In the past, it was said, “Never feed a horse treats or it will start biting”. Well, that can be true if a “marker” sound is not used to announce the giving of a treat. I always use a “kissing” sound before I give a treat. The horse hears the sound and soon makes the association that a treat is coming. The horse also then makes the association that when no “kissing” sound is heard, no special treat will appear.
If you just give a horse a treat from your pocket at any time with no special marker sound, the horse then associates you and your body as a treat dispenser. This is when the aggressive behavior can start. The horse will nuzzle your body looking for a treat and then that is when nipping can develop as the horse becomes frustrated because a treat can’t be found (and it appeared on the human just the other day!!)
Consistent use of a consistent marker sound either with your voice or clicker, will create an honest relationship with your horse. He will know exactly what is expected from him and what he can expect from you. I use the “kissing” sound when I don’t have a clicker with me, but using the clicker as a marker will lead to further training opportunities!