There should be a special order added that allows some one to make it so they reduce amount of supplies that the army eats in one day like going on half ration and quarter rations. Why i suggest this is because a lot of invasions that were made during the real civil war they would get supplies raided or have to abandon some to get away faster and it would make it really realistic to add a button to ration food supply.
However the cost of doing that should make it so coehesion goese down by what ever amount seems reasonable.