i have a yard that is fencened in good and its in two halves and when i let them in the other half one heads over to our barn door.which was open.well 30 minutes later i go to do the chickens and my horse is sitting there eating scratch/layer5 mix...i got her out and locked it up....well the other day they got out....we got them put up and then i went to feed the chickens and 3/4s of the mix was gone.we bought 2 bags of food for chickens the day before and 3/4s or more are gone?the horse ate all of that by herself!!!!!!!!!!!!!how do i stop her from eatting it?