No other nutrient is as important to horses as water. Under average resting conditions horses become severely dehydrated in as little as 72 hours. Under adverse conditions of heat and exercise this can occur in less than 12 hours. Not only is regular access to water essential to good health but the quality of water is also critical. Though much has been written on the proper watering of horses there has been little real study on the subject. Recently a group has studied the clinical, biochemical, behavioral, and hygiene effects of different watering techniques. This article presents theirs and others findings and discusses water quality used for horses.

A number of substances, which may be present in water, are quite toxic if sufficiently high concentrations are present. Toxic concentrations of water contaminants, including pesticides and herbicides, most commonly occur as a result of stagnant or runoff water that contains, feces, disease-producing organisms, or chemicals from agriculture and industrial wastes. A list of the recommended upper limits for some potentially toxic substances in drinking water for horses, and those not toxic but which, if present at concentrations above those given, reduce water palatability Some potentially toxic substances do not reduce water palatability.

Water Contaminants	Upper safe level in parts per million (ppm or mg/L)	Notes
Arsenic	0.2	-
Cadmium	0.05	-
Calcium	500	Not toxic at concentration given but may decrease water palatability.
Chloride	3000	Not toxic at concentration given but may decrease water palatability.
Chromium	1	-
Cobalt	1	-
Copper	0.5	Not toxic at concentration given but may decrease water palatability.
Cyanide	0.01	-
Fluoride	2 (for growing) - 8 (for adults}	-
Hardness	200	Water "hardness" indicates the tendency of water to precipitate soap or to form a scale on heated surfaces. Hardness is generally expressed as the sum of calcium and magnesium reported in equivalent amounts of calcium carbonate. Other cations such as strontium iron zinc and manganese also contribute to hardness.
Hydrogen Sulfide	0.1	-
Iron	0.3	Not toxic at concentration given but may decrease water palatability.
Lead	0.1	-
Magnesium	125	Not toxic at concentration given but may decrease water palatability.
Manganese	0.05	-
Mercury	0.01	-
Nickel	1.0	-
Nitrate	400	High levels are evidence of fecal contamination.
Nitrate nitrogen	100	-
Nitrite nitrogen	10	-
Potassium	1400	Not toxic at concentration given but may decrease water palatability.
Selenium	0.01	Lower levels have caused chronic toxicosis with long term exposure.
Silver	0.05	-
Sodium	2500	Not toxic at concentration given but may decrease water palatability.
Sulfate	2500	May effect palatability at around 300 ppm
Total Dissolved Solids	6500	TDS or salinity include the sum of the concentrations of all substances dissolved in water Salinity and TDS should not be confused with hardness. Highly saline waters may contain low levels of the cations responsible for hardness. For more on TDS see below.
Vanadium	0.1	-
Zinc	25	High levels may result where galvanized pipes are in contact with copper.

Total dissolved solids or salinity in the water in ppm or mg/L	Water Use
1000 - 3000	Satisfactory for all livestock and poultry. May cause mild and temporary diarrhea in livestock not accustomed to it but should not affect their health or performance.
3000 - 5000	Should be satisfactory for livestock although it might cause temporary diarrhea or be refused at first by animals not accustomed to it.
5000 - 7000	Can be used with reasonable safety for livestock. May be advisable to avoid water approaching the higher level of ppm for pregnant or lactating animals. Unfit for poultry and swine.
7000 - 10000	Considerable risk may exist in using this water for pregnant lactating or young animals or for any animals subjected to heavy heat stress or water loss. In general the use of this water should be avoided although animals other than these may subsist on it for long periods.
> 10000	Not recommended for use by any animal under any

Where water is high in nitrates, it will usually also be high in bacteria and algae. Drinking water containing some bacteria and algae may be harmful. Some species of blue-green algae (cyanobacteria), which grow on pond and lake water, may result in poisoning: therefore, water with heavy algae growth should be avoided. Heavy algae growth occurs most commonly during the summer months and is exacerbated by contamination of the water with fertilizers or decaying organic material. Horses that have acutely died of algae poisoning may have algae in their digestive tracts. Liver poisoning and photosensitivity have also been observed with exposure to blue green algae.

Most infectious diseases can be transmitted through water. The accepted indicator of water quality is concentration of coliform bacteria in the water. What is safe for horses is not known but levels up to 50 coliforms per 100 mls of water is acceptable in humans in some areas of the world. The US sets a standard of less than 9 per 100 mls as safe. Other potential pathogens are Giardia, Salmonella, and Cryptosporidiosis.