The color of your horse’s coat is so integral to his identity that it’s usually the first thing you mention when describing him: A chestnut mare. A palomino Quarter Horse. A leopard Appy.

But there’s so much more to the equine coat than color. A horse’s hair has important functions, both as part of his largest organ—the skin—and on its own. A year-round barrier to insects and the elements, the coat thins in time for hot weather and grows long enough to provide insulation in the winter. The equine coat is a marvel of adaptation.

For horse owners, a horse’s coat plays another important role—it’s a good indicator of his overall health. One of the rewards of taking good care of your horse is enjoying the beauty of his rich, lustrous coat. On the other hand, a dull, rough or sparse coat can be an early sign of illness, nutritional deficiency or hormonal imbalances.

Naturally, you want your horse to look good. But when working to make his coat gleam, it helps to take into account the structure and function of its hairs and the physiology behind its luster. Here’s a closer look at the science behind the shine.

ANATOMY OF A HAIR

Your horse’s coat is made up of millions of hairs, each consisting of distinct layers. If you were to look at a cross section of an equine hair shaft under a microscope, you’d see a tiny core, called the medulla. The medulla is filled with loosely packed cells that shrink when dehydrated, leaving air spaces. In general, the diameter of the medulla determines the diameter of the hair—thicker mane and tail hairs have larger medullas than thinner body hairs. In gray or white human hairs, the medulla is more prominent, but it is missing altogether in very fine hair.

The next layer is the cortex, which provides the bulk of the shaft. This layer is 85 percent keratin, a fibrous protein that also makes up hoof walls and your own fingernails. These protein fibers are long and parallel, cross-linked for strength. In addition to keratin, the cortex contains water, fats, melanin (the black or brown pigment that provides coloration) and minerals.

The outermost layer of the hair shaft is the cuticle, which is made up of overlapping cells that give it a rough, almost scaly surface. The cuticle anchors the hair shaft in the follicle and plays an important role in how we perceive the shine of a horse’s coat.

Each hair grows from a follicle, an organ that combines muscles, glands and blood vessels. Various follicles produce specialized hair cells: The ones on the crest, for instance, are equipped to produce thicker hair, perhaps of a different color, than are those on the flanks.

Attached to the hair follicles in a horse’s coat are the arrector pili muscles. When arrector pili muscles contract they lift the hairs off the surface of the skin, which helps trap air between them, providing insulation. Run your hand through the coat of a fuzzy pony on a cold winter day and you’ll notice a fuzzier, loftier feeling. That is the work of his arrector pili muscles.

Next to each follicle is a sebaceous gland, which produces sebum, a natural oil that coats each strand. Sebum not only provides a protective barrier for both the skin and hair, repelling water and inhibiting the growth of micro-organisms, but it also slicks down the “scales” of the cuticle, causing them to reflect light in a uniform way that our eyes and brains interpret as shine. Hair that lacks sufficient oil has a rough outer layer, reflecting light randomly, giving it a dull appearance.

At the deepest point of the follicle is the hair root, matrix and papilla, collections of cells in an area referred to as the “bulb” that regulate the growth and eventually the shedding of each shaft. The activity of these cells determine when your truck seats will become covered in horsehair each spring.

THE CYCLE OF SHEDDING

All hair, yours and your horse’s included, grows in three distinct phases:

• In the anagen phase the cells in the bulb are actively producing the hair shaft. Hair emerges from the follicle during this phase and grows longer. How quickly this happens and how long the strand grows depends on a variety of factors, including genetics.

• The relatively shorter catagen phase is a transitional period in which growth ceases and the follicle begins to shrink.

• In the final telogen stage, growth has stopped and the hair is no longer connected to a blood supply. The hair remains lodged in the follicle until it is literally pushed out by an emerging new hair in the anagen phase of growth.

In people, these three phases occur concurrently among different hairs. You’ll lose about 100 hairs from your own head every day, but many more are growing at the same time. Small changes occur in human hair growth rates with the seasons—peaking in late summer and early autumn—and hormonal fluctuations, but as a rule, human follicles are not in sync with each other. That’s why you have no “shedding season.” Your horse, on the other hand, certainly does.

Shedding in horses is controlled by photoperiods. Light receptors in a horse’s eyes relay changes in daylight length to the pineal gland in his brain, which produces melatonin. As hours of daylight decrease in the late autumn, melatonin production increases, which triggers coat growth.

These changes take time, however. Studies have shown that coat changes lag about five to eight weeks behind day-length transitions. Since days begin to shorten in June, by the middle of August—when winter may be the last thing on your mind—your horse’s winter coat is already starting to grow in. Similarly, in the depths of a miserably cold February, it may be heartening to remember that within your horse’s hair follicles, his sleek summer coat is already taking shape.

The effect of photoperiods is sometimes artificially reproduced to control coat growth in show horses. To delay the growth of a horse’s winter coat, for example, he may be kept in a barn that remains lit for several hours after sundown to “trick” his body by simulating a longer photoperiod. Similarly, a horse can be encouraged to shed his winter coat earlier by artificially extending the length of the day the horse perceives. Short of manipulating photoperiods, there are a few grooming techniques you can use to support the shedding process (see “Speeding Up Shedding,” page 50), but it will usually take four weeks or even longer.

HOW HAIR REFLECTS HEALTH

As you brush, shampoo and spray your horse’s coat, it’s easy to forget that its appearance is also influenced by factors beyond the reach of a grooming box. Hair follicles are complex, metabolically active tissues, subject to the same positive and negative influences that affect the rest of a horse’s organs and overall health.

