Here is a quick article Parelli wrote on this topic......maybe it will help you.
Horse Eating Grass
It's a common scenario; long, lush, enticing grass everywhere, and we want to play with our horse. Let's consider two things; when don't you want him to eat grass... and, what you can do to stop him that is fair, friendly and uses psychology and communication rather than punishment?
When don't I want him to eat grass?
I often hear people say, "I never let my horse eat grass when I'm riding", and guess what...their horse is one of the worst! He takes every opportunity to lunge at the grass, even unseating the rider to get at it.
If we think about it from his point of view, he's surrounded by delicious grass and then gets punished for wanting to eat it. This is just like taking a small child into a candy store and expecting him to have enough self-control to ignore all that candy. It's a pretty tough thing to ask, right?
By being more considerate, we could give him some time to eat before asking for his full attention, then allow him to eat it now and then, but only when you invite it. Be sure to give permission rather than just letting him plunge his head down. Invite him by using the Porcupine Game... simply lower his head to the ground when you want him to graze. It's a great way to improve Game #2!
How can I stop him when he does try?
The first thing is to clearly establish your alpha position in his herd through the Seven Games. You've also proved to your horse that you're not an unreasonable person, and that you're considerate of his desires. This prevents any resentment from forming as you become particular when he eats grass.
Now, use cause and effect. When your horse lowers is head, allow him to start munching without even touching the reins or rope, then smooch and lightly begin tapping his hindquarters, becoming firmer and firmer until his head pops up. Stop tapping right away and rub.
At first it may be a surprised reaction and his head may go straight down again. Just repeat the smooch and progressively firmer tapping until his head comes up, then rub him again.
Very quickly, a quiet smooch is all it takes to ask your horse to lift his head from eating grass. Your horse will learn to graze when you invite him to, and to stand respectfully until you do. Your horse will now think first whether he's been invited to eat before just hauling you into the brush for a quick bite.
Be prepared to out-persist your horse on this once your phases of firmness and timing of the rub is effective, the change is lasting, and sure beats the old habit of pulling on his reins or lead rope, not to mention his building resentment from your lack of regard for his desires.
This combination of knowing the horses desire, establishing our expectation, and having the technique to communicate without punishment is very effective. If you feel yourself getting mad, remember that it's nothing personal, and he's been munching grass much longer than you've been asking him not to. Hang in there with him; smile, whistle, and have fun with this project.