strong enough bit to keep her head up without fighting her. every time she puts her head down pull it back up and heal her into a trot. If healing doenst work go with stronger aids like spurs, slappers on the reins, etc. You also have to be consitent. Never ever ever let her eat with you in the saddle. Doesnt matter what other riders or horses are doing. DONOT let your horse eat while you are in the saddle. Never ever ever.
Id it is break time on a trail ride and a nice patch of grass comes up, or a stop and people are socializing I get off the horse and let him eat.