Volume 12: Livestock Farming
6. Cattle feeding and rearing
Digestive system of cattle

6.2 Cattle are ruminants. Their stomachs are divided into four compartments - rumen, reticulum, omasum and abomasum - of which the rumen is the largest. Beyond the abomasum are the small and the large intestines. Ruminants differ from other animals insofar as much of the breakdown of food is brought about by fermentation by micro-organisms inhabiting the rumen. The act of rumination, or chewing the cud, plays a part in breaking down the food and exposing it to digestion by micro-organisms. The products of digestion provide the basis for further growth of the rumen micro-organisms which themselves become an important food source for the ruminant.

6.3 Fermentation by the microbial population in the rumen allows ruminants to utilise a wide range of plant products and by-products which non-ruminants (such as pigs and poultry) and humans cannot digest or would not choose to eat.

6.4 Protein supplied to ruminants comes from two sources: microbial protein (produced in the rumen) and protein in feed sources which has escaped breakdown (or degradation) in, and therefore bypasses, the rumen. This microbial protein and the 'escaped' or undegraded protein are digested by enzymes in the abomasum and the small intestine. ¹

6.5 Microbial protein is sufficient to meet the ruminant's protein requirements for maintenance and low levels of milk production. However, cows have been bred since the Second World War to produce ever-greater milk yields, leading to the need for correspondingly greater precision about the diets they are fed to ensure they meet their genetic potential. Developments in the understanding of the protein requirements of ruminants in the early 1980s ² led farmers to seek to boost milk yield by feeding cows concentrates containing protein that bypassed the rumen. Indeed, the addition of protein concentrates to the diet of dairy cows appears to be an unavoidable consequence of breeding programmes to produce dairy cows with high milk yields. One vet told the Inquiry that cows need to be fed according to their genotype, and that feeding a high-yielding cow with a low-yield diet could result in the cow losing weight and becoming infertile. ³