Water Quality for Horses

Water, the essential nutrient for life should be freely available for horses. The amount of water consumed by a horse is the best measure of water adequacy. The average daily intake of an idle horse weighing 1,100 lbs. under thermoneutral conditions is between 6 and 9 gallons. Heavy workloads and high heat and humidity may double to triple the requirement to 12 to 18 gallons per day. Lactation also increases water intake to a minimum of 8 gallons per day. Diet will affect water consumption. Grazing of lush, green pastures in the spring tend to decrease water consumption. Horses fed all hay diets drink more water then horses fed mixed hay and grain diets. In the winter, water below 45^oF will reduce the horse’s voluntary intake and can lead to colic and/or impaction.

Figure 1. Horses Drinking Water. This Photo by Unknown Author is licensed under CC BY-SA

Interestingly the horse only spends 5–6 min/day drinking water over several visits to the water source. The bucket is the most common method of watering domestic horses. Heated buckets or adding warm water to the bucket help with water intake in the winter. Intake of water using automatic water systems is influenced by water receptacle and mechanism for dispensing water. Horses prefer float valve over pressure valve systems and prefer large bowls to small bowls.

Quality of water is important for a healthy horse. Water quality panel tests include nitrate/nitrite, total dissolved solids (TDS), sulfates, and coliform bacteria. The best indicator of water quality is total dissolved solids (TDS). The TDS sums the concentration of all substances dissolved in the water. The safe upper limit of TDS for horses is 6,500 ppm (parts per million or mg/L). Water below 1,500 ppm TDS is considered fresh water. Water greater than 5,000 ppm TDS is considered to be saline. Most human drinking water is less than 500 ppm TDS. The water can also be assessed by odor, color and temperature. Odor is affected by the amount of sulfates, manure or rotting vegetation. An increase of any of these can affect palatability and voluntary intake.

Coliform bacteria include total coliforms and fecal coliforms. These bacteria are considered indicators of the presence of animal wastes. Fecal coliforms are present in large numbers in intestinal contents of warm-blooded animals. Once outside the body they die fairly easily. Water containing fecal coliform bacteria should not be consumed without adequate treatment. Chlorinated water kills most bacteria. If the water is high in bacterial growth it may also be high in nitrates. Both may be due to surface contamination from runoff (fertilizers and manure).

Horses are more resistant to nitrate toxicity compared to ruminants. However, ingestion of large amounts of nitrates possibly due to contaminated water can cause gastrointestinal problems.  Water containing less than 400 mg/L of nitrate is generally safe, while a level over 1500 mg/L may be toxic. Nitrate can be converted to nitrite in the horse’s cecum. In horses, nitrite is 10 to 15 times more toxic, and concentrations exceeding 30 mg/L may be hazardous to a horse’s health. Generally, the only time a horse gets exposed to nitrite is ingestion of high-nitrite hay that was baled wet and the microorganisms changed the nitrate to nitrite. Thus, never feed moldy or wet hay to horses.

The Iowa State University Veterinary Diagnostic Laboratory will test the quality of water if you suspect a problem. The Water Quality Panel measures nitrate/nitrite, total dissolved solids, sulfates, and coliform bacteria.

1. Equine Applied and Clinical Nutrition. 2013. Saunders Elsevier.
2. Nutrient Requirements of the Horse. 6^th Revised Edition. 2007 National Research Council. The National Academies Press. Washington, DC.