The digestive anatomy and physiology of ruminants is markedly different to that of non-ruminant animals, including pigs and man. The ruminant has two additional digestive organs at the anterior end of the tract. The first of these, the rumen, contains in the dairy cow, for example, a liquid volume of 60–100 litres. This organ is essentially a fermentation chamber, containing about 1010 bacteria and 105–106 ciliate protozoa ml-1, together with an unknown, probably smalI, number of anaerobic fungi. The partly fermented food and the microorganisms then pass through the omasum, which is much smaller than the rumen (Figure 12.1). Its function is primarily to absorb both water and soluble nutrients.