If you are clipping the horse's 'whiskers' or vibrissae, you are essentially depriving him of an essential sensory organ needed for his safety and ability to detect food and other things in his environment. Did you know that each vibrissae is associated to a distinct part of the horse's brain?
From Evidence Based Horsemanship:
Did you know that horse whiskers actually have a name and a distinct purpose? They're actually sensory organs called "vibrissae."
They act as feelers for the horse and help protect vital areas like eyes, ears and lips by detecting things which may be in the horse's blind spots or too close for visual focus. While it may look pretty to clip these whiskers, you're actually increasing the risk of trauma to the face by removing these important sensors.