All feeds contain some water, ranging from around 10% in concentrates, 10%-15% for hay and up to 80% in fresh pasture. Horses grazing on fresh pasture with a high moisture content can derive virtually all their water requirement from the herbage. Water is also generated by several normal metabolic processes, so the total water intake derives from that internal source, as well as feed and drinking water. As a guide to total daily intake the NRC 2007 provides the following estimates for a 500kg horse.
Note the increase in water intake associated with the higher ambient temperature and exercise which is related mainly to sweat losses. High ambient temperature, solar radiation and high humidity can markedly increase requirements – up to three fold. Intake can also be influenced by water temperature and horses prefer tepid water at around 20o with a reduction of as much as 40% of intake when both water and ambient temperatures fall below 5o. Horses also prefer to drink water from buckets rather than automatically fed water bowls – when both bucket and pressure fed, or float valve bowls were offered the test horses took 98% of their water from the bucket. Around 80% of drinking occurs within two hours of eating.
Ideally water should be available ad lib throughout the day, but this is not always feasible and intermittent provision of water in buckets works fine so long as the amount is sufficient.