Overweight or obese horses are at the same types of health risks as overweight humans, dogs and any other type of animal. Since in the wild horses would be continually required to graze and forage, the chance of a horse in the wild becoming obese is almost non-existent. In captivity, horses are restricted to small areas, fed high carbohydrate, protein and fat diets, and only exercised for small periods of time per day, all leading to obesity problems.
Obesity puts more strain on all the various aspects of the horse's body from breathing and respiration through to digestion and cardiac functioning. The more weight the horse is carrying the greater the stress will be on the cardiac and respiratory system, especially when the horse is being exercised. Since obese horses rarely get routine exercise, this difference in respiration and heart rate, especially in hot or humid weather can quickly lead to heat stroke and heat intolerance. The excessive body weight will also prevent the dissipation of heat in the natural body cooling process, further leading to problems with heat stroke and stress.
The more that the horse weighs the greater the chance of leg and joint problems. Additional weight places greater pressure on the ligaments and tendons, as well as on the hoofs themselves. Obese horses often develop osteoarthritis as they age due to wear and tear on the joints and poor muscle tone compared to the overall body weight. Obese horses have a greater chance of developing back problems and sway backs as they mature and age.
Horses and ponies that are obese have more likelihood of developing colic, laminitis and founder, especially when placed on lush pastures or fed good quality supplements outside of the regular ration. Owners may not realize that they are putting these ponies and horses in greater risk by actually overfeeding them to begin with. Colic, a digestive problem, can occur because of gas build up, digestion problems and blockages that are more problematic in overweight horses.
Ponies are actually more prone to becoming obese than horses. This is because many of the pony breeds were developed and bred to adjust to very poor grazing conditions and their metabolisms have slowed down to allow them live on very low quality and quantity feeds. When these ponies are brought to areas where there are high quality pastures and supplements, the pony's metabolism still works the same as when he or she was on poor pasture or how it has been genetically geared to work. Even moderate amounts of high quality feeds can cause a pony to gain weight in a very short amount of time.
Careful monitoring of your horse's weight and diet as well as regular exercise is very important to ensuring that obesity is not a problem in your horse or pony.