Section A. Physiology of dairy cattle: Part II. Physiology and biochemistry of rumination

Abstract

Studies of rumination have now reached the interesting stage where an attempt at synthesis of empirical data may profitably be made without too much reliance on unsupported hypotheses. The modern researches stimulated by the approach of the recent World War, and actively continued since in many countries, have been a vindication of the inspired guess known as the hypothesis of Zuntz(l), who suggested that the abundant bacterial flora of the rumen were beneficent symbionts rather than mere commensals. This beneficent symbiosis is probably due to the age-long adaptation of ruminants to a diet rich in cellulose, of which a cow may digest more than 60% of its large intake (2). Ruminal bacteria perform many functions of value to their host. They bring about the digestion of cellulose, a process for which the host animal has no appropriate enzymes. They enable the host to cover a not inconsiderable proportion of its amino-acid requirements by means of such simple nitrogen compounds as ammonium salts, glycine or urea(3). Finally, they render the host independent of exogenous supplies of many of the water-soluble and at least one of the fat-soluble vitamins (vitamin K(3)). These are all major advantages which have probably ensured the survival of the ruminant during the geologically recent vicissitudes of the earth's climate. There are also certain minor biochemical advantages in the possession of a rumen with its abundant microflora. Thus phytin, which is abundant in all seeds and which in non-ruminants is rachitogenic, is hydrolysed completely into non-toxic products in the rumen (4). Defatted soyabean (so-called soyabean oilmeal) unless heat-treated to deactivate the antitrypsins which it contains is toxic to the rat (5). The ruminant, however, tolerates it well. Rumination has, of course, its disadvantages too. Thus a ruminant is not as efficient a digester of its preferred diet as is, say, a carnivore, of its preferred diet. With bacterial aid it can digest cellulose which the carnivore can hardly digest at all, but even with bacterial aid its digestion is comparatively slow, and a large fraction of the cellulose normally passes through the alimentary tract undigested. The ruminant's great merit from the economic viewpoint of the animal husbandman is that it digests a diet so rich in cellulose as to be useless to other domestic animals (2,6,7). It will tolerate far more crude fibre (cellulose) in its diet than will chickens, pigs or even rabbits(2). In comparison with the pig or the dog, the ruminant has a large appetite which is a necessary consequence of its large intake of food of low digestibility. To facilitate bacterial digestion the ruminant has a large 'fill'(8), that is to say, the ratio of the weight of food and food residues in the animal at any time to the weight of the animal's living tissues is large. This is due to the delay of the food, during its passage through the animal, in the rumen, the caecum and the colon, in all of which chambers bacterial fermentations are active. Thus the ruminant carries ...

Section A. Physiology of dairy cattle: Part II. Physiology and Biochemistry of Rumination

“…At the time the previous review (75) was written there was still some doubt as to the relative importance of volatile fatty acids (VFA) and carbohydrate in ruminant nutrition. Recent work would seem, however, to confirm the prognostications of earlier workers in this field.…”

“…In the previous review by the present author it was stated that the tissues of the ruminant seemed to have evolved in adaptive relation to the fermentation occurring in the rumen. Bound up also with this peculiarity is the ruminant's exhibition of a respiratory quotient in actively lactating mammary tissue, greater than unity (75,83,86). Thus the adult ruminant has a peculiarly low blood sugar and a blood VFA titre which is much larger than those of man or dog (80).…”

“…It was obvious that not only the liver but also the rumen wall participated in some kind of active metabolism of butyric and propionic acids, and in partial support of this hypothesis it was found that as volatile fatty acid was absorbed through the rumen wall it was in part replaced by incoming chloride and bicarbonate. It is convenient at this point to stress a distinction, drawn by the reviewer in earlier reviews (75,107) in this series, between animals like the average sheep which spends a considerable portion of its life on a plane of nutrition not far above maintenance and such an animal as a cow yielding over 4 gal. Parthasarathy (92) and Garton(93) further investigated the mineral metabolism of the rumen wall.…”

The use of chromium sesquioxide to measure the digestibility of carotene by goats and cows

“…Lignin is not entirely indigestible (McAnally, 1942;Forbes & Swift, 1943;Davis, Miller & Lindahl, 1947;Pazur & DeLong, 1948;Tscherniak, 1950), and is not a definite chemical entity (Bondi & Meyer, 1948;MacDougall & DeLong, 1948;Snowden & DeLong, 1949), nor is it so readily determined chemically as has sometimes been supposed. Peculiarities of lignin metabolism have been discussed recently in a review by one of us (Owen, 1951).…”