Utilisation of metabolisable energy

The slope of the line relating retention to intake is a measure of the efficiency of ME utilisation. For example, if the ME intake of an animal was increased by 10 MJ and its retention increased by 7 MJ, then the efficiency of utilisation of ME would be calculated as 7/10 = 0.7. (Conversely, the heat increment would be calculated as 3/10 = 0.3 of the ME.) These efficiency values are conventionally called k factors, with the letter k carrying a subscript to indicate the function for which ME is being used.The commonly used k factors are shown in Box 11.6. 

The term kf has been used to denote both the specific efficiency of fat deposition (as indicated above) and the general efficiency of energy deposition in what were once called fattening animals. Today, the preferred term for the latter use is 1, as animals are now considered to grow rather than fatten. 

BOX 11.6 Efficiency factors (k) used to describe the efficiency of metabolisable energy (ME) utilisation 

An additional feature of Fig. 11.5 is the shaded area on either side of the lines. This is intended to indicate that the efficiency of ME utilisation is quite variable. We shall see later that the principal causes of this variation in efficiency are, first, the nature of the chemical compounds from which ME is derived (hence the nature of the food and the manner in which it is digested) and, second, the function for which these compounds are used by the animal. 

The efficiency of free energy capture when body fats are oxidised and ATP is formed can be calculated from the reactions shown in Chapter 9 to be of the order of 0.67. For glucose, to take an example of a nutrient, the efficiency is similar, at about 0.70. One would therefore expect that glucose given to a fasting animal would be utilised without any increase in heat production, or in other words with apparent (calorimetric) efficiency of 1.0. Table 11.5 shows that this is approximately true. In sheep the efficiency is reduced through fermentation losses if the glucose passes into the rumen, but these losses are avoided if it is infused directly into the abomasum. 

---

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

Other factors affecting the utilisation of metabolisable energy... 

Erom the calorimetric work of Forbes, Fries and Kellner, an efficiency of utilisation of metabolisable energy for milk production (kj) of about 0.70 is indicated. More recent estimates of k] have varied widely from 0.50 to 0.81, but the majority cluster around 0.60-0.65. There is considerable evidence that much of the variation is due to differences in the energy concentration of the diet. Van Es has suggested that the efficiency of utilisation of metabolisable energy for milk production is related to the metabolisability of the diet, defined as the ME (MJ/kg DM) at the maintenance level as a proportion of the gross energy (MJ/kg DM). His implied relationships for (a) Dutch and (b) American data are ... 

The efficiency of utilisation of metabolisable energy is influenced by the level of protein in the diet. When protein content is inadequate, body tissues are catabolised to make good the deficiency, a process that is wasteful of energy. AVhen protein content is too high, excess amino acids are used as a source of energy. Since protein is used relatively inefficiently for this purpose, the overall efficiency of utilisation of metabolisable energy is reduced. 

There is some evidence (Fig. 16.4) that the efficiency of utilisation of metabolisable energy for milk production is influenced by the proportion of acetate in the fatty acids produced during rumen fermentation. 

In calculating the energy requirements of the dairy cow, cognisance must be taken of the decline in the efficiency of utilisation of metabolisable energy with increasing level of energy intake. In order to do this, the calculated requirement has to be increased accordingly. The procedure, which involves the use of a correction factor, is best illustrated by an example, as shown in Box 16.2. 

Values for the efficiency of utilisation of metabolisable energy for maintenance and for milk production are related to the energy concentration of the diet and are very similar. 

Wilkinson (1984) presents information showing that the amount of metabolisable energy utilised on dairy farms can vary by more than a factor of four. He suggests that high levels of efficiency can be achieved by ... 

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

Table 11.6 Typical values for the efficiency of metabolisable energy utilisation for growth in pigs...

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

The effects of the relative proportions of nutrients in a diet have been partly covered above. However, a fattening animal will tend to use metabolisable energy more efficiently if it is provided as carbohydrate rather than protein. Similarly, if a growing animal is provided with insufficient protein, or with insufficient amoimts of a particular amino acid, then protein synthesis will be reduced and it wUl tend to store energy as fat rather than protein. In this situation, the efficiency of ME utilisation will probably be altered. 

Efficiency of utilisation of dietary metabolisable energy for maintenance (k j) may be calculated as follows ... 

Other metabolic effects. In addition to enabling glucose to pass across cell membranes, the transit of amino acids and potassium into the cell is enhanced. Insulin regulates carbohydrate utilisation and energy production. It enhances protein synthesis. It inhibits breakdown of fats (lipolysis). An insulin-deficient diabehc (Type 1) becomes dehydrated due to osmotic diuresis, and is ketotic because fats break down faster than the ketoacid metabolites can be metabolised. 

---