The potentcy of certain vit. and minerals are slowly lost over time after exposed to environment. We make our own hog feed and buy vit. and min. and mix our own, so I know about these things.Just because your dog takes four months to go through a bag of dogfood, don't think that she is getting all she needs. Shelf life of certain ingred. in dog food are not affected by exposure to air, like corn,soy and wheat. I feed a dog food with just one source of protein, chicken,fish or lamb and ONE source of carbs, Potato or brown rice. and this feed has to be eaten quickly or you loose the effectiveness of some of the vit. and min. I don't buy dog food where the first ingred, is listed as anything other than whole chicken, fish or lamb, no soy, no corn, no wheat, no by products, no bht, no preservatives, added colorings, etc. Vit E, is one of the vit. that will be affected the most by being on the shelf too long. Good luck with whatever you decide to do. Patricia in La.