The local German papers featured adverts. for the brew in late winter and into early spring.
(I'm not a beer fan, but as I understand it, back when city animal ordinances were more lax, enterprising home brewers would feed all the necessary ingredients to their goats and then collect the warm, foamy, golden, organic brew for annual festivities. If not properly filtered, drinking Bock beer could leave a prickly mustache ;-). Bring back the goats!
More accurate information can be found at: http://en.wikipedia.org/wiki/Bock