The hotter and more humid the air around them, the more heat accumulates inside of them, raising their core body temperature. 

When THI increases, the cow’s body heat cannot be thrown off the cooler air around her, and that air is as hot or hotter than she is. The heat is trapped, causing core body temperature (CBT) to rise.

Normal CBT: ~38.6°C (101.5°F)
Heat-stressed CBT: 39.7°C+ (103.5°F+)

A rise of even 1°C (1.8°F) in CBT is dangerous.

Why? Because CBT controls everything:

• Fertility: High CBT reduces conception and increases embryo loss
• Milk components: Butterfat and protein drop as CBT rises
• Immune function: Cows become more susceptible to mastitis and disease

Fun fact: Research shows CBT is one of the best indicators of early heat stress, even before you see outward signs.

THI Is a Silent Threat
Here’s the challenge: heat stress doesn’t always look obvious. Your cows may still be eating, walking, and producing—while their internal temperature is rising above safe levels.

Visible signs of heat stress:

• Panting or open-mouth breathing
• Standing instead of lying down
• Drooling or bunching

But by the time you see these signs, the damage is already happening inside the cows. Most systems talk about heat abatement; the definition of abatement is to bring it back to where you want it to be.

A cooling system needs to be proactive, meaning that the cow's CBT never rises in the first place. It works proactively to prevent the impact of rising THI throughout the day. 

Cow CBT zones:

• Comfort (38.3–38.8°C / 101–102°F)
• Danger (38.9–39.6°C / 102–103.3°F)
• Critical (39.7°C+ / 103.5°F+)

Why Cooling the Barn Isn't Enough
Many farms install fans or sprinklers to cool the barn—but that only works if it actually cools the cow.

Let’s compare:

Reactive cooling - Yard soakers or feed bunk sprinklers

Temporary cooling - Doesn’t protect  CBT during rest or overnight

Proactive, THI-based cooling:Runs automatically based on real-time THI
Targets cows while they lie down (10–13 hours/day) and where they eat 10 – 13 small meals a day (grazing).
Keeps CBT stable and within thermal comfort zone

The Thermal Comfort Zone: Why Lying Cows Are Your Best Indicator
When CBT is controlled, cows lie down more, chew cud, rest, and ruminate. When CBT rises, cows stand up, breathe harder, and stop ruminating.

This is why cow behavior can be one of your best indicators of whether cooling is effective.

The goal isn’t just cooling the air. It’s cooling the cow from the inside out.

Real-World Damage from Poor THI Management
The impact of heat stress is not just about milk yield. It affects your entire operation:

Reproduction losses: Conception rates can drop by 20–30%
Embryo damage: Early heat stress leads to smaller calves and lower survival rates
Health problems: More retained placentas, mastitis, and lameness
Milk quality: Declines in butterfat, protein, and total output
*Source: West, 2003 – Effects of Heat Stress on Dairy Cattle

Practical Takeaway: Know the THI. Focus on and monitor the core body temperature.
Whether you’re in a tropical climate or a summer heat wave, the message is the same:
Start managing heat before it becomes stress.

• Monitor THI daily, starting at 68 (20°C)
• Use tools that activate automatically based on THI
• Focus on CBT, not just barn temperature

Because when cows stay in their thermal comfort zone, they eat more, reproduce better, and stay healthier.

Need Help Keeping Your Cows Cool From the Inside Out? We believe the best way to protect cows is to cool the core, not the barn. Want to learn how THI-based systems can help you do that? Email Nancy at nancy@corecoolsystems.com or send a message on WhatsApp: +1-330-717-8852

We’re happy to answer your questions and help you protect your herd. Cool Cows are Core Cool Cows.