Largely, the subject line is a grabber (it did get you looking, or you wouldn't be reading this

) but what I am really trying to accomplish is the following:
I am working on a set of rules that will use 15mm figures, but will be more strategic than tactical, with each figure probably representing a battalion/squadron. This will be fought out on a table representing two imaginary nations, complete with villages and a capital.
So why the cows? I want the game to have an economic aspect, where the smaller towns will generate production each turn (represented by the cows) which must be moved to the capital, where they are then converted into troops. The cows will move like any other unit, can be attacked and destroyed by the enemy, or captured and converted to his own use, etc.
I was originally thinking of getting some type of small, cheap toy truck to reperesent production convoys, but the advantage of the cows would be that they could be used to represent production in virtually any period (I would like to give basic rules for an ancient or medieval war, with upgrades to WWII and maybe even sf and fantasy) inasmuch as soldiers always gotta eat, whereas trucks could really only be used for the 20th century+.
Of course, I could use other things instead. Maybe sheafs of wheat? Bails of some kind? Chickens would probably be too small, unless multiple chickens could be mounted on the same base. Carts, of course, would suffer the reverse problem from trucks.
Any ideas? Whatever it is has to be small, easy to paint or modify, cheap and plentiful, and look reasonably in scale with 15mm.
Thanks