Bob Judd, DVM, DABVP (Equine Medicine), DABVP (Canine and Feline Practice)
Multiple diseases can cause your horse to drink excess water. Drinking excessive water in horses is not common, but when it occurs, the reason could be important.
Dr. Emily Barnett, from the University of Minnesota, says in the publication Equine Disease Quarterly that a healthy average size horse weighing about 1,100 pounds will drink about 13 gallons of water each day, and produce about three to four gallons of urine daily.
Horses classified as excessive water drinkers usually drink at least two times more than horses that drink the 13-gallon amount, sometimes more. If it is determined that your horse is drinking excessive water, one of the most common causes could be psychogenic polydipsia.
Psychogenic polydipsia means there is no real medical reason, and your horse is drinking excessive water because they want to, not because they have to do so. In other words, your horse is not sick but just wants to drink excessive water. This can be caused by stress relating to diet, management, or housing situations. Cushing’s disease is very common in older horses, and 30% of the horses with this disease drink excessive water.
Chronic kidney disease will certainly cause horses to drink excessive water, but the amount of water consumed is not as great as those with other diseases and may go unnoticed.
Your horse can develop diabetes, but this disease is much less common in horses than in small animals. Other causes of increased drinking are high salt diets, some sedatives or other drugs like corticosteroids, and drugs that can cause kidney failure, like some antibiotics or nonsteroidal anti-inflammatory drugs like Bute or Banamine.
If you notice your horse drinking lots of water and flooding the stall overnight with urine, call your veterinarian for an exam.