Cities supply factories and HQ ... factories supply fields. You have to take out the head of the chain to make a difference.
So if you want to really pork a strat factory and not have it re-supplied before you land, you have to take the Cities out.
So if you reduce the Cities to 0%, and then reduce the Ammo factory to 0%, and then take out the ammo bunkers at a field, that field will not have ammo for quite some time ... up to 3 hours.
If a city is taken down to 0%, it will take 3 hours for that city to regenerate. So, while the city is completely down, it will not supply the factories and the factories will not supply the fields ... very simple in concept ... hard to accomplish.
I don't know how many time I have told people bombing the factories that there efforts are in vain unless they take down the city.