Stocking up is pretty normal for a lot of horses, especially if yours spends most of its time outside. I read an article on it recently....here is some info from it:
Several other factors can contribute to stocking up. Just as in humans, a horse’s circulation diminishes with age. In rare cases, heart disease may be involved. And a horse will be more likely to stock up if he has a history of cellulitis or lymphangitis--even a single episode many years in the past. In both conditions, infection causes inflammation of the skin and subcutaneous tissues, which clogs the leg’s lymph vessel "sewer system." Eventually, this leads to scarring of the lymph vessel walls, reducing their long-term elasticity and diminishing their function even more.
The good news: Stocking up may be unsightly, but it is generally harmless and painless. What can you do about it? First check to be sure your horse isn't dealing with a more acute condition, such as an infection or tendon, ligament or joint injury. If the swelling disappears after exercise or turnout, you can be pretty sure it was just the result of stocking up. If swelling remains, occurs only in one leg (stocking up is invariably bilateral), is painful or is accompanied by a fever, ask your veterinarian to investigate further.
Many horsemen like to use standing wraps to prevent stocking up, but bandaging can actually exacerbate the problem. It prevents air and sun from reaching the skin and concentrates sweat, opening the pores to dirt and bacteria and increasing the risk of infection. And bandaging also can create pressure rubs, which compromise circulation. Over time, the lymphatic system can become dependent on routine bandaging, so the next time you stable your horse without bandages, he'll stock up even more.
Similarly, most topical treatments (poultices, liniments, etc.) are counterproductive. By disrupting the natural balance of healthy bacteria on the skin, they make it more prone to infection.
The best solution to stocking up is to let Mother Nature take care of it. Turn out your horse and exercise him as much as possible. If you have a paddock with shade trees or a run-in shed, turn him out there on hot days instead of stabling him. If there’s no outdoor shade available or it’s too buggy to keep him pastured around the clock, stable him for just the hottest, buggiest hours of the day.