8. Citations Bell, Alan W. Encyclopedia of Animal Science . Ed. Wilson G. Pond. Marcel Dekker, Inc., 2005. Krogh, David. Biology: A Guide To The Natural World . 4th ed. Boston: Benjamin Cummings, 2009. Nutrition and Feeding of the Cow-Calf Herd: Digestive System of the Cow." Virginia Cooperative Extension . 20 Jan. 2009 <http://www.ext.vt.edu/pubs/beef/400-010/400-010.html>.