The majority of a mature horse’s body weight is composed of water, 62 – 68% to be precise! Unsurprisingly, water plays a key role in maintaining many different systems, including: blood and plasma volume, gastric motility, gastrointestinal tract lubrication and secretions, thermoregulation, connective tissue, skin, muscles, ligaments and tendons, as well as acting as a transport mechanism for dissolved substances. Therefore, it is vital to maintain adequate hydration levels and this is where electrolyte supplementation can have a beneficial impact (Lindinger and Ecker, 2013).>
Electrolytes are minerals that have a positive or negative charge when dissolved in bodily fluids, allowing them to conduct electrical charges, hence they are able to influence circulatory fluid balance, acid-base balance, nervous impulses and facilitate muscle contraction. The key electrolytes (in order of the quantity they are contained within plasma) are as follows: Chloride 3.5g/l, Sodium 3.3g/l, Potassium 0.16 g/l, Calcium 0.12g/l and Magnesium 0.0024g/l (Robert et al, 2010).
The requirements for electrolytes vary according to certain factors. Horses at maintenance in a thermoneutral environment will be subject to obligatory water and electrolyte losses (excretion of faeces and urine and respiration) meaning they will have a daily water requirement of about 41-67ml/kg bw/day (Mitchell, 2018).
When a horse is in work there is an increased demand to thermoregulate. At moderate exercise a horse may not sweat, but at increased intensity a very effective mechanism for thermoregulation is through the evaporative heat loss of sweat. A horse’s sweat is hypertonic, which means it is more concentrated with electrolytes than the blood plasma. Subsequently, the type and intensity of exercise will significantly impact water and electrolyte balance. For example, endurance horses will experience a rise in temperature of 0.25˚C/min, where as a racehorse will experience a rise of 0.8˚C/min (Marlin et al, 1995). This directly correlates to the rate at which electrolytes are lost as sodium and chloride are lowest at the onset of sweating whereas potassium is highest and this will reverse as duration increases. As a result, racehorses are likely to lose more potassium and endurance horses are subject to higher sodium and chloride losses (Lindinger and Ecker, 2013 and Spooner and Nielsen, 2010).
If horses become electrolyte deficient it can have serious implications on muscle integrity and has been linked to recurrent bouts of Exertional Rhabdomyolysis, as well as lethargy, fatigue and causing horses to go off their feed and water. In more severe cases it can cause metabolic conditions such as synchronous diaphragmatic thumps and hypocalcaemic tetany, as well as collapse and even death (Harris and Riverio, 2017).
There are practical solutions that can be implemented to ensure adequate water and electrolyte levels are maintained. Firstly and most importantly, there has been ample research to suggest that horses will rehydrate better if they are being fed electrolytes on a daily basis as the intake of sodium will help trigger the thirst stimulus and encourage drinking, thus shortening recovery periods and prolonging the onset of fatigue. It is also important to consider the basic rules of thumb when introducing electrolyte powders to feed, for example, introduce slowly and gradually, especially with fussy eaters, ensure access to water at all times and feed electrolytes daily to horses in work. Commercial feeds and forages will not provide adequate levels of salts, in the correct ratios and despite contrary opinion it is has been shown that horses cannot successfully self-regulate their salt intake from a salt block (Butudom et al, 2002 and Hess et al, 2008).
References
Butudom, P., Schott, H.C., Davis, M.W., Kobe, C.A, Nielsen, B.D and Eberhart, S.W. (2002) Drinking salt water enhances rehydration in horses dehydrated by frusemide administration and endurance exercise. Equine Veterinary Journal. Vol 34: 516-518
Harris, P And Rivero, J. (2017) Nutritional considerations for equine rhabdomyolysis syndrome. Equine Veterinary Education. 29 (8), pp: 459 - 465
Hess, T.M., Greiwe-Crandwell, K.M., Waldron, J.W., Williams, C.A., Lopes, M.A., Gay, L.S., Harris, P.A and Kronfeld, D.D. (2008) Potassium-free electrolytes and calcium supplementation in an endurance race. Comparative Exercise Physiology. Vol 5: 33-41
Lindinger, M and Ecker, G. (2013) Gastric emptying, intestinal absorption of electrolytes and exercise performance in electrolyte-supplemented horses. The Physiological Society. Vol 98: 193-206
Marlin, D., Harris, P., Schroter, R., Harris, R., Roberts, C., Scott, C., Orme, E., Dunnet, M., Dyson, S., Barrelet, F., Williams, B., Marr, C. and Casas, I. (1995) Physiological, metabolic and biochemical responses of horses competing in the speed and endurance phase of a CCI**** 3‐day‐event. Equine Veterinary Journal. 27 (20) pp: 37-46
Mitchell, B. (2018) The importance of hydration in horses. Equine Health. Vol 2018 (42).
Robert, C., Goachet, A.G., Fraipont, A., Votion, D.M., E. Van Erck, E. and Leckerc, J. (2010) Hydration and electrolyte balance in horses during an endurance season. Equine Veterinary Journal. Vol 42: 98-104
Spooner, H., Nielsen, B., Schott, H and Harris, P. (2010) Sweat composition in Arabian horses performing endurance exercise on forage based, low Na rations. Equine Veterinary Journal. Vol 42: 382-386