2+

Cows produce babies.
Chickens are babies with confused pasts (they don't even know if they came before or after the egg).
If they were born on The Farm, chances are they don't know what came first.

Therefor, 30 cows gave birth to 28 chickens (ignoring multiple births to the same cow), so a minimum of 2 cows did not give birth to the next generation of farm products.

We are talking about the CIA, right?
Because all I know about actual chickens is that they have a unhealthy fascination with spaghetti.