Did you know that the horse is the only other mammal, along with us humans, that uses perspiration through the skin as the main way of cooling down. Most other mammals including primates have sweat glands, but only humans and horses use sweat glands as the main mechanism for regulating body temperature. We can see this clearly in performance horses when they sweat heavily producing a ‘white lather’. Unlike us, horses secrete a protein in their sweat called latherin, together with water and various mineral salts. Latherin is a surfactant protein and acts like a detergent to break down the waterproof coating on the skin, allowing the sweat to evaporate and cool the skin down. Another feature of our skin and that of a horse is that the outer couple of millimetres of skin cells actually ‘breathe’ directly with the surrounding air (rather than oxygen and carbon dioxide transfer through the blood, circulatory system and hence the lungs). This is called cutaneous respiration and may account for up to 2% of carbon dioxide removal from the body. With these facts in mind, its easy to see why keeping a working horse’s skin clean with good grooming, managing the use of rugs or cleaning and changing rugs regularly is important to ensure sweat can evaporate and the skin can breathe.Sweating is induced in horses by a number of factors, including increased environmental temperature, emotional stress such as anxiety, fear or pain, increased workload, increased food intake and heavy winter coats retaining heat. At the highest levels of workload, an endurance horse may lose 25 litres of sweat during a 75 to 100 mile desert endurance race, losing up to an incredible 250g of salt from the body. That is why pain, stress and work can knock many kilos of weight off a horse in a short period of time, literally hours – most of that weight is fluids. You can reduce the amount of fluids lost through sweating by sponging a horse with cold water on its neck and between its legs (ie the places where it would naturally produce most sweat). Sponging or soaking a hot horse with cold water on the flanks, groin and hindquarters can cause muscle cramping or a chill to the kidneys and liver. Horses that are ‘hide bound’ are suffering from dehydration, whereby the fluids lost through excess sweating have not been replaced through sufficient drinking of water. You can test to see if a horse is hide bound by pinching the skin on the side of the neck. If the horse is well hydrated, the skin will rebound immediately, but if the horse is dehydrated, the skin will only slowly return to a level surface. You can do the same ‘hide bound’ test on your own skin on the back of your hand (although age will affect the skin’s elasticity to a certain degree too).A horse’s sweat contains mainly water with sodium, chloride and potassium as the key electrolytes and in lesser quantities magnesium and calcium, plus latherin protein. The more a horse sweats, the more of these electrolytes a horse loses from his body tissues. Pure white salt is sodium chloride, composed of approximately 39% sodium and 61% chloride, and research shows that horses can self regulate their intake according to need. Too little salt in the diet and a horse will often demonstrate ‘pica’, licking and chewing soil, wood, walls, your hands, etc. Too much salt in the feed and a horse can show signs of salt toxicity including loose stools, colic, cramping, excessive urinating, muscle weakness and lying down a lot.A large horse (500kg) at rest needs approx 20g to 30g of salt per day, and will need more if in hard work or hot weather. Grazing and forage contains very little salt, so horses would obtain it in the wild by eating soil and licking rocks. For domesticated horses we can provide salt in a number of forms. The easiest to buy is table salt from a supermarket, but unfortunately most of these processed salts contain anti caking agents (these are mainly cyanide and alumininum containing compounds to stop the salt from clumping). White rock salt, mined from salt quarries in the UK is mainly sodium chloride with natural traces of other minerals. Pink rock salt from the Himalayas is popular as it contains slightly higher levels of trace minerals, hence providing it with its pretty colours.One myth, largely fuelled by social media, is that Himalayan rock salt contains excessively high levels of iron. The pink rock salt contains on average about 40 parts per million iron, that’s just 40mg of iron in a 1kg block of rock salt. The daily recommended rate for a 500kg horse is 400mg of iron per day. So a horse would have to eat ten 1kg salt licks per day before it went above its daily iron levels, and a 20g daily portion of pink rock salt will contain just 0.8mg of iron. Some soils can be very high in iron (up to 200 parts per million – or 200mg per kg) so a couple of good mouthfuls of high iron soil would provide much more iron in the horse’s diet than a few licks on a Himalayan salt lick. This is especially true since all horses naturally ingest soil as they graze.Another myth is that horses cannot obtain enough salt from a solid block as their tongues are too sensitive to lick enough, but horses can easily grind off 20g to 30g of salt per day from a salt block, using their teeth. Salt blocks can be purchased that also contain added amounts of other minerals such as copper, zinc, manganese, iron, etc. Usually these licks provide inadequate trace minerals to be of sufficient value to the overall diet, and at worst they can contain sweet molasses, preservatives and synthetic flavourings which encourage a horse to eat excessive amounts of salt.Organic approved sea salt is mainly sodium chloride with natural traces of other minerals and is supplied in free flowing white granules. It is best offered separately in a second feed bowl so that the horse can self select how much salt it wishes to eat, as and when required.Finally, avoid feed and supplements containing added salt as firstly, it’s quite a cheap ingredient, secondly the horse cannot self regulate its salt intake if it is incorporated into its daily feed ration whether it wants it or not, and lastly it can compromise the shelf life and stability of the feed. Salt attracts moisture which will degrade vitamins and other essential nutrients.In summary, hang up a pure rock salt lick or keep a separate bowl of sea salt granules available for your horse to self select, and self regulate their salt intake.