Is there any problem more frustrating than packing up for a show or trail ride, running a few minutes behind, rushing to load your horse.. only to find the horse won't walk in the trailer!?
Walking in to a trailer is a difficult task for a horse, who has every instinct telling him to flee from cave-like spaces.
There are two reasons that a horse will not walk into a trailer: fear or disrespect. Some horses legitimately find trailer loading terrifiying, while other horses refuse to walk in a trailer simply because they think if they refuse strong enough, they won't have to. How do you tell which your horse is? A hore truely afraid to load onto a trailer will appear tense and distraught, the whites of his eyes may be showing, he may appear to be ready to bolt, he may poop as soon as he sees the trailer (a sign of anxiety in horses). A horse who is simply testing boundaries will appear calmer. More likely to stand sedately outside the trailer while you try to load him, or may barge over or around you to avoid loading.
If your horse is legitimately afraid, give them a chance to get familiar with a trailer. Many trainers recommned parking a trailer in the horse's turnout area (keep it hitched! Horses should not be allowed to load in an unhitched trailer) tying the doors open, and feeding the horse out of the trailer- starting on the loading ramp, then eventually feeding in the farthest corner of the trailer. This method allows the horse to familiarize themselves with the trailer on their own terms, and provides a reward (feed) for interacting with the trailer.
For the simply willfull horse, we prefer a simple exercise. It isn't a "quick fix" to get your horse loaded in time for a show, but is lower stress than other methods and each time you attempt this method it will take less and less time.
We simply make being outside the trailer the uncomfortable place!
Prepare your trailer- get it ready and open, get the horse ready to load, walk them up to the trailer and give them and opportunity to load themselves. If they refuse or try to turn away, BACK them up about 10 steps (never turn them away from the trailer) then make them start to halter work: backing up, turn on haunches, trotting in hand, etc. This does two things: 1. tells the horse you are the boss! 2. Gets them wondering, like any horse, how to get out of having to work.
So present them with a way out: walk them back up to the trailer and give them the opportunity to load. One try. If they refuse or try to walk off, back them up and start it all again. Repeat this cycle as many times as it takes and try to make it increasingly difficult or physically exerting for them. For example, in hand work the first few times, then in hand work with lots of backing up, then lunging at a trot for 2-3 minutes, then lunging at a trot for a longer period, then if you get to that point, cantering on the lunge, etc.
As soon as they load, they get to rest- and most learn that really quick. Most problem loaders take 30 minutes to 1 hour with this method the first time (horses that could take hours previously) but each time the method requires less and less time as they realize they can avoid work by walking in the trailer.
There are many training methods that work to get a horse to walk into a trailer when they don't want to, but this is one that has worked extremely well on our farm. The only "trick"- with any trailer loading method, really- is just keeping your cool. Prepare yourself to spend all day, if that's what it takes, relax, and adopt a "it'll take as long as it takes" attitude. More often than not, your attitude will help the horse decide to walk into a trailer more than any other factor.