Lots of people like their horses to look super neat (I'm not one of them, but no judgment). And though there's something to be said for clipping a horse's hair coat in general for any manner of reasons (showing, high summer temperatures, etc.) there is one are of the horse's body that should definitely NOT be trimmed: their whiskers.
If you're familiar with cats, dogs or other animals that have whiskers, you may already know that those whiskers play an important role in their ability to sense the environment around them. And it's absolutely the same with horses. Technically called vibrissae, a horse's whiskers around its eyes and muzzle help it to gauge distance from objects and in some cases, might be used to test an electric fence before touching it*. Generally, they help provide tactile information and are considered extremely sensitive by the scientists who have studied them.
Trimming whiskers has already been outlawed in Germany, Switzerland and France because they are deemed an essential part of the horse's anatomy. And while it may not be illegal here in the US, it should certainly be considered taboo out of a concern for horses' well-being. Bottom line: what we humans might deem unsightly or messy needs to be put in the context of the animal's ethology (natural way of being, outside of domestication). The whiskers were there first, and our opinions, alongside domestication, came after.
*McGreevy, Paul, Equine Behavior Second Edition (Saunders Elsevier, 2012), 49.
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