WD 40 for the butt bar. High value food in the trailer. Walk the horse so he is straight behind the trailer, a few feet from the ramp, and stop quietly. Quietly put the long lead shank over his back just behind the withers. Give the signal to walk on (whatever you usually do) and walk toward the trailer. His path should be straight into the stall. Your path should stay just left of his - ie don’t get between the horse and the space on the trailer. Hopefully he keeps walking onto the trailer and you stop walking before you hit the trailer wall. If he backs out, you put up your hand and pluck off the lead rope. It takes some horses longer than others but every horse I’ve ever met has learned to self load. Just keep everybody calm and keep working at it. Good luck!
Well I did work on this yesterday. I loaded both horses by leading them on and walking out the other (empty) stall while I still had the lead rope attached. And then I got the one horse, who is the harder to load (not that mine are really hard) to self load on the first try.
In my mind, this was a little too easy. I think maybe they were just having a good day. I want to keep practicing. I want it to be where they see the open trailer and walk on, and don’t even think about it. They’ve always been worse about loading once we are out. If they load super easy, 100% consistently at home, then they will probably load decent when we are out.
Also I only loaded each of them by themselves; now I need to load them both.
I taught mine to self load. I walk to the ramp, toss the lead rope over their back, and in they go. I do the butt bar, put up the ramp (or close up shop if it’s my step up LQ) and shut whatever else and off we go. I never ever tie until at least the butt bar is up.
My late ottb, I just led him in (didn’t tie, he stood in there on his own) and then just walked around and put up the butt bar.
My new guy, I taught him to self load. I tried leading him on but he isn’t always 100% reliable yet to stand there on his own. For him the self loading works better.
In the beginning I walk on with them, leave the lead draped over the chest bar, and walk round to the back and catch them as they back out. They’re quiet, a little anxious about being in the trailer, and I just let them back out and reset until they stay in.
Then I bang the bum bar against the trailer wall or divider and catch the horse as they back out, until they stay in.
Then I walk across the ramp behind them, and catch them as they come out, until they stay in.
Then I do up the bum bar and let them back into it, even if I have to go to the front and suggest they back into it. I’m going up front to encourage the calm step forward anyway, Then get the step forward from behind.
Anyway, once they’re good with that I will swing the divider over and go partway in myself to get the horse all the way in. Or go partway in the other stall if the divider is low enough to reach over (mine isn’t so it must swing). I can stay further and further back until I’m staying outside with the divider in position.
Then I stand beside the ramp like so
Use a lunge line.
I “self-load” mine into their stalls daily with the cue “step up.” Miss Mare is easy peasy, Mr. Gelding not so much but light years better than he was.
I trailer by myself all the time in my straight load and my horse now self loads but didn’t come that way.
First you have to make sure you have the basics. Does the horse load into a trailer and stand quietly? Are they comfortable with the trailer? Make sure that’s all good first.
Next, without the trailer in the picture, does your horse understand all the basic ground handling cues from the ground, can you move their feet and position their body, back them up etc. And the big one, send them forward without you leading. Then test the sending forward in various situations, like between a barrel and arena wall.
Then you should be able to send them forward into the trailer. I stand outside, lead rope over the back and send my horse forward. I’m standing at the butt bar ready to do it up. Then walk around to clip in front.
To unload, I unclip, throw lead over the horse’s back, then walk around to back of trailer. I wait until horse is standing quietly to undo butt bar. Then gently encourage to back up and grab lead rope as the horse is about halfway out.
I will say I started with a horse who was already very good about trailering so that helped a lot.
I’ve always taught my horses to self load and it’s invaluable. I do use ‘high value’ treats in the hay bag, they don’t usually get much grain so a few McCauley pellets on top of the hay always makes their day.
The important thing teaching self loading is take your time, be very clear and don’t get emotional. “Don’t encourage a horse who is already trying” is one of my favorite and most important sayings. If your horse is looking IN the trailer, ears up, considering getting in, DO NOTHING. If they are turning away, backing up, whatever, redirect them to looking back IN the trailer. Even one step forward deserves a break in the action. if they turn their head towards you block them with your arm (because the next step is they may just step into/onto you to leave). If they turn to the right like they are going to leave the other way bring their head back until they are straight. Break. Then ask again for forward. I usually swing the end of the rope (I love the yacht ropes), if I need to tap them with it because I’ve got no reaction, I do. Just always be aware of whether they are ‘trying’ or not. A try can be very small and missed by a lot of people (even pawing a ramp is a try so don’t discipline for that!). Take a walk in a circle break after a good try sometimes. Re-establish the forward.
There are some good videos out there on this subject. And yes, once they are in do up the butt bar and then go secure the head and I may give another treat 
My horse does not self-load. I walk all the way in with him. I have him trained to stay there while I duck under the chest bar and out the people door, walk around, and do the butt bar. Then I go back and tie him.
To unload, I untie him and snap the lead rope to his halter, walk to the back and let the butt bar down, then walk back to his head and ask him to back out. He’s not allowed to walk out without me.
Originally, I wanted to teach him to self-load but he preferred I stay with him at his head and this works, so I stopped trying any other way. The only problem is he needs to load first so there’s enough room for me to walk next to him if we’re also taking a friend. However, my friends have all been accommodating and my horse is small.
