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Horses have a strong flight instinct and often resist being loaded into a trailer.  Start slowly, introducing your horse to the trailer without forcing him to get in. Walk him around and let him check out the trailer. Reward all positive steps towards the goal of loading including sniffing the trailer, not backing away and exhibiting curiosity. Pat or rub him and give verbal encouragement. Do not use force or violence as this will create a bad association for your horse and make him even more resistant to the trailer. It may take many tries to get your horse comfortable enough to load. Let him back out whenever he wants to. The goal is make him comfortable and relaxed so that he will load easily. When the horse loads for the first time do not shut the doors as this may panic him. Once he is completely comfortable, close the butt bar and doors. You want the horse to walk out of the trailer, not bolt.  Use patience when teaching him to back out of the trailer. Use verbal cues or a small tug on the tail to signal that it’s time to back out. If he wants to rush out of the trailer, use the lead rope to slow him down.  Do not attempt to pull or push the horse. Do not stand behind the horse. Untie the horse from the trailer before releasing the butt bar or opening the back door.  When he unloads properly, reward him with praise and patting or rubbing. Some horses are very resistant to being in a trailer.  Your horse may be afraid of the trailer because of past bad experiences. Ask a horse professional for assistance if your horse is particularly resistant or reacts with violence (such as bolting or rearing). Do not lose your patience or react with violence as this will exacerbate the problem.
Practice loading. Practice Unloading. Know when to get help.