The most important nutrient in the horse’s diet is one that is rarely thought about in detail: water.
An adult horse’s body is composed of roughly 70% water, which equates to about 95 gallons (360 liters) of water for the average 500-kg horse. Foals have even higher water content, roughly 80%.
The amount of water required daily by the horse is determined by the magnitude of water losses from its body. These losses occur through feces, urine, respiratory gases, sweat, and, in the case of the lactating mare, milk. An individual horse’s water requirement is also greatly dependent on the age, environment, temperature, humidity, exercise intensity and duration, fitness level, and type and quality of feed consumed, among others. In the case of the lactating mare, stage of lactation also plays a large role in water requirement.
Average water intake for a 1,100-lb (500-kg) horse, according to the National Research Council:
- Mature, idle, 68° F (20° C): 5.5-7.5 gallons (21-29 liters)
- Mature, idle, 86° F (30° C): 11- 14 gallons (42-54 liters)
- Lactating mare: 10.5-17 gallons (40-63 liters)
- Moderate exercise, 68° F (20° C): 7-12 gallons (26-46 liters)
- Moderate exercise, 86° F (30° C): 19-24 gallons (72-92 liters)
Environmental temperature has a dramatic effect on water consumption. When temperatures rise, intake rises; the same applies when temperature drops, which can lead to increased incidence of impaction colic. The addition of salt to the diet will increase the horse’s desire to consume water.
Water temperature also plays a role in voluntary water consumption. Water around 50-65° F (10-18° C) seems to be the sweet spot to encourage consumption. Studies have shown a drop of 38-41% when horses were offered near freezing water. In many parts of the world where temperatures are often below freezing, farm managers install heated waterers or troughs to encourage water consumption.
Aside from temperature, the freshness, purity, and palatability of the available water also influence water consumption.
As a rule, water intake is proportional to dry matter intake, but the composition and digestibility of the feed is also a factor. Horses consuming all-hay high-fiber diets drink more water than horses fed a grain-based higher carbohydrate diet coupled with hay. Diets higher in fiber, protein, sodium, and potassium may increase water intake and subsequently result in increased urination.
Monitoring horses’ water intake on pastures can often be misleading, especially when pastures are lush and green. Green pasture grasses can be 65-80% water, which means many horses can consume much of their water requirement simply by eating pasture. A study of the intake of pregnant and open mares showed a pasture intake of 134-165 lb (61-75 kg) and 87 lb (39.5 kg), respectively. The pasture was measured at 79.6% moisture, equivalent to 68-132 lb (31-60 kg) of fluid and close to the horse’s requirements.
The drinking behavior of horses is episodic and circadian in nature. These episodes can be modified by the water source, quality, temperature, availability, and the age of the horse.
Water quality plays a major role in not just determining intake but also the health of a horse. Measuring water quality can be broken up into two distinct parameters: physical and chemical/ biological. Physical parameters monitored include odor, color, temperature, turbidity, and total dissolved solids (TDS). Chemical and biological factors include hardness (cation exchange), pH, minerals, protozoa, algae, bacteria, and chemical residues.
Water quality is an important driver in water intake. Odors from organic material, minerals such as iron and sulfur, can negatively affect palatability. Temperatures below 40° F (5° C) and over 86° F (30°C) have shown significantly reduced intake. Color seems to be more of an issue to humans than to horses.
Turbidity is an optical determination of water clarity. Turbid water will appear cloudy or murky. Turbidity can come from suspended sediment such as silt or clay, inorganic materials, or organic matter such as algae, plankton, and decaying plant material. Suspended solids and dissolved material reduce water clarity by creating an opaque, hazy, or muddy appearance. Turbidity measurements are often used as an indicator of water quality based on clarity and estimated total suspended solids in water.
Total dissolved solids (TDS), as the name suggests, is the combination of all inorganic and organic substances present in a liquid in molecular, ionised, or microgranular suspended form. The safe upper limit of TDS for horses is 6,500 mg/l. Water below 1,500 mg/l TDS is considered fresh water. Water greater than 5,000 mg/l TDS is considered saline. As a guide, most drinking water is less than 500 mg/l TDS.
With respect to pH, water should be in the range of 6.5 to 8.5. If the pH is more acidic (less than 5.5), acidosis and reduced feed intake may occur. Highly alkaline water (over 9) may cause digestive upsets and diarrhea.
Biologicals such as protozoa, algae, and bacteria are quite important to monitor, and some degree of vigilance must be maintained to ensure they are not affecting water quality. Protozoa are single-celled organisms such as cryptosporidium and giardia which can be introduced into the water system after a deluge of rain if your water comes from dams, creeks, or rivers. Signs of protozoal problems in horses include diarrhea, fever, and dehydration.
Many types of algae are present in the water system and generally bloom due to eutrophication. One of the more notable species is blue-green algae (cyanobacteria). If consumed in large enough amounts, blue-green algae can cause muscle tremors, respiratory distress, seizures, diarrhea, death. Other algae can produce hepatotoxins (toxins affecting the liver) that can cause acute death or delayed death after signs of rapid-onset liver failure. Photosensitization, a skin condition affecting unpigmented areas of skin, can occur in animals that survive the acute stages of liver damage.
Bacteria such as salmonella and Escherichia coli can be an issue in horses consuming poor-quality water with major health and reproductive problems ensuing.
Testing of water is not expensive and provides a snapshot of your water quality which, when added to pasture testing, nutritional feed analysis, and veterinary diagnostics, can provide an accurate assessment of farm health.
If you do choose to test your water, find an accredited laboratory that is able to interpret results and provide advice. If you choose to test periodically, such as seasonally, use the same laboratory to ensure consistency. Take advice on how to sample, where to sample, and how to store samples before delivering them to the laboratory.
Depending on the size of the farm, water-quality management may be as simple as regular cleaning and maintenance of tanks and troughs or the addition of filtration systems designed to reduce organic matter. Ultraviolet light systems, water softeners, and chlorine are also used to boost water quality. More substantial treatment options are available for those establishments with problematic water.
A common product used to assist with trough hygiene is copper sulfate or products containing copper sulfate. Many people overdose this, which can lead to extremely high copper levels in the water. Overconsumption of copper in the total diet can affect some mineral relationships, like zinc, leading to deficiencies and possible issues with growth, immunity, and general health.
