Does alfalfa make horses hot?
The reason alfalfa gets a bad rap for making horses “hot” is because alfalfa is higher in digestible energy and lower in acid detergent fiber (ADF) and neutral detergent fiber (NDF) than grass hay. (ADF is negatively correlated with digestibility, while NDF is negatively correlated with intake; the higher each number is, the less digestible the hay is.)
An average grass hay in the USA has an ADF of 38.76%* and NDF of 62.42%* and will provide 909 kcal/lb*. The average alfalfa has an ADF of 30.68%* and NDF of 38.87%* and provides 1,193 kcal/lb*. While alfalfa is lower in simple sugars, it is composed of more easy-to-digest complex carbohydrates than grass hays, and therefore provides more energy per pound, but less fiber. This is great for hard keepers or horses that need lots of energy to fuel work or growth. Horses with high energy requirements can eat enough alfalfa to meet both energy and fiber requirements. This is not good for easy keepers, as you exceed their energy requirement while trying to meet their fiber requirement, which leads to fat or hot horses.
Protein, on the other hand, is an inefficient source of energy for horses. Much of the excess protein is urinated away. So while alfalfa does contain higher protein it is not the culprit when it comes to the increased energy levels that might make a horse “hot”. What is true is that protein “burns hotter” than fats and carbohydrates, so if a horse is using protein as an energy source they produce more internal body heat. This can be detrimental in hard-working horses where keeping their body cool is an issue, such as endurance or event horses. This is why endurance and event horses tend to utilize high-fat diets, as they burn cooler.
The take-home message: Alfalfa can be added to the diet when additional energy is needed to fuel work or weight gain without causing excitability, but if a horse is already in positive energy balance and has enough get-up-and-go to do their job, adding alfalfa could cause the horse to get fat or have more “energy” than is desired by the rider.
*Source for average energy levels in forages: Equi-Analytical Laboratories, 730 Warren Road, Ithaca, New York 14850