A couple of weeks ago, I came across some photos on Facebook that someone had posted from a recent horse show. In several of them, her horse could be seen tied to the trailer with the chain over its nose!
A horse should never, under any circumstances, be tied with the chain over the nose (or under the chin, for the matter). The further down the nose you go, the more sensitive the facial structures become. A chain that slips down (it can slip down with the noseband even if it's wrapped around) can easily reach the more sensitive structures and cause a lot of damage should the horse pull back on the chain. Even higher up on the nose, the full weight of the horse against the chain would likely have very bad results.
I am not against using a lead chain over the nose for leading; I use one on my own horse at shows just in case the atmosphere gets a bit too exciting. One downside to the chain is that when you need to apply pressure, it can tighten against the nose and not release on its own. When there is a human at the other end of the chain, this pressure can quickly be released by hand and the maximum weight that the horse can pull back against is that of the human, approximately 10-20% of the horse's own weight. There is a risk that the horse could get loose and step on the lead, but that is a small risk that can be weighed against the benefits of the extra insurance that the chain provides.
When you tie with the chain, however, the force when pulling back can be up to 100% of the horse's weight, a big difference to those sensitive parts of the face! There is also no one to release pressure when the chain tightens, meaning that the horse can be punished for extended periods simply for standing still.
If your horse does not tie well, the answer is not to tie it to the trailer! Such a horse probably shouldn't be tied at all without a wall behind it, especially not to an object that could possibly be tipped or dragged. A chain would likely just make the horse panic more in addition to damaging the nose if the horse were to pull back. On a fairly cool day, you could keep the horse tied on the trailer (always with a bar or wall behind), or in warmer weather rent a day stall or simply hold the horse for a few hours.
Some things are just not worth risking your horse's well-being for, and tying your horse to anything with the lead chain over the nose is one of those things. Some people might get away with it some of the time, but when it goes wrong, it will go very wrong.
Have you witnessed a dangerous situation related to showing that you would like others to be warned against doing? You can leave a comment on the blog, tell me on Facebook or send me an e-mail at email@example.com.