The summer months in Australia are normally hot and dry. In hot weather, especially in some northern areas where the tropical heat and high humidity combine, the ambient heat in the air can reduce the ability of a horse to lose heat absorbed.
How do horses absorb heat in hot weather?
Horses absorb heat directly from the sun, the surrounding hot air and heat reflection from the ground during the hot parts of the day, particularly in temperatures above 35°C. Ponies, miniatures and foals are likely to absorb more heat from the ground surface, especially bare ground as their legs are short and their body is closer to the ground. On a hot weather day, a grassy yard provides more protection from heat radiated from the ground surface as opposed to a bare yard. Similarly, grassed riding surfaces or dampened sand surfaces will provide more protection from ground heat.
Generating Heat from Exercise
Horses generate heat during aerobic exercise, up to 80 % of energy metabolised using oxygen produced in the working muscles is expended as a by-product of heat. Heat production of 1,400 Kcals per hour can be generated during a light to medium exercise session, with even higher amounts of up to 2,700 Kcals during a few minutes at the gallop and approaching 7,000 Kcals/hour in an endurance horse. Exercise adds to the heat ‘load’ on the horse’s body. The friction of hooves on the working surface also generates heat and can raise the hoof temperature up to 43°C.
Heat from Digestive Processes
Heat is also generated during microbial digestion of fibre in the hindgut, especially when fed high protein or highly fibrous diets. Although this is of benefit to aid body warming during cold weather, it adds to the overall heat load during hot weather and exercise.
Water within the body cells, gut and other tissues contribute 60 % of the total body weight of a horse, or about 300 kg of total water by weight in a 500 kg horse. In the average well hydrated horse, about 60 litres of water is trapped in the fibrous digesting mass in the large bowel (hindgut). It is used as a valuable reserve of water to replenish the blood and body to reduce the risk of dehydration through sweat loss, but also as a ‘heat sink’ to store excess heat under hot conditions. This allows a horse to dissipate the heat through sweating, radiation and expelling heat in expired air from the respiratory system during and after exercise and when resting.
How Horses lose heat in Hot Weather:
- Sweating and Radiation from the Skin
Sweat is a skin secretion of primarily water and salts as well as some soap-like lathering compounds (saponins). It evaporates off the skin to remove heat from the body, leaving the salts and other residues in the hair. Each litre of sweat as it evaporates, removes approximately 200 Kcals of heat. A healthy, well hydrated horse or pony loses 50 – 60 % of heat absorbed on a hot day, or produced during exercise, through sweating. During exercise, sweat output can increase to 5 – 11 Litres per hour in a 500 kg horse, which if not replaced with water the drink, can quickly lead to dehydration.
Horses can also radiate heat from their bodies to aid cooling, but this is reduced when they are moving slowly as they graze out in the hot sun. Ideally, a horse’s paddock should have some shade trees, or even a shelter with a high roof to allow air flow. There should be a 2-metre space between your horse’s back and the roof of the shelter to allow air flow and reduce roof heat radiation.
Horses also lose heat through the process of convection, which occurs when air passes over the horse’s body. If a horse has had a hard workout and becomes excessively sweaty, then it is at risk of overheating quickly once the airflow over the body stops. In this case, walking the horse on a lose rein for 3 – 5 minutes will aid the process of convection. The cooler air will help to remove heat, except on a very hot or humid day when the air temperature is above 35°C in the sun.
Horses will pant to cool themselves if they are overheated and unable to sweat. A horse may also start to pant if they are unable to sweat because they are dehydrated. Panting helps to expel heat from the lungs. After exercise on a hot day, your horse will offload up to 30 % retained body heat by panting. A horse standing under a tree or in the shade on a hot or humid day which appears to be panting is absorbing or retaining too much heat. The horse should be sponged and scraped off with cool water and hand walked to assist with cooling.
Summer Feeding Tips for Hot Weather
General care and management strategies to help horses cope with the hot weather are widely used. However, optimising your horse’s ration for summer weather can also help to reduce heat load. This is especially important for horses which are sensitive to heat stress, for example, those in moderate to intense training, older horses and ponies, and horses with common conditions such as EMS, Cushings and anhydrosis.
Roughage (ie grass, hay, chaff) is a critical component of your horse’s ration – it should make up more than half their total feed to reduce the risk of digestive problems and particularly, gastric ulceration. Fibre, as part of roughage, traps water in the bowels and provides a beneficial fluid reservoir for exercising horses. For example, 1 kg of hay absorbs and holds 3 kg of water in its structure as it digests in the hindgut.
However, providing a high roughage diet is a balancing act, as the digestion of fibre in the hindgut is a fermentation process that produces quite a lot of heat. Hindgut heat, or the heat produced as the roughage is digested, can be a hidden factor in heat stress.
Fats, such as oil, are a useful addition to your horse’s ration for coat condition and as a non-fizzy, but also cool energy source. Oils produce less heat as they are digested than other feeds such as grains, bran, chaff and hay. Reducing the amount of high fibre feeds and grains in your horse’s hard feed and substituting with 75 – 125 ml of good quality oil can help a horse which often becomes overstressed by heat. Kohnke’s Own Energy Gold is a cold-pressed, virgin Australian oil blend with balanced omega-3 fatty acids for optimum health, coat shine and cool, non-fizzy energy.
Provision of Protein
Protein is another important consideration in summer. An adequate protein level is particularly important for your horse’s health, including maintenance of muscles and top-line. However, too much protein can cause increased heat load. This is because excess protein in the diet is dumped into the hindgut, producing a lot of heat (40% heat waste) when it is digested there (rather than in the small intestine where it is normally digested). Providing too much lucerne hay is a common cause of excess protein and fibre levels, resulting in higher hindgut heat waste and possible heat stress.
Lucerne hay, as a roughage, contains both highly digestible fibre and high protein (17%) compared to grass hay and if fed in excess, can significantly add to hindgut heat load. All that is needed for a lightly to moderately worked horse is 1-2 biscuits of lucerne hay per day, any more hay offered should be grass-based or from a cereal crop, such as oaten hay.