Fermentation Heat from Forage: The Horse's Internal Furnace

How the Hindgut Generates Internal Heat

One of the most effective and fascinating cold-weather adaptations in horses is their ability to generate internal heat through the fermentation of forage in the hindgut. Unlike simple-stomached animals, horses are hindgut fermenters, meaning they rely on microbial populations in the cecum and colon to break down fibrous plant material. This microbial activity not only releases energy-rich volatile fatty acids (VFAs) for absorption but also produces significant heat — a critical source of warmth during cold weather.

What is Fermentation Heat?

This process, known as "fermentation heat," functions much like an internal furnace. Long-stem forage, such as hay, promotes longer fermentation times and thus more sustained heat production compared to concentrated feeds like grain. Studies have shown that the digestion of high-fiber diets increases the horse's core temperature and helps maintain thermal balance during exposure to cold.

Winter Feeding Requires Extra Forage

Because of this, winter feeding strategies should prioritize forage over concentrates. Horses with free-choice access to hay, particularly during cold spells, are better able to meet both their energy and thermoregulatory needs. In practical terms, for every 10°C (18°F) drop below a horse's lower critical temperature, it may need an additional 2–4 pounds of hay daily to maintain body condition. A horse's lower critical temperature (LCT) is the ambient temperature below which it must expend extra energy to maintain its core body temperature. This temperature varies widely, from about 40°F for clipped horses to as low as 5°F for a healthy horse with a dense winter coat. Key factors that influence a horse's LCT include coat condition (clipped vs. winter coat, wet vs. dry), age, size, health, and access to shelter.

Grain vs Forage: Who's the Winner?

In contrast, grain is digested earlier in the digestive tract and generates minimal heat. Therefore, although concentrates may be necessary for particular horses (e.g., hard keepers or athletes), forage remains the cornerstone of winter feeding — offering both nutritional and thermal benefits.