Using stocks to treat or examine a horse is common in veterinary practices, and on some large farms and ranches. Stocks help contain a horse, freeing up a vet or other staff who would otherwise have to hold a horse.
However the improper use of stocks (or the use of unsafe stocks) can be dangerous to horses and handlers. Do not use stocks with horses that resist pressure on their head and will not tie, as they may react violently to restraint within stocks.
Halter the horse with a well-fitting halter and lead rope. With an assistant at the back of the horse, ask the horse to enter the stocks as you would ask them to load in a trailer. Ask with pressure on the lead rope and instantly reward a "try" with slack.
If the horse sets back, keep gentle pressure and have your helper drive the horse from behind. Instantly release when the horse makes a "try" toward the stocks. If the horse rears, try to keep pressure (unless the horse continues to go higher in which case you may need to release to save the horse going over backwards), but drive harder from behind.
Consider whether you want to put a rearing horse in the stocks at this point. Horses that rear when being asked to move forward have not been properly halter trained. It is much safer and wiser to train the horse the basics in a safer location.
Once the stock doors are closed, an experienced horse handler should always stay at the head of the horse to keep it from getting its head on either side of the upright bars or otherwise getting into trouble.
Tips for safety & Success
Stocks can provide a false sense of security. You must pay attention to the horse at all times. Do not become complacent or distracted.
If the horse is sedated, pay attention to the placement of the head and neck. If your horse lowers its head too far, it may rest its throat on the bar of the door. This could place pressure on the arteries to the brain causing the horse to pass out and collapse.
If you have not used the stocks before or have not used them in a long time, inspect them thoroughly and test the latching mechanisms before you load your horse.
A horse falling down in the stocks is a dangerous situation. If a horse falls down in the stocks, move all equipment you can out of the way. Then open all stock doors to allow it to get to its feet. If this is not possible because of the design of the stocks, you may need to carefully drag the horse by its tail into a position where it can rise.