Today’s guest blogger is By Anja Dijkstra, DVM, from the Netherlands
The article first appeared on the Vetsweb website.
Water is the first-limiting nutrient for horses but the amount of water a horse requires depends on internal and external factors such as ambient temperature, diet, work, milk production, health and physiological or pathological water loss.
Approximately 60% of total body weight in a normally hydrated adult horse is water. This is called Total Body Water (TBW) and decreases with age. The TBW in a foal is around 70% of body weight and TBW is therefore relatively higher than in an adult horse. Physiologically water is lost through feces, urine, respiratory gasses, insensible skin loss and sweating. Water can also be lost due to pathological circumstances e.g. blood loss, renal failure, diarrhoea or gastric reflux or even a relative loss by intestinal pooling. Health problems occur when excessive body water is lost or if the horse cannot drink enough.
Water intake
Drinking is induced through stimulation of receptors in the oropharynx which detect dryness of the mouth and controlled by osmosensitive neurons in the hypothalamus (Kohn and Hansen, 1998). The thirst center is situated in the hypothalamus and water intake via drinking is regulated to maintain homeostasis. If extra water, from e.g. diet, or less water is consumed, the thirst center will respond by respectively reducing or increasing thirst. Water available to the horse comes from drinking or feed as well as water that is produced by metabolic breakdown of carbohydrates, fats and proteins. The latter can provide up to 10% of the water needs (Roussell et al. 1989). Horses at rest in a moderate climate will generally consume between three and seven liters of water per 100 kg of body weight. However, water requirements are more proportional to metabolic body size and body composition than weight. A pony requires relatively more water than a large horse and a fat animal appear to require less water than a lean one.
Influence of diet
The effect of diet on voluntary water intake and requirements depends on composition and digestibility. Horses that only eat hay drink more than horses on a diet consisting of concentrate and hay or a complete pelleted feed (Cymbaluk, 1989). When a horse consumes fiber intake (Pagan and Harris, 1999). Fecal water loss is greater with higher forage intake due to a larger water holding capacity of plant fibers and poorer digestibility. The increased water loss will in turn stimulate water intake. Other dietary effects are, increased water consumption when protein intake is above the horse’s requirement, this also results in increased urinary output to get rid of excess nitrogen. High salt intake increases urine output as well as increasing water intake by stimulating thirst.
Ambient temperature, work and production
Horses typically drink less water in lower temperatures. Higher temperatures increase water requirements, especially in exercising horses. A study done on adaptations in trained thoroughbred horses to daily exercise in hot and humid ambient conditions (33-35ºC and 80-85% relative humidity) for 22 days, showed that over the 3 week period of heat and humidity, mean 24 hour water consumption increased from 26.0 +/- 2.1 liters to 39.5 +/- 3.2 liters, largely reflecting a 2-fold increase in water intake during the 4 h period of heat exposure (Geor et al., 1996). Water consumption of weanlings kept in a non-heated barn (mean temperature -5.2 +/- 1.72ºC) was as much as 14% lower than water consumption of weanlings housed in a heated barn (mean temperature 10.9 +/- .66 ºC) (Cymbaluk, 1990). In addition to these factors duration and intensity of work, and production (milk) affect water consumption. Total water intake of a 500 kg horse in work can rise to more than 90 liters per day, depending on the conditions in which the horse is exercised (National Research Council, 2007). A combination of fluid losses associated with milk secretion and increased consumption of feed to enable milk production accounts for increased water intake in lactating mares. These factors and the previously mentioned factors (diet, ambient temperature etc) result in lactating mares drinking up to 75 litres per day (National Research Council, 2007).
Dehydration
Dehydration is the loss of total body water, this occurs when the body loses more fluid than it takes in. This condition can result from loss of water caused by the already mentioned physiological or pathological factors or by decreased water intake. Horses deprived from food or water for 72 hours had an average weight loss of 10.7% of body weight, decreases in extracellular fluid volume and plasma volume. After the experimental dehydration period 62% of the lost weight was replaced within one hour just by drinking water (Carlson, G. P., Rumbaugh, G. E., et al. 1979). Dehydration can be detected clinically. However physical function is already compromised before clinical signs of dehydration are visible. A horse that is mildly dehydrated (5-7%) will be depressed, has dry mucous membranes and a prolonged capillary refill time. When dehydration is moderate (8-10%) next to depression and an even more prolonged capillary refill time, the horse will also have a weak pulse and tachycardia, poor jugular distensibility and decreased skin turgor. In the case of more than 10% dehydration the horse will be in hypovolaemic shock, capillary refill time will be longer than 5 seconds, skin will remain raised when pulled, the extremities are cold, the horse is recumbent and moribund.
How to hydrate?
Horses should always have free access to good quality, palatable water. The exceptions being, straight after strenuous exercise horses need to cool down first before being allowed to drink and in case of medical indications (e.g. post sedation, reflux, oesophageal obstruction). Too much cold water has to be avoided as well in horses to prevent colic or laminitis. Horses that have lost water and electrolytes through sweating can be encouraged to drink more, after cooling down, by making sure palatable lukewarm water is available. Oral administration of electrolyte pastes can improve voluntary water intake and recovery, especially when NaCl is a component of the electrolyte paste (Schott, H. C., 2nd, S. M. Axiak, et al. (2002). Mild dehydration can be corrected orally, when oral fluids are not contraindicated and the horse is willing to drink. If the horse doesn’t drink (enough) a nasogastric tube can be used to hydrate the horse. However in case of severe dehydration immediate intensive intravenous fluid therapy is indicated.