The dairy farm is the source of cheese for civilians to eat. The farmer must bring up cattle before cheese production starts. Once started, cheese is produced faster and in larger quantities than apples, but it falls short to bread production.
Cows are useful in more ways than being a food source. Tanners can take away cattle to cull and skin for their leather, which is manufactured into leather armour for crossbowmen and macemen. One cow provides enough leather for three sets of leather armour.
Cows can also be used as 'ammunition' in siege warfare. Catapults and trebuchets can fling cattle carcasses to a location, which spreads disease clouds in a short vicinity. Cows used this way will disappear from a random farm, and do not have to be killed or transported to the engine that fired them.
Note that cows have to be replaced and overly high demand will hinder food production from dairy farms.