Q. We’ve recently been experiencing very cold weather. I feed my mare a performance feed and have been giving her extra to make up for any additional calories she’s burning as she tries to keep warm. A friend recently told me I should increase her hay not her grain. Isn’t the grain a better choice because it is going to give her more calories per pound?

A. This is a great question and certainly timely for winter. While the National Research Council (NCS) guidelines for feeding horses take into account age, work level, and body weight when considering calorie needs, we must consider other factors, as well. One of these is weather conditions. Horses definitely burn more calories to maintain their core body temperature when the weather is cold. If we don’t account for this additional loss, over time the horse will lose weight.

Horses’ weight is maintained by balancing calories consumed against calories burned. So, when calories burned exceeds calories consumed, the horse loses weight and vice versa. So, you are correct in thinking that you need to either reduce the calories lost through management techniques such as blanketing and providing adequate shelter, or you need to provide more calories. Sometimes you need to do both.

Calories come from different sources. Most calories in the equine diet are from various types of carbohydrates as well as fats. Fats provide the most calories per gram, and carbohydrates such as starches and sugars are easily digested and absorbed. Therefore, fats and carbs are good energy sources to choose when you need to increase your horse’s calorie intake. However, another consideration when feeding for warmth is the ease with which the calorie sources are digested and absorbed. This might make other options, such as structural carbohydrates, better choices.

Structural carbohydrates, such as cellulose and hemicellulose (which are abundant in hay), require microbial fermentation by the microbial population that live in the horse’s hindgut. This process isn’t particularly efficient. As with most inefficient systems, not all the energy contained within the fuel (structural carbohydrate) is captured. Some is lost to the environment, which is the horse’s hindgut.

Fermentation in this instance can be defined as the chemical breakdown of a substance by bacteria that typically results in the release of heat. Instead of capturing all the energy contained within the hay’s structural carbohydrate, some of it is lost as heat. This heat helps maintain the horse’s core body temperature. As a result, the horse expends fewer of his stored calories trying to keep warm and, therefore, maintains better body weight.

So, increase hay intake first when you experience a cold snap, and if this isn’t enough to maintain weight, increase your performance feed. Be careful though that you don’t end up feeding too much performance feed in one meal, which can upset the hindgut microbial population. As a rule of thumb, keep grain meals to less than 5 pounds each for an average-sized horse. This will be the case with some horses, especially those with higher rates of metabolism that burn calories to keep warm.

This is why if you’re feeding with the goal of keeping your horse warm by reducing the calories he’ll need to burn in the act of keeping warm, hay is a better choice over high-calorie performance-type feeds even though performance feed might provide more calories pound for pound than the hay.