NOTE: This article originally appeared in a Canadian horse magazine, so all temperatures are given in Celsius.           

             Despite the fact that many of us do not ride our horses much during the winter, we often need to provide them with something extra in the grocery department when the mercury takes a nose dive. However, determining just how much we should feed them and when they really need it is often a bit of a mystery.
             During cold, wet and windy weather, horses require more energy to maintain their internal body temperature. Just how much energy an individual horse needs depends in part on the environmental conditions, but it is also affected by the horse’s overall body condition and the state of his winter coat. For example, a horse with an insulating layer of fat and a thick coat will conserve energy better than a thin horse with a short coat. While differences of this nature must be taken into consideration, there are some general guidelines to help you gauge when to start forking out the extra grub.

Leaving the Neutral Zone

When environmental temperatures are within the horse’s “thermal neutral zone” (10-15 C in dry conditions), the body doesn’t have to do much to regulate heat generation or heat loss. It’s when temperatures drop below the bottom of that neutral zone that the body revs up the chemical engines that produce heat. That tipping point is known as the “critical temperature,” and it is an important part of understanding a horse’s winter feeding requirements.
 However, the critical temperature may not be the same for each horse, and it is likely to change somewhat from day to day. Experts estimate that the critical temperature at which horses start requiring extra energy lies somewhere between -1 and 15 degrees Celsius, depending on the actual temperature (including wind chill), hair coat, body condition and wetness.  Wet conditions raise a horse’s critical temperature by approximately 5-8 degrees C, meaning that he will start getting colder at a higher temperature. Knowing what the critical temperature is for your horse on any given day allows you to estimate the changes in his nutritional requirements. Table 1 illustrates what some critical temperatures might be.

Table 1. Estimated Critical Temperature for Horses in Moderate Body Condition
Hair Coat
Critical Temperature (C)
Wet or short

For each .555 degree C below the critical temperature, your horse’s digestible energy (DE) requirements go up 1%. For example, if you had an 1,100 pound horse in moderate body condition with a moderate coat, and you were looking at a dry day with an environmental temperature of –1 C, with a wind chill that made the actual temperature –4 C, you would do the following calculations:

Critical temperature = 7 C
Actual temperature = -4 C
Difference = 11 degrees
11  divided by .555 = 19.82% increase

Your horse would therefore need approximately 20% more digestible energy to maintain normal body temperature on that day. If you had the same temperature but the day is a really wet one, your figures would look like this:

Critical temperature = 15 C (7 C base + 8 C for wetness)
Actual temperature = -4 C
Difference = 19 C
19 divided by .555 = 34.23% increase

Now you know how to figure out what percentage increase you need to make, but you also need know understand how that number translates into different types of feed. We’ll get to that in a minute, but first, you need to know what a megacalorie (Mcal) is, and what the basic Mcal requirements for different horses are.

The Mighty Mcal

            When we talk about a “calorie”, as it applies to the human diet, what we are usually actually talking about is a kilocalorie (which is also correctly called a Calorie with a capital C), which is the amount of energy required to raise the temperature of one kilogram of water one degree Celsius. When we talk about the diet of a large animal like a horse, we use the Mcal, which denotes 1,000 kilocalories. According to the National Research Council, the basic daily Mcal requirements for a horse whose mature weight is or will be 1,100 pounds are as follows:

            Weanling (4 months old) 14.4 Mcals
            Yearling (12 months old – moderate growth) 18.9 Mcals
            Two-year old (not in training) 18.8 Mcals
Two-year old (in heavy training) 26.3 Mcals
Adult at maintenance 16.4 Mcals
Working horse (light work) 20.5 Mcals
Working horse (moderate work) 24.6 Mcals
Working horse (intense work) 32.8 Mcals
Lactating mare (first three months) 28.4 Mcals

If your horse is larger than 1,100 pounds, you would add approximately 1-1.5 Mcals for every additional 100 pounds.
Going back to our example of the 1,100 pound horse on the dry cold day with an actual temperature of –4C, we know that his basic needs would be fulfilled by 16.4 Mcals a day, so if he needs an extra 20% based on the weather, we have to get and extra 3.28 Mcals into him. One would think that you could just increase whatever forage and grain the horse gets by 20%, and that might indeed work out just fine. However, there are some differences in how forages and concentrates work in the body that are worth considering.

Body Heat

Most importantly, forages and concentrates vary in the amount of internal heat they produce during the digestive process. Forages are digested by microbes in the cecum and large intestine, producing more heat than concentrates, which are digested by enzymes in the small intestine. Therefore, the digestion of hay actually gives off significantly more heat than the digestion of concentrates, even though the latter contain more Mcals per pound. Thus, the best way to increase internal body heat is generally to increase the intake of hay. 
On average, hay contains about 1 Mcal per pound (2.2 Mcal per kilogram), with grass hay typically a little lower and legume hay and alfalfa a bit higher. However, if you really want to know what the DE of your hay is, you would have to have it analyzed by a laboratory that can give you the calculations for horses, as DE numbers for cattle are different for the same hay, due the way they digest their food.
Once again looking at our example scenario, if we assume that our hay is about 1 Mcal per pound, we would need to give our 1,100 pound horse 3.25 pounds of extra hay to cover his requirements on our –4 C dry day, and 5.61 pounds on our –4 C sopping wet day.
One additional advantage to increasing the amount of hay in your horse’s ration is that the increase in dry matter will encourage him to drink more. Inadequate water consumption is a serious and common problem in cold weather, and has a direct association with and increased risk of impaction colic.

When Hay is Not Enough

In some situations, however, your horse may simply be unable to consume enough hay to get the additional energy he requires. If you have extremely cold weather, older horses, thinner horses, horses with dental issues or horses that don’t have an adequate winter coat, you may need to add concentrates to the mix.  Unfortunately, most feed manufacturers do not put the DE (usually expressed as Mcals per pound) of their feed on their labels, so you may need to call the company and talk to their equine nutritionist to get that figure.
One last thought: some people say that one way to reduce the amount of extra energy your horse requires during cold weather is to outfit the horse with a blanket (see our article on blanketing on page ___). While that may indeed be the case, a leaky or otherwise inadequate blanket can actually make the situation worse, so if you do choose to blanket, make sure you go with a well-fitting, high-quality, effective product.

SIDEBAR: Ten Tips for Cold-Weather Management

 • Monitor the weather forecasts: knowing when the cold is coming helps you to prepare.
• Give your horse a head start: increase feed as necessary 24 hours prior to forecasted cold conditions.
• Make sure your horse is in good body condition going into the winter. For a metabolically normal horse in moderate condition, a 5% increase in body weight over his summer weight can help him stay warm.
• Determine each horse’s critical temperature and adjust DE intake accordingly.
• Use hay for increases for horses in good body condition and “easy keepers.”
• Use hay and concentrates for horses in poor condition and “hard keepers.”
• Offer a minimum of 10 gallons of warmed water per horse daily: horses can easily become dehydrated if all they have to drink is cold water or whatever snow they can eat.
• Feed concentrates as a warm, moist mash during cold periods: horses will like the warmth and you will sneak some extra water into them.
• Provide adequate shelter for your horses. A good windbreak and a place to get out of the rain can help your horse’s winter coat do its job better.
• Run your hands over your horses regularly to feel their body condition. Fuzzy winter coats can hide the fact that a horse is dropping weight. 

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