As the days get warmer, it’s important for those training horses to know that the horse has to work harder the warmer the weather gets. So, a 2 mile medium speed canter in an air temperature of 20°C around the lunchtime will be harder than the same exercise when it was perhaps only 2°C first thing in the morning. The effect of temperature is also greater the longer the horse works. For a horse doing a fast canter/gallop for 1 minute, there will be little difference if the air temperature is 5°C or 35°C. In fact, when it comes to performance, it’s likely to be better at the higher temperature for short events. But for a Dressage horse working for an hour, the difference will be important.
Horses, like their human riders, can acclimatise to (get used to) heat but only if they are exposed to it on a regular basis. The majority, perhaps 75%, of the heat acclimatisation a horse can gain comes from regular daily exercise over 2-3 weeks. The horse will get some acclimatisation from living in a warm climate (or even heated stable) but this will not be as effective as exercise. A horse that is acclimatised to heat will perform better in heat than one that is not heat acclimatised.
To acclimatise to heat a horse must get hot during exercise. How do I know that my horse is hot? Sweating and blowing after exercise are the main indicators. The myth is that horses blow after exercise because “the oxygen levels in the blood are low”. This is completely untrue and a horse’s breathing after exercise is mainly controlled by its body temperature. Of course another way to assess if your horse is getting hot is to take its rectal temperature before and after exercise. Temperatures in excess of 40°C (104°F) indicate your horse is moderately hot. Rectal temperature lags slightly behind the central body temperature. So if you take it immediately after exercise and it’s 39°C, if you take it 5 minutes later you may find its risen another degree.
The transition from a cooler climate, e.g. Northern Europe in September to a warmer one, e.g. Kentucky USA, does mean that many horses will not be immediately ready for competition in the warmer weather. This would be especially true of horses that are trained in the early morning, where temperatures can be close to freezing in September in some parts of Europe and then have to compete in the hottest part of the day where temperatures can reach into the 20°C’s. Heat stroke appears to be much more common in racehorses when they race in the Spring, i.e. before they are heat acclimatised. In fact , in the 1990’s there was a particular year at Badminton when the weather had been very cool leading into the event and then the temperature shot up the few days before with the result that many horses struggled on the cross-country.
Fit, thin, Thoroughbred type horses will also cope with heat better than lower fitness, well-covered, Warmblood types. An interesting observation is that the sweat of fit and heat acclimatised horses should be watery with very little if any white foam.
Another reason for avoiding hard exercise in cold weather is that we now know that this causes airway “irritation” and that some horses are affected more than others. This is similar to the condition known as ski asthma that develops in human athletes who take part in winter sports, especially cross-country skiing. There is also some evidence that exercise in the cold may worsen bleeding (EIPH; exercise-induced pulmonary haemorrhage).
Between about 5°C and 20-25°C horses are able to control their body temperature with minimal effort. But above and below this range they have to use additional energy for thermoregulation (Body temperature control) not only during exercise, but also at rest. Thus, horses actually need more energy when training and competing in temperate conditions, but as it gets warmer they require more energy, which means more feed or more energy dense feeds, such as oil. This is very important for horses travelling from a cool climate to warmer climates to compete.
As the temperature goes up, horses rely mainly on sweating to get rid of heat. Human sweat is mainly water with a low level of electrolytes. However horse sweat is very high in electrolytes, particularly sodium, chloride, potassium and calcium, and the level is higher than in the plasma (the yellow-watery part of the blood that you see above the red blood cells if you let a blood sample settle out). In fact horses lose around 11g of electrolytes for each litre of sweat. So a horse working reasonably hard (e.g. cantering, jumping, flatwork) for 1h on a warm day could easily lose 15 litres of sweat (a bucket’s worth!), which would equate to 165g of electrolytes. Typically riders may supplements 50g a day, so it will take around 3 days to replenish the body stores.
The first sign that a horse may have an electrolyte deficiency or imbalance can be something as subtle as a slight drop in performance. Electrolytes are rarely if ever the first thing someone will look at as a cause of poor performance.
It’s not uncommon to hear people say that horses “cannot store electrolytes”. This may be the reason why many riders only start to supplement a few days before competition. This is not a good strategy. The horse can store electrolytes in the body. What it cannot do is store much above what are the “normal” levels. If a horse with normal levels of electrolytes is given an extra large dose of electrolytes in its feed, a number of things can happen. Firstly, it may not eat it. Secondly, if it eats it it may develop signs of gastro-intestinal discomfort. Third, it may develop a strong thirst and drink excessively. Finally, it will produce large amounts of urine as a way to excrete this large load of electrolytes. However, if a horse is deficient in a particular electrolyte or electrolytes, it may takes many weeks of regular feeding of electrolytes to allow it to get back to “normal” levels. So my strong advice is to feed electrolytes daily in the feed and don’t increase them around the time of competition. Of course it’s also essential when discussing electrolytes to discuss water. In general horses will drink around 25 litres per day, but in warm-weather and after hard work and transport, this could increase to as much as 75 litres (nearly 5, 15 litre buckets).
How can I tell if my horse has an electrolyte deficiency or imbalance? Blood samples are not very useful, except if a horse is clearly ill. The definitive way is for a vet to take a blood sample and a urine sample for a creatinine clearance test. Another approach which is not as good as the creatinine clearance test but better than a blood test is to have the diet analysed and then measure weight loss during training and competition. It’s then possible to estimate electrolyte intake from the diet and electrolyte loss in sweat and adjust accordingly.
When it comes to competition, travel can contribute to an increased risk of heat exhaustion and poor performance. Horses lose around 2-3kg per hour of transport depending on the climate and whether or not the lorry is air-conditioned. The loss is around the same by air. For a ten hour journey in warm-weather in a lorry without air-conditioning a horse can easily lose 30kg, which could take 2-3 days to recover, even with the use of intravenous fluids. Heat exhaustion is not something that occurs just as a result of a short period of hard exercise on a hot day e.g. a cross-country round. It can occur due to prolonged periods of exposure to warm conditions e.g. travel, stabling, warm-up and competition itself. From a physical point of view, horses will warm-up quicker in warm weather so the warm-up time can be reduced. It’s also important to cool horses appropriately after training or competing in warm conditions. Cooling speeds recovery, reduces electrolyte and water loss and also reduces the risk of heat exhaustion or collapse. This will also allow horses to perform at their potential.
In summary, performance in the heat for short events (less than 1-2 minutes) may be improved as muscles function better in warm conditions. For longer events, acclimatised horses that are on the lean side will have the advantage. Finally, good management, including careful travelling, allowing sufficient recovery after travel 1 day for every 2h of flying, adjusting warm-up, appropriate electrolyte supplementation and use of cooling and shade can all help to ensure that horses perform to their potential and are at reduced risk of heat-related illness.