Metabolisable energy efficiency

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).

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

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

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

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. 

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

In the UK metabolisable energy system for ruminants, animal requirements are expressed in terms of net energy (NE) and food energy values are expressed as metabolisable energy (ME). Animal ME requirements are calculated using efficiency factors (k) for different productive processes (e.g. maintenance, lactation or growth), which depend on the ME concentration of the diet (M/D). 

For housed ruminants, the increases in heat loss associated with a 1 °C fall in environmental temperature are comparable (10-20 kJ per kg per day) to those for pigs and poultry, but are much greater (20-40 kJ) for ruminants kept out of doors and exposed to wind and rain. In ruminants it is often possible to influence heat production by changing the quality of the diet. Metabolisable energy derived from low-quality forage-based diets is used with a lower efficiency (k) than that derived from high-quality concentrate-based diets, and thus more heat is liberated to keep the animal warm. 

A hen weighing 2.0 kg has a fasting metabolism of about 0.36 MJ/kg per day, or 0.60 MJ/day, and utUises metabolisable energy for maintenance and production with a combined efficiency of about 0.8. Its requirement for 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. 

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

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. 

The trend towards the Holstein breed in the UK over the last 20 years has led to problems in relation to milk quality. While these cows have the potential to produce high yields by the greater partitioning of nutrients into milk, their efficiency in converting metabolisable feed energy into milk has not increased... 

Plants use carbohydrates in their own metabolism, as do the animals that eat the plants. As the carbohydrates are metabolised back into carbon dioxide and water, the energy they contain is liberated and used by an organism to grow, move, reproduce, devise more efficient ways of killing other organisms—and write books about porphyrins. 

---