With the hot weather upon us, providing access to good clean water is even more vital now than at any other time of year, due to the increased requirement for water to cool a horse’s body down in high temperatures.

How much water should a horse have access to and how much does a horse drink? There are no simple answers to these questions as water requirements are affected by many factors, including environment, diet, body size, metabolic rate and even the fat content in the body. As an example, it has been shown that horses increase their water intake 15%-20% when the ambient temperature increases from 13°C to 25°C.

A “normal” horse requires approximately 50-60ml/kg body weight/day (25-30 litres/500kg) to replace the fluids required/excreted in the process of conducting basic bodily functions such as metabolism, respiration, and elimination of wastes from the body. If “water in” doesn’t equal “water out”, homeostasis (maintenance of the body’s equilibrium) cannot be maintained, and the horse will suffer if this inequity is not rectified in a timely manner.

Typically, in horses, water is obtained by voluntary consumption, predominantly from drinking but with a smaller quantity obtained from food intake. Water is lost from the body in urine, faeces, sweat and respiration. In the warm to hot weather, water lost through sweat can be 50-100% higher than it is during the winter, as horses rely on sweat to cool their bodies. Water lost via the respiratory system is low compared to water loss through sweating, defecating and urinating, but still needs to be considered when assessing fluid requirements and does increase in hot, humid weather. Manure contains a variable amount of water and can account for 30-55% of the water lost from the body, and will vary according to the diet.

Generally, horses on a high roughage diet will require more water than horses on a high grain diet. Bacteria resident in the hindgut require large volumes of water to break down the roughage ingested and produce nutrients from this roughage that the horse can absorb. Horses eating lush grass will appear to drink less when compared to a horse eating hay, however, this is because the horse eating lush grass obtains a greater part of his daily water requirement from the grass than the horse eating dry feed. Horses that are stabled or yarded have been found to drink most of their water intake within an hour or two of eating their feed, giving evidence to the close relationship between diet and water consumption.

Horses lose 20-55% of water through urination; however, this may be less in hot weather than cold weather, as the body will need to conserve fluid if water losses through sweating and respiration are increased. Horses that are fed a diet high in protein will require larger volumes of water to compensate for the increased levels of nitrogenous waste that are produced from metabolising the proteins that subsequently need to be eliminated from the body in the urine.

The kidneys play a major role in maintaining the water balance in the body by altering how much water that is either resorbed through the renal tubules or excreted out in the urine.

Homeostasis is reliant on the kidney receiving messages from the brain to regulate the body’s fluid so that the osmolality, or number of molecules and ions in the plasma, is constant. The mechanism for the control of water passing out through the kidneys is governed by the release of a hormone called antidiuretic hormone, or ADH, that is produced in a part of the brain and stored in the pituitary gland. Any change in the osmolality detected by osmoreceptors in the brain (hypothalamus) will trigger a response to either increase or decrease the water resorbed back through the kidneys and into the blood.

When the body senses an increase in the osmolality in the blood, ADH is released into the bloodstream and travels to the kidney where it increases the amount of water resorbed from the renal tubules. ADH works by changing the permeability of the tubule membrane to water, allowing water to move back into the blood, effectively producing concentrated urine. As the name suggests, ADH will decrease the amount of urine produced (a diuretic increases urine production). If osmolality is low, there is no release of ADH, and more water will be passed out in the urine as the tubule membrane becomes less permeable to water, producing dilute urine.

ADH will also be released if there is a significant decrease in the blood volume, even if the osmolality is within the normal values. Receptors in the blood vessels pick up a change in the pressure if the blood volume decreases and this triggers ADH release. If ADH is insufficient to replace blood volume or to change the osmolality, thirst receptors are triggered in the brain that stimulate the horse to drink water. This can be seen sometimes when a horse has lost a large volume of blood through an injury and looks for water to drink.

Apart from the kidneys, the body also relies on resorption of water from the hindgut to increase blood volume or decrease blood osmolality. As stated above, the hindgut is home to millions of bacteria that are required to help digestion of certain feed substrates, and this fluid also acts as a reservoir that can be accessed when the body becomes dehydrated and requires additional fluid. This works well as a short-term supply of water, however, if not replaced or if an excessive amount of water is resorbed, the intestinal contents or faeces become dry and impacted and this can lead to colic. I would typically see more colic in January and February than any other months of the year and this is most likely due to horses resorbing this reservoir of fluid, resulting in very dried faeces.

There are numerous incidences where the daily water intake is increased above the basal rate. Exercise increases the demand for water, especially when the horse is worked in hot, humid conditions. Evaporation of sweat is the predominant manner in which the horse deals with heat loss, and exercising in any weather conditions will utilise more water through evaporation and through respiration because exercise increases the respiratory rate. The actual increase in the water required can be two to threefold on a standard 20-30 litres given to a non-exercising horse and will vary according to the duration, the intensity, and the environment in which it exercises.

Lactation is another scenario that greatly increases the demand for water and, depending on the age of the foal, can increase the need for water by 50% or more. A mare may produce 15 litres of milk a day to feed her foal, so not only are there an extra 15 litres of water required to meet the volume of water lost this way, but additional water is also needed to digest the extra feed required to produce this milk and to eliminate the extra faecal output produced with a larger feed and to remove the increase in nitrogenous waste in the urine from this larger diet.

Illnesses, especially diarrhoea, will increase the demand for water for the obvious reason that the water content in the manure is so high compared to healthy faeces. In some cases, the horse will increase its voluntary water intake to cope with the increased demand. In severe cases of diarrhoea, the consumption of water cannot meet the demand for fluid, or more often, the horse is too sick to eat or drink and intravenous fluid is needed to maintain hydration.

In cases of kidney failure, where the kidney loses the ability to resorb the water adequately, the horse quickly becomes dehydrated and needs to compensate by drinking excessive quantities of water. These horses require uninterrupted access to water and are what is known as Polyuric/Polydipsic (PU/PD) meaning they urinate a lot and drink a lot. Horses with kidney failure, however, should not be confused with horses that have Pituitary Pars Intermedia Dysfunction or PPID and show signs of excessive drinking and urinating, as PPID horses have high cortisol levels that inhibit the release of ADH and therefore prevent the regulation of water resorption. Horses with kidney failure usually have a poor prognosis for long-term survival whereas PPID horses can often be managed successfully for many years.

Checking your horse’s water supply daily is vital, as scenarios such a dead bird or rodent contaminating the water, a dislodged plug in a trough or small hole in the bucket that allows the water to leak, or an automatic waterer that fails to replenish the water supply after the horse has drunk, can all lead to a decrease in water consumption and interfere with the body’s ability to maintain adequate hydration. Unfortunately, there have been several incidences of horses found dead in paddocks or yards simply because their human caretakers failed to check that water was freely available, a mistake that should never happen.

This has been a very simplified version of the mechanisms involved in maintaining hydration in the horse and the importance of water intake being adequate to cover water lost through varies bodily functions. The combination of complex interactions between electrolytes, feedback loops and other metabolic pathways all play a role in fluid balance and osmolality and are probably outside the interest limits of most readers. EQ

YOU MIGHT ALSO LIKE TO READ:

Electrolytes: Getting Out What You Put In – Equestrian Life, January 2022