Water, the number one nutrient

Horses grazing at the WCVM’s Goodale Research Farm. Photo: Michael Raine.

During a hot July in 2006, the owners of a southern Saskatchewan farm made a grim discovery when they found five of their 19 horses dead, in or near a slough. The remaining horses were severely dehydrated and had diarrhea.

The local veterinarian’s first suspicion was anthrax, a prevalent disease in the area at the time. But blood tests for anthrax were negative.

The case soon involved veterinarians from the Western College of Veterinary Medicine (WCVM) who performed multiple tests and launched a farm investigation. Blood analysis showed the typical findings of a dehydrated, diarrheic animal. Toxic plant ingestion and bacterial and parasitic infections were ruled out.

So what caused this sudden outbreak of diarrhea and death in this horse herd? What the investigative team eventually discovered was that a gate had been inadvertently closed, barring access to the farm’s automatic water troughs and forcing the thirsty horses to drink slough water. Water samples from the slough revealed that poor water quality — specifically high salinity and high sulfate concentration — was the culprit.

Don’t forget about the water

Because the body of an adult horse is 70 per cent water, good health maintenance requires access to water that’s suitable for drinking. “Water is the number one nutrient,” says Dr. Katharina Lohmann, a large animal internist at the WCVM. “Horse owners are concerned about feed and supplements, but sometimes they forget about water.”

Ideally, horses should be given water from a source that’s fit for human consumption such as municipal drinking water or well water that’s been tested, suggests Lohmann. If natural water sources are used, she recommends testing the water at a provincial laboratory before allowing horses to drink up.

Water quality measures include total hardness, specific conductivity, presence of algae and/or bacteria, pH and coliform count. But the major factor contributing to water quality is the amount of total dissolved solids (TDS) in the water. Several elements contribute to TDS including calcium, phosphate, nitrate, sodium, potassium and chloride.

How much TDS is too much? In her 1999 presentation, “Water: the overlooked nutrient” at the Alberta Horse Breeders and Owner Conference, equine veterinary specialist Dr. Nadia Cymbaluk specified that a TDS value greater than 5,000 milligrams per litre (mg/L) is considered unacceptable for horses.

Environmental factors

During the summer, hot and dry conditions evaporate water resulting in water reduction and a higher concentration of TDS. Horses also tend to consume more water during hot weather, increasing the amount of contaminants ingested.

Warm temperatures, coupled with increased nitrogen levels, can cause rapid growth of blue-green algae that’s toxic to animals. Signs of blue-green algae intoxication develop suddenly in horses and can include bloody diarrhea, muscle tremors, seizures and difficulty breathing.

Water supply must also be considered during the winter season. Although snow feeding can occasionally supplement water intake, both Lohmann and Cymbaluk stress that snow isn’t a substitute for water. Use of heated watering bowls prevents water from freezing, but horse owners need to check their automatic waterers daily to ensure that they’re functioning properly.

Out with the bad, in with the good

Supportive therapy counteracts the effects of contaminated water. In the case of the Saskatchewan horse herd, the main treatment was intravenous fluids to correct the horses’ dehydration and electrolyte imbalance caused by the diarrhea.

Lohmann suggests seeking immediate veterinary attention if dehydration results from drinking poor quality water or from refusing to drink at all. According to Cymbaluk, a test of hydration that owners can perform on their horses is the skin tent. Pinch the skin on the neck or shoulder, twist it and observe how long it takes to return to its normal position – longer than two seconds indicates mild dehydration.

Manage your horse’s water

Horses need their water intake monitored. In general, their average water intake for maintenance is 5.5 litres per 100 kilograms of body weight per day. “It’s absolutely important to have enough water for all animals,” stresses Lohmann, adding that water intake varies depending on ambient temperature, humidity, activity level and water content of feed. Here are some recommendations for managing water intake and quality at your farm or acreage:

    • test all natural water sources available to horses.
    • if tested water has high mineral levels, adjust mineral content in the feed accordingly.
    • apply water treatments if necessary. For example, copper sulfate controls algae blooms.
    • check water supply daily.
    • ensure that water doesn’t freeze in the winter.
    • avoid stagnant water during a hot, dry season.
    • monitor horses for signs of dehydration such as a prolonged skin tent, sunken eyes, dry feces, low urine output and dry mouth.


Burgess BA, Lohmann KL, Blakely BR. 2010. “Excessive sulfate and poor water quality as a cause of sudden deaths and an outbreak of diarrhea in horses.” Canadian Veterinary Journal. 51(3): 277-282.

Cymbaluk N. 1999. “Water: the overlooked nutrient.” Fred Pearce Memorial Lecture, Alberta Horse Breeders and Owner Conference, Red Deer, Alta. Cymbaluk is director of veterinary research for Pfizer’s Linwood Equine Ranch in Carberry, Man.

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