I want to take apart the story about the horse and the trailer and the food and leadership, so we can see the actual laws at play within that situation rather than the story created around it. It’s only when we see clearly, that we can make informed choices for ourselves as trainers and for our horses as learners, so let’s begin.

First, we have to look at the nervous system.
I like to describe the nervous system to my clients as the scales of justice, a scale on each side hung from a central point. When one side of the nervous system gets heavier (or activated!), it hangs down a bit lower and the other is lifted a bit higher. The two sides counter-balance one another.
Essentially, one side is for threat preparedness, and the other is for return to homeostasis and relaxation. To make it easier to remember, think of your horse having a “survive” side of his nervous system and a “thrive” side of his nervous system.
Officially, the “survive” side is called the sympathetic nervous system or SNS and the “thrive” side is called the parasympathetic or PNS. We all have both and we need both to be alive.
Based on your horse’s behavior, you can observe which side of their nervous system they are operating out of. It’s good to know, and it matters a lot when you are trying to work on re-training something previously stressful like trailer loading.

On the thrive side we have: rest, digest, feed/eat, and breed.

On the survive side we have: fight, flight, fidget, faint, and freeze.

When we are truly worried about our survival, we don’t: lay down to sleep, stop for a bite to eat or to check out a love interest as we run for our lives.

When we feel safe and unthreatened, we do not: fight, run away from others, pace around, faint or remain frozen or immobile (like standing in front of a trailer.)

So, in the story about the horse who loses interest in food when asked to get on a trailer, what that detail really tells us is the horse was worried enough about being asked to step onto a trailer to be pushed into the sympathetic side of his nervous system.
His survival instinct just got triggered and when that happens, eating goes offline.

But does that mean food is useless when it comes to teaching and maintaining the skill of trailer loading? Oh my gosh, NO! It means you have to know how to use it. And to really understand the power of food, you need to understand reinforcement histories.

So, imagine this: Each time you ask your horse for a behavior, they perform the behavior and you feed them (ABC). Each time, they experience a little jolt of pleasure in relation to the behavior you asked for. Over time, your horse will grow to have a general impression, or “classically conditioned emotional response” to being asked to perform this behavior.
This emotional response is the composite of every time he performed the behavior and the consequence that followed. So, if each time you asked your horse to walk forward on a lead and they complied, you stopped and reinforced them with some grain, they would have a very positive emotional affect when asked to walk forward. Walking forward predicts good things. So, they feel good when you ask them to walk forward. This is their reinforcement history for going forward on lead. It contains the depth of multiple repetitions, rather than the shallowness of one bucket of food in the present moment.

So, cool! It would seem you were all set to walk your horse forward, which they LOVE, and into the brand new trailer you just bought!
But here is where folks go wrong. Walking forward near the trailer or into the trailer, ALSO needs to predict good things. This is a separate reinforcement history. (This is compound now. Walking forward+trailer = ?) If every time I walk into the trailer, my person closes me in and takes me on a long bumpy ride and then I’m away from home and my friends for days, then my reinforcement history for walking forward and onto the trailer is going to be poor. Getting on the trailer will predict unpleasant things.
To fix this, just breaking the ratio of loading to actual trips helps tremendously. If I load up ten times for every one time that I actually go somewhere, then I won’t worry so much about getting on the trailer. I’ll probably get on quite easily as it usually will predict a nice big flake of alfalfa and then unloading to go back to my paddock. And I’ll eat the whole time, because I’ll be lounging around in the “thrive” side of my nervous system.

So what of the horse who refused to get on the trailer and ignored the nice bucket of food? He wasn’t lacking for a leader and he, contrary to the post, DOES find the trailer to be a the problem. Trailers are small, often dark and when a horse steps in they don’t know how long the ride will be and where they will end up. Unpredictability is, by nature, punishing. Think of blindfolding an adult human and telling them you are taking them in the car for a “birthday surprise.”Lots of people panic, pull off the blindfold or get really angry about the situation. Honestly, it’s no different for the horse.
They see the trailer, their survival mechanism gets invoked and they go into (often) an extended freeze response to avoid loading up.

Their reinforcement history is insufficient to the task being asked.

So what do horses need?
Horses just need an observant, educated human to assess what part of their learning history needs to be re-worked if they are not loading into the trailer. The time to train trailer loading is not at the horse show or clinic or vet hospital. Yes, there are emergencies and natural disasters and other situations that come up and require urgent loading. And in those moments you do the best you can with multiple tools: chutes, panels, etc, because those are “oh crap!” situations, not training scenarios. But most days aren’t emergencies. Most days are calm and open and perfect for getting to work building deep reinforcement histories. Build yours carefully and deeply enough, and that reinforcement history will always lead right into the trailer.