You don't want her to stop entirely, you just want her to eat with your permission. Allow your horse some grazing time before riding. If she still persists on the trail, try this. Let her munch, but then using a kissing sound, tap her hindquarters, becoming firmer until she lifts her head. Stop tapping immediately and give kissing sounds with petting. She learn to associate something pleasant when she stops grazing. It takes patience and being consistent. But your horse will eventually learn she can't stop to eat without your permission.