Horses do not hibernate like squirrels or hedgehogs, but they do have a metabolism that is adapted to the seasons. Research shows that the body temperature and heart rate of horses drop in the winter, especially when there is less food available. When it's cold, horses that are kept outdoors also move less to conserve energy.

Summer metabolism and winter metabolism

All our horses still have a seasonal clock, inherited from the time when they lived in the wild. In the summer, the 'summer metabolism' is active and that changes in the fall. Under the influence of shorter daylight hours, the hormone balance changes. Your horse begins to grow its winter coat and switches to the 'winter metabolism'. This natural process is important for many functions. Therefore, it is important to take this into account in the feeding and management of your horse.

Summertime: building up reserves

In the summer, the body temperature and heart rate of horses are slightly higher. The grass is rich, it is light for longer, and horses eat many hours a day. They build up reserves: the sugars and other carbohydrates from the grass are stored as fat. Some fat-soluble vitamins, such as vitamin E and A from the grass, are also stored in fat tissue and in the liver. This way, the horse has a reserve for the winter.

Generating heat from roughage

When we humans are cold, we drink hot chocolate with lots of sugar. For horses, it works very differently, as they have a very different digestive system than we do. In horses, heat is mainly generated in the large intestine. There is a whole ecosystem of fungi and bacteria (gut flora) that help the horse convert cellulose (plant cell walls) from roughage. Fibers keep your horse warm: the coarser, drier, and more stalky the hay, the harder that gut flora has to work. And heat is released in the process. Therefore, in winter, it's not a bad idea to feed less rich hay, even if it goes against your intuition. But this strategy is much more natural. In the wild, horses usually don't find rich grass in winter, but instead reeds, dry stems, and branches. And their intestines turn that into heat. So, it's not helpful to give a horse (extra) concentrated feed, muesli, or mash if you think they are cold... they need fibers!

When to feed extra to stay warm?

A healthy adult horse can keep itself warm down to about -15 degrees Celsius. If it gets even colder, you might need to feed them extra. Such conditions are very rare in the Netherlands, especially not for horses kept in the stable at night. For foals, it's different, as they have a much higher 'critical temperature', so early foals may need a blanket sometimes. The lower limit for yearlings is -11 degrees Celsius. Most horses can keep themselves warm just fine without extra feed. For older horses, the threshold often shifts a bit, and you might need to put a blanket on earlier or provide some extra hay. This varies from horse to horse.

Using winter metabolism wisely

We all know deep down: many of our horses are too fat. So, if they naturally lose a little weight in the winter, that can actually be useful. You don't have to worry too quickly that your horse will get too cold, as this only happens for most healthy horses at temperatures well below freezing. If it's very wet and windy, protection from the rain may be nice for some horses. But basically, in winter, your horse is using up the reserves it built up in the summer. Stored vitamins are released from fat tissue, but waste products are also removed. Weight loss therefore provides a natural 'cleansing'. By the time spring arrives, your horse switches back to the summer metabolism and begins building up reserves again. Then they naturally become a bit fatter again.

Conclusion: Weight loss is not a concern, as long as it's not excessive

A horse's summer metabolism stores energy in the form of fat tissue. Also, reserves of different vitamins are stored. These reserves are used by the horse in the winter. A horse's winter metabolism is focused on poorer and more fibrous food. A horse keeps itself warm by digesting coarse fibers, not by digesting many calories. Modern horses are often a bit overweight, especially after a rich and lush grazing season. Therefore, winter is a favorable period to get your horse back to a proper weight. There are exceptions to this standard for very young, old, and sick horses.

Sources:

Nadia F. Cymbaluk. Thermoregulation of horses in cold, winter weather: A review.

Livestock Production Science. Volume 40, Issue 1,1994, Pages 65-71, ISSN 0301-6226,

https://doi.org/10.1016/0301-6226(94)90266-6

Cymbaluk NF, Christison GI. Environmental effects on thermoregulation and nutrition of horses. Vet Clin North Am Equine Pract. 1990 Aug;6(2):355-72. doi: 10.1016/s0749-0739(17)30546-1. PMID: 2202497.
https://pubmed.ncbi.nlm.nih.gov/2202497/

Lea Brinkmann, Martina Gerken, Alexander Riek. Adaptation strategies to seasonal changes in environmental conditions of a domesticated horse breed, the Shetland pony (Equus ferus caballus). J Exp Biol (2012) 215 (7): 1061–1068.

https://doi.org/10.1242/jeb.064832