Exposure of food to the bacteria and protozoa of the rumen has both advantages and disadvantages; it enhances the nutritional value of poor foods but may detract from the value of highly nutritious foods that could be digested without the aid of the micro-population of the rumen. This state of affairs has clear benefits for freeliving ruminants as it assists them to survive during winter or during the harsh, dry conditions of warm climates. T h e extravagant use of foods when plenty is available probably does not matter very much to wild ruminants. It does however matter to the farmer who has to grow or buy food for his beasts. It is for this reason that considerable attention has been given recently to the protection of food constituents from the effects of the rumen. T h e first method is to make use of the oesophageal groove mechanism of young animals to convey liquid foods direct to the abomasum and the second is to process dietary additives with the object of increasing their resistance to the processes that go on in the rumen.
The oesophageal groove mechanismT h e fact that milk bypassed the reticulum and rumen in the young calf was noted by early authors who attributed this to the oesophageal groove. T h e experiments of Wester (1930) established that closure of the groove and the passage of milk or other fluids to the rumen was a reflex involving excitatory areas in the mouth or pharynx. Since atropine inhibited closure of the groove, he contended that vagal efferent fibres were involved. Later, Comline & Titchen (1951) established that stimulation of the superior laryngeal nerve of calves caused reflex closure of the groove and that efferents were confined largely to the dorsal abdominal root of the vagus-an observation also made by Duncan (1953) with conscious lambs. Inhibition of the reflex was induced by central stimulation of the vagal branch to the abomasum and also by stimulation of the peripheral end of the splanchnic nerve. More recent work by Newhook & Titchen (1969) provides evidence that although the abdominal roots of the vagi include the efferent fibres concerned with closure of the groove, not all are atropine-sensitive and the efferent arm of the reflex appears to be more complicated than the earlier evidence suggests.Watson (1941) and Watson & Jarrett (1944) made a detailed study of the behavioural patterns of lambs in relation to drinking or sucking milk or water from nipples. They found that the behavioural pattern of the animal is closely conhttps://www.cambridge.org/core/terms. https://doi