Q. With the extreme heat this summer I’m struggling to keep weight on my horse. I’m not used to dealing with summer weight loss. Our pastures are pretty dried out, so we are feeding a good amount of hay. Typically, I just feed a balancer, but this doesn’t seem to be enough. Do you have any advice?

A. We often think of weight loss in horses as a winter issue, when they need more calories to keep warm. However, we don’t often consider that horses also require calories keep cool in hot weather.

Thermal neutral zone (TNZ) is the range of environmental temperatures horses need for minimal metabolic regulation to maintain their internal temperature. When horses are within their TNZ, the basal rate of heat production equals the loss of heat to the environment. They neither heat up nor cool down.

If the ambient temperature drops below the thermal neutral zone, horses reach their lower critical temperature (LCT), which requires an increase in metabolic rate to generate the heat necessary to maintain body temperature as heat is lost to the environment. Conversely, when temperatures reach above the TNZ, the body uses evaporative cooling to lower its temperature. If, however, temperatures rise too far above the upper critical temperature (UTC) and heat gained from the environment is greater than can be lost, hyperthermia can occur.

Experts believe horses’ TNZ ranges from about 5 degrees Celsius/40 degrees Fahrenheit on the lower end (LCT) to about 25 degrees Celsius/77 degrees Fahrenheit on the upper end (LCT). The National Research Council’s Nutrient Requirements for Horses estimates that in cold weather the horse’s digestible energy (DE) requirements increase by about 2.5% for each degree Celsius below -10 degrees Celsius (or 14 degrees Fahrenheit). However, there’s little data about the impact on DE requirement when temperature exceeds the UCT.

Even if the increase in digestible energy required was as little as 0.5% for increase in degrees Celsius above the UCT, this would result in a need for 5% more digestible energy if the ambient temperature increased from 25 degrees Celsius to 35 degrees Celsius. It therefore becomes clearer as to why a horse might lose weight in hot weather if the dietary digestible energy intake remains the same.

Coping with high temperatures above the UCT is most difficult initially, as it takes horses two to three weeks to acclimate. Once they’ve acclimated, their metabolism returns to near temperate metabolism. Not only will high temperatures increase DE demands but they also tend to reduce feed intake. So just when your horse needs to consume more calories, his desire to do so might decline.

The challenge, therefore, becomes getting more calories into the heat-challenged horse, especially one who might have a reduced appetite. While my first go-to when dealing with weight loss is generally to feed more forage, that might not be the solution here. Remember that fermenting forage generates body heat. This is welcome in the winter but not great for the horse already dealing with heat stress. Feeding horses in hot climates typically means keeping forage intake on the lower end of desired levels and relying instead on more energy-dense concentrates. In particular, high-fat concentrates are beneficial because fat reduces heat load.

If you’re already feeding a balancer in combination with forage, and all mineral and vitamin needs are being met, adding oil to the diet would be the easiest and least taxing way to increase DE. You can gradually build up to a maximum of 2 cups of oil a day if needed for an average-sized horse. The alternative is replacing the balancer with a high-fat performance feed, although this might require more total feed intake for the same result. Be sure to follow the feeding instructions to ensure you’re feeding enough to meet your horse’s mineral and vitamin requirements.

Avoid feeding excess protein, as it might have an adverse effect on body heat production. For this reason, equine nutritionists don’t advise feeding large amounts of alfalfa in the summer to horses that struggle to maintain weight. The metabolic demands of breaking down excess protein might result in greater calorie loss, and the horse will need to consume more water to excrete the excess nitrogen. Horses in hot climates might already struggle to drink enough water to meet their needs and replace fluid lost in sweat. Horses not consuming enough water will have reduced total feed intake, so keeping your horse hydrated should be a key part of your weight management plan.