[A_Brandybuck]

I think there is no doubt, that the food, which is cultivated in Isengard, is not enough for the thousands of orcs and similiar things.
In the meantime I doubt, that the food came the whole way from Mordor to Isengard. It is too risky for them. Such a supplying could be easily discovered by the 'enemy'.
What is about the theory about the 'lands' beyond the gap of Rohan? He could have bought the food in this lands. From the Dunlendings or later from the Shire (like the tobacco).

Or I remember, that I read somewhere, that there is the possibility to cultivate some food on plains in the mountains (I mean the Dwarves did that). Saruman could have lied his acres in Isengard idle to give noone the possibiliy to be suspicious. Instead of cultivate in Isengard, his could have 'out-sourced' the production of food to the near mountains.