Simplistically stated, when energy in equals energy out, body condition is maintained. This means that if the energy content of the diet you feed your horse exactly meets your horse’s energy requirement it will maintain a constant weight. This is called being in energy balance it’s a state of equilibrium. Conversely, if the energy in the diet is less than the horses energy requirement the horse will lose weight, and if the dietary energy is greater than the requirement the horse will gain weight as fat. The same is true for humans; it is the basic rule of energy metabolism.
Knowing this, the question then becomes, what factors affect a horses demand for energy. These factors include; reproductive state (pregnancy, lactation), level of activity, age, overall health status, and weather (ambient temperature, wind chill, humidity). A horses lowest energetic state is called maintenance and applies to horses and ponies that are not in work, are not pregnant or lactating, are in good health and do not have to deal with major climatic demands. Dietary maintenance energy requirements differ by the size of horse as obviously it takes less energy to maintain a Shetland pony than it would a Shire horse.
The equation used to calculate average maintenance requirements is as follows:
Digestible Energy (Mcal per day) = body weight in kilograms x 0.0333
To convert your horse’s estimated body weight from pounds to kilograms divide the weight in pounds by 2.2. For example, a 200 kg (440 lb) pony has a maintenance requirement of 6.7 Mcal (for explanation of Mcal see The Skinny on Energy Units and Terms) per day compared to 16.7 Mcal per day for a 500 kg (1100 lb) horse. Assuming an energy density in hay of 0.8-1.0 Mcal/lb their Mcal for maintenance approximately equals the numbers of lbs of hay needed per day to meet that requirement.
Each time you add a factor such as work, your horse’s energy demand increases, and therefore to remain in energy balance, the diet must include more energy or the horse will lose body weight and condition. As was mentioned above, weather can be such a factor. Cold weather requires that the horse burn more energy to keep warm and therefore less energy is available to maintain other functions unless dietary energy intake is increased to compensate. This is why horses tend to lose weight over the winter.
So how do you know if your horse is in energy balance? The best way is to assess his/her body condition and fat deposition is through use of a body condition scoring system. This involves manually palpating and visually assessing certain areas of your horses body that are known to be locations of fat deposition, namely; the loins, ribs, tail head, the side of the wither and neck and behind the shoulder. By running your hand over these areas and paying attention to what you feel, you gain a much more objective sense of your horses overall fat cover. This is particularly important in the winter when a horse’s hair coat can be long and give the illusion that the horse is in good weight when in fact they are not. There are 5, 9 and 10 point scales the most widely used being the 9 point scale developed by Hennecke et al in 1983. The scale used is less important than the fact that the same scale be used each time and be used on a frequent, monthly, basis allowing constant assessment of body condition over time. Details of the 9 point condition scoring system and methodology can be found here.
The best thing you can do to insure that your horse comes out of the winter in the condition you want is to insure they go into the winter in good condition and then provide enough energy in the diet to meet requirements. As was mentioned above, during the colder winter months, horses have to expend extra energy to keep warm and that means that the maintenance requirement we talked about earlier increases. Fat acts as an insulator and so a modest covering of fat going into the winter will actually help your horse use less energy to keep warm. Plus, should the dietary energy levels fall below their requirement they can burn their fat stores as an energy source until the dietary energy levels increase. This is potentially useful for horses that need to lose weight, and mirrors horses in the wild that lose weight over the winter and regain it in the spring when the grass returns.
The key is not to lose so much weight that an undesirable condition results. So the horse that condition scores now at a 5 or 6 is going to have an easier time staying warm and maintaining a desirable body weight than the horse that goes into the winter with a score of 4. As a general rule, to raise the condition score of a horse from a 4 to a 5 over a 90 day period requires an increase in energy intake per day of about 25%. So our 200 kg pony from earlier would need 8.4 Mcal each day and our 500 kg horse would need 20.9 Mcal per day over the 90 day period. Of course there are those horses that have to work intense jobs over the winter who need to be at a condition score of 4 and they will require particularly close management to insure that they do not drop more condition.
It is possible to successfully maintain body condition but it takes a close eye and constant assessment. So I strongly encourage you to start a monthly plan of body condition scoring so you can catch changes in body condition and take action sooner rather than later. To regain that lost condition scoring point come spring can literally take months.
Understanding Energy Balance by Dr. Clair Thunes and Summit Equine Nutrition LLC is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.