Horse bones are incredibly dense and fairly difficult to break, but when they do break they do not heal well, easily, or quickly and are very prone to infection. Most would die a slow painful death from infection, even with antibiotics and other medical care. Those that survive would most likely not have full use of the leg, and have a leg prone to break again.

There are several reasons:

Horses are pretty weird anatomically – they have no muscles below the knee. Part of their ability to run super fast is this really incredible tendon/ligament system – muscles higher on the body load the tendons and ligaments with huge amounts of power (picture a spring). Because of this, the bones in their lower leg (which is almost always where they break) are relatively very thin but also generating massive power. This ratio is much more extreme in the horse than other animals, so it means that solving it with a few pins like you might in other animals won’t do the trick. In addition, this massive power makes it more likely that the horse will basically shatter the bone rather than just a simple break or fracture . Also, they’re just really big. foundering – if the horse can’t move his legs around, he can’t circulate blood through properly and it somewhat pools in the hoof and the attachments of the hoof to the bottom of the leg weaken. Compare this to a human or dog, where we can wiggle our toes and whatnot to help move circulation through – remember, horses have no muscles below the knee. Trying to keep a horse from moving around for any extended period of time is a nightmare. There’s a saying I heard that all horses have two goals in life: homicide and suicide. They will colic and die if a bird looks at them wrong. In addition to the possibility of foundering, a horse is pretty likely to just lose his marbles if you try to keep them sedentary for too long – they may try to climb out of their stall, break whatever apparatus they’re in, colic (severe and sometimes fatal stomach ache), get ulcers and whatever else. You can’t force them to lay on their side for very long at all – they’re too heavy and will have difficulty breathing and damage nerves.

When deciding whether or not to save an animal, quality of life must always be considered. A dog may do perfectly well and have a wonderful life in a cast for a while and then with a moderate limp – this is not true for horses. Further, cost can be very prohibitive; it’s worth it for very very expensive horses (think Barbaro) but for your average pleasure horse, the cost and pain is just not worth it or generally feasible.

The physical difficulty of getting an isolated horse bone to heal well is what leads to the difficulty in treating it. It’s incredibly physiologically stressful on the horse. Healthy horses spend like 18-22 hours of a given day standing on all four legs and meandering. If a horse breaks its leg, it must be literally immobilized for weeks in order to not put any weight on the leg, until the healing process is stable. But horses who cannot stand, who are kept laying down, develop a multitude of other problems – laminitis, difficult breathing, sores, lameness in other ligaments/tendons/muscles/etc. You’d be extinguishing a grease fire in the kitchen, only to turn around and realize the wildfire from the forest had consumed the rest of your house.

horses are resistant to anything containing their leg. When you put support wraps on a horse who’s about to take a long trailer ride or be stabled for a while (to help with circulation when they can’t walk around), a horse who has never had wraps on will spend some time kicking their legs out and high-stepping, until they figure out the wraps aren’t going to hurt them. That doesn’t even bring the hoof into the equation – horses with items on their feet (even if it’s a boot with medicinal stuff like poultice or antibacterial soak that’s making them feel better) will go to extreme lengths to get That Thing off their foot – kicking, stamping, flinging, chewing at their own leg. If you were going to effectively cast a horse with a broken leg, it may need its foot casted, which it would not like, and it’d probably need to spend the whole time sedated.

And, lastly, you may have seen other animals with broken legs that have healed, right? Dogs, cats, etc. However, those animals don’t need to put weight on their leg, and don’t need it for their gait in the same way that a horse does. Cats and dogs frequently find a way to walk on three legs, but due to a horse’s skeletal structure and the way they’re gaited, they simply can’t walk or support themselves long-term on only three legs, let alone trot/canter/pace/etc. If you were going to go to extremes and consider amputation, you’d have to look into expensive and sophisticated prosthetics.