As the forsythia blooms and your horse begins to shed his winter coat, you may find that theres not as much horse left underneath. Your horses wintertime metabolic changes can burn more calories, causing winter weight loss, as seen here.

Horses have a remarkable ability to withstand winter weather. But wintertime can sometimes exact a price on horses in terms of calories. If you found there wasn’t as much horse left underneath that thick winter coat when your horse shed out this spring, you’re not alone.

Winter Metabolism
Your horse’s body uses several different strategies to keep warm in the winter. His hair coat of course is a big part of it, but some breeds are better at growing the dense, insulating coats than others. Good coat or not, when it gets very cold, or your horse gets wet, he needs other ways to generate heat.

To avoid heat losses through the skin, blood flow to the legs decreases. Heat generated by the organisms fermenting fibrous foods in the horse’s large intestine helps warm from the inside out. But that’s not all. The horse’s metabolism also changes.

Levels of active thyroid hormone normally increase in the winter. This is a metabolic strategy found in many species. This makes your horse burn his feed more quickly—"fast metabolism"—and also makes the burning of feeds less efficient in producing energy for the cells. When energy production is less efficient, more of your horse’s calories are lost as heat, which helps keep your horse warm. As essential as this extra warmth is, it also means fewer calories are available to maintain weight.

If you understand your horse’s winter metabolism, then it’ll make sense when you find your horse is a little lighter in the spring. As the weather warms up, his metabolism will return to normal. In many cases, you don’t really have to "do" anything for your horse to gain back the weight he lost over the winter. If the diet you were feeding him all winter had kept him at a good weight the prior summer and fall, he’ll return to that quickly with no change in feeding.

Causes of Weight Loss
If your horse had an excessively large weight loss last winter—greater than in previous winters and much greater than other horses—without the excuse of very extreme weather, then you need to do some detective work. First, go over the details of your winter feeding program with your veterinarian. If the calorie and protein levels in his winter diet weren’t low enough to explain the degree of weight loss, there has to be something else going on with your horse.

• If your horse was in a group setting, was he able to effectively compete for food?
• Is there a lameness issue keeping your horse from eating and drinking well?
• Does your horse have signs of parasitism, such as a big belly or a poor hair coat?
• Are you sure your horse can chew efficiently?

A horse with low social status in a herd often does poorly over winter. When grass is available, it’s much easier to stay out of the way of the more dominant horses but still find enough to eat. Over winter, when food is put out in a smaller area, the horse may not be able to compete for his fair share. Be especially suspicious of this if some horses in the group maintained a great weight but a few did not.

As your pasture comes back in, this problem will eventually correct itself. In the meantime, if possible, separate out any thin horses for supplemental feeding for a few hours each day. This will also give you a chance to observe this skinny group for any other problems, such as lameness, not eating well, or poor chewing ability.

Parasites always need to be considered when there is weight loss. This is especially true for the very old or young horses, but there is also considerable variation among horses in how well their immune system can deal with parasites. While there is much less risk of parasite exposure in winter, small strongyles can be dormant in the walls of the horse’s intestinal tract for many months, emerging to cause problems during the winter. Horses with very severe cases of parasitism also have diarrhea and even colic, but significant weight loss may occur without those symptoms.

A positive fecal exam is reason to deworm, but a negative result doesn’t necessarily mean there are no parasites. Fecals only check for the presence of adult, egg-laying worms. Tapeworms are difficult to detect this way, immature parasites will be missed, and a delay in examining the manure (which should be fresh and kept refrigerated) can lead to eggs hatching and a false negative test.

A dental exam should also always be part of the investigation of weight loss. In older horses, the appearance of the teeth is not necessarily an indicator of efficient chewing. A recent study found that older horses develop a different angulation to the surface of their teeth from dental wear, which can make chewing less effective. And chewing is the first step to good digestion, since it breaks up the diet into small pieces, exposing more surface area to the action of stomach acid, digestive enzymes, and the organisms inside the intestinal tract. An older horse who has not wintered well because of dental issues will do much better on grass, which is about 80% water and easier to chew, than he will on dried hays.

Weight Gain Strategies
If the weight loss your horse experienced over last winter is mild, try to resist the impulse to push a lot of extra food at him. Spring grass is nature’s best weight gainer. If your horse doesn’t have access to grass, try free-choice hay first. Liberal hay, with the appropriate supplements, is the most healthful way to put weight back on your horse. Add or increase grain only if your horse doesn’t start to gain weight on free-choice hay. Beet pulp—or a 50:50 mix- ture of beet pulp and oats for horses on an exercise regimen—is a good way of providing extra calories in the form of an easily fermented fiber that’s friendly to your horse’s digestive tract.

If your horse has had a dramatic weight loss, it’s very important to rule out the issues mentioned above before deciding on a diet. For horses who rank low in the social order or who have lameness issues, protecting them from bullying by instigating separate feeding arrangements may be all that you need to do. A parasitized horse won’t gain weight well on any diet until that issue is addressed. Ask your veterinarian about safe deworming approaches. If your older horse doesn’t have access to pasture—or still needs to gain weight despite pasture access—you’ll probably find he does best with a moistened diet. Senior feeds are convenient and can be fed wet, but you can also soak hay cubes or pellets, beet pulp, and wheat bran.

If you don’t see improvement in your horse’s weight within about two weeks of implementing a new diet, it’s time to involve your veterinarian to perform a thorough work up looking for underlying illness that could be causing the weight loss problem.