Q. I’ve heard that alfalfa is a good hay choice at night for horses because it helps generate heat and keep them warm. Where I board my horse, the barn managers feed grass hay at night and told me that it keeps horses full longer. So, which is better for winter night feedings: grass or alfalfa hay?

—Julie, via e-mail

A. As we head toward the colder winter months, you’re not alone in wanting to make sure your horse stays warm overnight. When temperatures drop, feed requirements increase for your horse to consume enough calories to maintain condition. Staying warm requires calories beyond those needed for regular maintenance. Horses have different ways to regulate their body temperature depending on the ambient temperature, wind chill, and other climatic challenges they face.

Thermal neutral zone is the range of temperatures at which metabolic heat production doesn’t need to be altered to remain thermally neutral. When temperatures drop significantly, horses cross a boundary known as the lower critical temperature (LCT). Below this temperature, horses need to increase their metabolic heat production to maintain body temperature. Similarly, an upper critical temperature (UCT) exists, above which measures are taken to reduce heat production.

At exactly what point horses cross into the LCT varies based on a number of things, such as age, whether their coat is clipped, and the weather conditions they’re used to. Upon reaching LCT, the question becomes what form of extra calories should you provide your horse? It might be tempting to increase horses’ daily grain intake because it’s the simplest way to add more calories. However, as you have heard, feeding more hay might offer advantages.

Forages such as hay require microbial fermentation in the hindgut to maximize their use in the digestive tract. This isn’t a completely efficient process, and fermentation results in energy being lost as heat. This heat acts as an internal central heating system, helping keep your horse warm. Thus, any kind of forage can help keep your horse warm in winter. Less digestible types might result in greater amounts of heat being produced.

Because grass hay is a lower-calorie hay, you can feed horses larger rations of it than you can alfalfa hay, especially with easy keepers. Many people do believe alfalfa is the best hay to feed in winter for warmth. I suspect this is due to alfalfa’s high protein content and the fact that metabolizing protein isn’t the most efficient process—thus, it generates metabolic heat. Another reason is that people misguidedly believe protein is a good energy source.

Horses tend to eat grass hay slower than alfalfa, and so there’s the possibility that grass hay will last longer during the night, resulting in a more continuous stream of fuel for the fermentation process. The bottom line is both types of hay will result in microbial fermentation in the hindgut. It comes down to which is the best type of hay for your specific horse. While in some cases I will pick the alfalfa, more often than not I choose to feed more grass hay.