Longer. Bake it longer.
I've had success with modifed goat and horse treat recipes. Whole wheat flours, rolled oats (small quantity), rye or buckwheat flours, pumperkickel, peanut butter or similar binder, dried fruit (cranberries can often be found sulfite free and at reasonable price), molasses as a sweetener. Makes a VERY stiff dough. Force into a shallow pan (or shape on thick aluminum foil) then bake till dry. Now keep baking. REALLY dry. Closer to an hour.
If there is any moisture in it, the cookies will be quicky colonized by molds and mildews. Not good eats. and since its a treat, you can't use it quickly unless you have a large flock. So it MUST be cooked to near deser quality - less than 10% moisture total.