Q – How exactly does eating hay keep my horse warm in the winter?

A – Eating hay provides two ways to help your horse stay warm. The first is simply through providing calories (energy), which can be used in metabolism to help maintain body temperature. When environmental temperatures start to drop in the winter, horses increase their metabolic rate as needed to maintain a stable body temperature. When feed is consumed, digested and metabolized, calories are made available for use.

Different feeds have different amounts of energy (calories) per unit weight, based on their digestibility and chemical composition. While hay does have a relatively low energy density (Digestible Energy or Mcal/kg) compared to feeds such as vegetable oil or cereal grains, because a horse can safely eat a lot of it, those calories quickly add up.

The number of additional calories your horse needs to consume on a cold day depends on how cold the temperature is, but may be as much as 50 per cent more! So, an average 500kg horse, who typically eats about 17Mcal per day, may need upwards of 25Mcal on a very cold day. This would work out to 4-5kg of extra hay per day (above what they normally receive), or about three to four flakes more.

Eating hay can also keep your horse warm indirectly. When hay is eaten, the normal enzymes of the horse can’t actually break it down in the digestive tract (because hay is largely fibre, which is indigestible to mammals). This is why horses have formed a symbiotic relationship with the microbial organisms that reside in the large intestine (cecum and colon) – these microbes have the ability to break down fibre through fermentation. If you’ve ever been to a brewery and seen the fermentation vats (or even baked bread), you’ll have noticed that when carbohydrates are fermented (by microbes and yeasts), it results in the production of gas – and heat. When hay is fermented in the large intestine, heat is produced as a by-product, and is available to the horse.

This “heat increment of feeding” is present for most foods, but is significantly higher for something like hay, compared to a feed that is high in fat (see sidebar). This is why in the winter, offering additional hay is often recommended.

The bottom line is hay is a good source of Digestible Energy, which can be needed to help maintain body temperature. Further, hay digestion produces additional heat which can be useful to help maintain body temperature during the winter. So, go ahead and toss some extra hay into your pasture (assuming it is covered in snow) or purchase a good round bale (just make sure it isn’t mouldy!). On the flip side, in the summer heat you might want to opt for higher quality (lower fibre) hay, to avoid any additional heat stress to your horse (but don’t not offer them hay in the summer, as the horse needs to consume hay to keep the microbes happy and ward off digestive upset!).

Remember to work with your qualified equine nutritionist to ensure your horse is meeting his nutritional requirements as well as getting some extra heat benefits where needed.

The Science of Turning Hay Into Heat

The basic energy content of any type of feed is called Gross Energy (GE), which, scientifically speaking, is the amount of energy released by the feed when combusted (burned), and refers to the potential available energy, expressed for equine feeds as Mcal/kg. Different compounds have different Gross Energy values: carbohydrates (which include sugar, fibre, starch etc.) have 4Mcal/kg, fats have 9Mcal/kg and protein has around 5Mcal/kg.

When your horse consumes any feed, not all of the energy is available, as some of the potential energy passes through the digestive tract and into the feces. The Digestible Energy (DE) content of feeds depends not only on their Gross Energy (which depends on how much carbohydrate, fat or protein is in them), but also how digestible they are. As such, Digestible Energy actually refers to the Gross Energy of the feed minus the Gross Energy of the fecal material. A horse weighing 500kg, for example, requires approximately 17Mcal of Digestible Energy per day. It is, therefore, important to consider where the calories are coming from in order to best serve your horse.

Additional potential energy is lost through urine, as well as to the gas produced by microbes in the digestive tract, which break down carbohydrates. The term Metabolizable Energy (ME), therefore, refers to the Gross Energy of a feed, minus the energy lost in feces, urine and gas.

The energy that is lost as heat is called the heat increment of feeding (HI) and differs for different feeds. The fat found in vegetable oil, for example, is broken down fairly easily by the digestive tract and converted to fatty acids, which are a very efficient source of energy. So, the heat increment for fat is fairly low. In contrast, fibre needs to be broken down extensively through fermentation, which gives off significant heat. The heat increment of feeding is, therefore, larger and can be useful for horse to help maintain body temperature in the winter.

After all of these processes, the remaining energy that is actually available to the horse for maintenance of body functions (heart beating, lungs breathing, etc.) is the Net Energy (NE). This can be further classified as truly the Net Energy for maintenance (NEm) or for work (NEwork), growth (NEgain).