Food consumption and combat effiency.
An army doesn't march on a empty stomach, unless in Stormfall, and it doesn't make sense. If there is no food, there should be starvation, famine....
So how about rather than troops just consume food, they consume rations, and level of rations would decide their effectivity. If your food levels not enough to cover your troops, then they get decreased rations to avoid go in negatives. Less rations less effective they are.
Cool you got a huge bad ass army, shame they survive on water and a piece of bread a day and as effecitve in combat as a pesant with a pitch fork.
There need to be a consequence of running in shortage of food. You shouldn't be able to buy new units if you running in negatives, or doesn't have a posetive food stock.
I don't really care about how the game is a fantasy game, and players enjoy it more if there is no penality for running with food shortage, it's not right. Bottom line is you need food/fuel to sustain life.