Ruminants utilisation

The D-3-hydroxybutyrate may then be used as a source of energy by a number of tissues, notably skeletal and heart muscle. In non-ruminant animals, but not ruminants, utilisation by the brain increases markedly under conditions of glucose shortage. The reactions involved in energy production are shown in Fig. 9.9. 

Doreau, M., and Ferlay, A. (1994) Digestion and Utilisation of Fatty Acids by Ruminants, Anim. FeedSci.Technol. 45, 379-396. 

The use of radioactive magnesium in tracer studies indicates that the magnesium present in food is poorly absorbed from the alimentary canal in some cases only 50 g/kg of the herbage magnesium can be utilised by the ruminant. AVhy this is so in ruminants is not known. Since adult animals have only very small readily available reserves of body magnesium, they are dependent upon a regular dietary supply. 

Fig. 11.5 Efficiency of metabolisable energy utilisation (an example based on metabolisable energy utilisation by a growing ruminant).

Table 11.5 also shows that, as one would expect, dietary fat is used for maintenance with high energetic efficiency. However, when protein is used to provide energy for maintenance, there is an appreciable heat increment of about 0.2, which is in part attributable to the energy required for urea synthesis (see Chapter 9). In ruminants, energy for maintenance is absorbed largely in the form of volatile fatty acids. Experiments in which the pure acids have been infused singly into the rumen of fasting sheep have shown that there are differences between them in the efficiency with which their energy is utilised (Table 11.5). But when the acids are combined into mixtures representing the extremes likely to be found in the rumen, the efficiency of utilisation is uniform and high. Nevertheless the efficiency is still less than that for...

Very few experiments have been conducted to determine the efficiency of ME utilisation of foods for maintenance, and these few have been restricted almost entirely to ruminant animals fed forages. A selection of the results is presented in Table 11.5. 

Table 11.7 Efficiency of utilisation of metabolisable energy from various nutrients and foods for growth and fattening in ruminants...

Table 12.1 Efficiency of metabolisable energy utilisation by ruminants for maintenance, pregnancy, growth and lactation...

Current systems for the evaluation of food protein for ruminant animals involve determinations of the degradabUity of protein in the rumen, the synthesis of microbial protein, the digestion in the lower gut of both food and microbial proteins, and the efficiency of utilisation of absorbed amino adds. The methods used to determine these components of the system are described next, after which their use in the systems will be illustrated. 

Table 13.10 Comparison of estimates of the efficiency of utilisation of truly digested true protein made in some protein evaluation systems for ruminants...

It contains 408 g N/kg, equivalent to 2550 g CP/kg. Biuret is utilised by ruminants, but a considerable period of adaptation is required. Adaptation is speeded by inoculation with rumen liquor from an adapted rumen. The nitrogen of biuret is not utilised as efficiently as that of urea, and it is very much more expensive. It has the considerable advantage that it is non-toxic even at levels very much higher than those likely to be found in foods. 

Table 3. Comparison of ATP synthesis and utilisation in response to increasing protein supply or increasing ruminal propionic acid supply in one experimentJ... 

For estimating the marginal protein utilisation of post-ruminally infused casein (mean 0.34 kg/d SD = 0.16) a meta-analysis of published data (75 diets, 40 comparisons) was conducted. The mean DM intake and milk yield were 15.9 and 22.8 kg/d, respectively. Dietary CP concentration in the control treatments was 152 (SD=16) g/kg DM and milk N efficiency, calculated as MPY/CP intake, was 0.282. The regression equations included a random intercept and slope effect for each study in the database (St-Pierre, 2001). Casein infused was converted to MP by assuming values of 90 and 100% for the CP concentration and intestinal digestibility, respectively. The latter value was estimated by the Lucas test from the data of Whitelaw et al. (1986), who infused casein at four graded levels. 

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