For instance, nutritional deficiencies can lead to dull or brittle hair, although by the time an insufficient diet affects a horse’s coat, signs of malnutrition are usually apparent in other ways. Nonetheless, reviewing your horse’s diet to ensure he’s receiving appropriate amounts of protein, fat, copper, iodine, zinc and vitamins A, E and C is a good way to rule out his ration as a source of a less-than-lustrous coat.

On the flip side is selenium toxicosis, which results from an excess in the trace mineral selenium. This condition, which can occur when a horse grazes on pasture in areas where the soil contains high concentrations of selenium, makes the mane and tail hair brittle. The excess selenium, it is thought, replaces sulfur in the amino acid chains of the hair, reducing its strength. Thin, sparse mane and tails and poor quality hooves are often the outward signs that trigger investigation and an eventual diagnosis of selenium toxicosis.

In earlier eras, before diligent deworming programs were the norm, heavy parasite burdens would lead to a dull coat as the horse’s nutritional resources were sapped by the worms he carried. These days, it’s unlikely that a well-tended horse’s dull coat is parasite related, but it can’t hurt to perform a fecal egg count to rule out the possibility.

Sometimes an illness can lead to more than just dull or brittle hair. In fact, some diseases cause hair loss. In a condition called anagen defluxion, a physiologic stressor—such as a high fever or infectious disease—interferes with the anagen phase of hair growth. This causes growth to cease immediately and hair begins to fall out within days. A similar condition, called telogen defluxion, has a similar effect, but hair loss usually doesn’t occur for one to three months after the initial stressor. The resulting baldness can be widespread but more often is seen only in patches. In most cases of defluxion, the hair grows back normally once the underlying factor is resolved.

Defluxion is different from “patchy” shedding, where the winter coat is lost in distinctive patterns every spring. Many horses shed first from the head, shoulders and rump, while the process takes longer for the rest of the body. Why this happens isn’t fully understood, but it isn’t anything to worry about, although the horse’s coat may not look its best until the process is complete.

COAT-CONSCIOUS GROOMING

All of this doesn’t mean that your grooming practices won’t have an effect on how your horse’s coat looks. In fact, quite the opposite is true. Although the foundation of a handsome coat may be good health, it takes a good grooming regimen to produce the head-turning “bloom” of a truly stunning coat.

As you select and use various tools and products, consider how they will support the overall and long-term health of your horse’s coat.

• Currycombs not only loosen deep or caked-in dirt, making it easier to brush away, but they massage the skin, which encourages circulation and stimulates production of oils that produce a natural and lasting shine. Choose a comb with long, flexible “fingers” and use it regularly. The more you can curry a horse, the better. If your horse is ticklish or sensitive, consider using a “cactus cloth,” a rough towel woven with agave fibers that offers the same benefits.

• Softer, finer brushes do a better job of distributing coat oils than do those with thicker bristles. So even when faced with a thick winter coat that a soft brush couldn’t possibly penetrate, don’t skip that final finishing step.

• For your horse’s mane and tail, use a comb with the widest teeth you can find to minimize hair break-age. Mane and tail hair grows constantly but slowly. Every lost long tail hair takes years, literally, to replace. When combing, start at the bottom and work your way up, dividing the hair into sections if necessary, and removing tangles by hand (a spritz of detangling spray can help). Trying to comb straight through hair from the crest or dock will just lead to frustration and broken hairs.

• Less is more when it comes to shampooing your horse’s coat. Shampoos work by “grabbing” dirt particles, allowing them to be rinsed away easily. Shampoos, however, also grab oil particles, including the oils that protect your horse’s skin and produce a shiny coat. Shampooing your horse too much can result in a less-than-lustrous coat. If your horse is sweaty but not particularly dirty, rinsing him clean with plain water in lieu of a bath may suffice. Save the sudsing for special occasions (it takes a few days for natural oils to return, so if you can shampoo well before the big event, even better) or when the dirt is beyond the scope of a simple water rinse.

Make sure to use a shampoo formulated for horses, never dish detergent, which can be hard on equine skin. Also, use shampoos according to the directions on the label. You’ll notice that many call for adding the product to a bucket of water and then sponging the diluted soap onto a horse, not putting shampoo directly on the coat. This ensures that the suds will rinse clean. Soap residue leads to a dull coat and can even irritate a horse’s skin.

• Equine coat conditioners restore oil that may have been stripped from skin and hair by shampooing. They can also help detangle hair and combat “frizzies” on a molecular level by neutralizing negative charges that can make hairs stand on end. If you regularly shampoo your horse, following up with a conditioner is a good idea. Again, make sure you follow the label directions. Some conditioners are formulated to be left on for a few minutes before rinsing for maximum effect.

• Shine sprays, which typically contain silicone, keratin or panthenol, work by bonding to hair shafts and smoothing the cuticle, which makes it reflect more light. These products can add a “finishing” touch to an already clean and well-groomed horse, but they can’t replace a conscientious long-term grooming regimen. Be careful when using them because they can make the coat slick, affecting saddle stability. Spraying a clean towel and then wiping the horse can reduce risk of inadvertent application in an area where tack will sit.

As you step back to admire your well-groomed horse this spring, take a minute to appreciate not just his sleek and gleaming coat, but the complex physiological process that produced it and the many functions beyond beauty it serves.

This article first appeared in EQUUS issue #463

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