Energy evaluation systems

Early energy systems

Energy systems have a history going back to the first half of the nineteenth century but did not provide an adequate description of energy utilisation until methods of animal calorimetry came into use in the second half of that century. About 1900, H. P. Armsby at the University of Pennsylvania and O. Kellner at the Möckern Experiment Station in Germany used the results of their calorimetric studies to devise energy systems based on the net energy value of foods. The systems differed in some respects, most evidently in the units used. Armsby expressed net energy in terms of calories (the unit ante¬dating joules), but Kellner - believing that farmers would have difficulty in understanding calories - expressed net energy values of foods relative to the net energy value of that common food constituent, starch. For example, if the net energy value of barley was found to be 1.91 Mcal (megacalories) per kilogram, and that of starch, 2.36 Mcal/kg, then 1 kg of barley was stated to have a starch equivalent of 1.91/2.36 = 0.81 kg. Both Kellner's and Armsby's systems encountered difficulties caused by the differences in net energy values of foods for maintenance, growth, etc. and used approxima¬tions to avoid these difficulties. Kellner's starch equivalent system was used (mainly in Europe) as the basis of practical rationing systems until the 1970s.
Armsby's net energy system was incorporated in what was at one time the standard reference work on the feeding of livestock in the USA, F B Morrison's Feeds and Feeding, but was not much used in practice. The pre¬ferred system in the Americas was for many years the total digestible nutrients (TDN) system.

Recent systems

All three components of a modern energy system, the energy content of foods, the energy requirements of animals and the interface linking them, can be given greater accuracy by introducing additional factors.
As the food intake of an animal increases, the metabolisability of the food energy declines. Food intake can be defined as the level of feeding, which is the metabolisable energy intake relative to that required for maintenance; thus if the level of feeding is 60/33=1.82. For growing cattle the level of feeding is commonly 2-2.5, but for lactating cattle it rises to 3-4. The energy requirement of lactating cows is increased by 1.8 per cent for each unit increase in the level of feeding. Thus for a cow with a level of feeding of 3 the initial estimate of energy requirement would be increased by 2 x 1.8 = 3.6 per cent to allow for the effects of increasing food intake on the energy content of the ration. The same correction is made for lactating ewes, but not for other classes of ruminants.
Increasing the level of feeding may also reduce the efficiency of utilisation of ME (i.e. reduce the k factors). Refinements of the system include corrections for this effect. For example, for growing cattle kg is assumed to have its normal predicted value when the level of feeding is at twice the maintenance level, but if the level of feeding is greater than this, kg is reduced, and if the level of feeding is less than twice the maintenance level, kg is increased. For example, if cattle on a diet containing 10 MJ ME/kg DM were fed at 2.5 times maintenance, kg would be reduced from 0.43 to 0.39. The same correction is used for growing lambs.
There is also evidence that when metabolisable energy is used for growth, the efficiency with which it is utilized varies with the nature of the diet. For example, when diets containing 11 MJ ME/kg DM are made up from either high-quality forage alone or poorer-quality forage plus concentrates, the diet containing concentrates will have a kg value about 5 per cent higher than that of the diet of forage alone. Among the forages, there is evidence that kg values are greater for first growths (i.e. spring growths) of temperate herbages than for later growths having the same metabolisable energy concentration, and are higher for temperate than for tropical forages. However, it is difficult to classify diets into different categories for the purpose of predicting kg.
Although it seems undesirable to introduce an arbitrary correction factor to an otherwise logical system, it is important that the system should accurately predict the animal growth rates that are achieved in practice.
In 1988 the European Association of Animal Production carried out a survey of the feeding systems in use in European countries. The report on energy systems for ruminants demonstrated the wide variety of systems in use in Europe, and it is not possible to describe all of them here. The Netherlands, Belgium, France, Germany, Switzerland, Italy and Austria have systems with many features in common, and the Dutch system will be described as an example. The metabolisable energy content of foods is calculated from digestible nutrients and is then converted to a net energy value. For growing animals, the basis for this conversion is that the animal production level is assumed to be constant, at 1.5, and hence that kmp, has a unique value for a food of known metabolisable energy concentration. Each food can therefore be given a single net energy value for maintenance and production (NEmp), but this is converted to a unit value by dividing it by the presumed NEmp of barley (6.9 MJ/kg, or about 8 MJ/kg DM).
For lactating cows a corresponding net energy value for maintenance and lactation is calculated by assuming that kl is 0.62 and the net energy value of the food for lactation (called NEl) can also be calculated.
After using a system of total digestible nutrients for many years, the USA changed to net energy systems for beef and dairy cattle, these being described in publications of the National Research Council. The Hungarian system is almost the same. Metabolisable energy is calculated as 0.82 x digestible energy and digestible energy is calculated as 18.45 MJ per kg TDN. For beef cattle, foods are given two net energy values, for maintenance (NEm) and gain (NEg), these being calculated from the metabolisable energy (ME) con¬tent of dry matter in each food by means of the following equations:
NEm = 1.37ME - 0.033ME2 + 0.0006ME3 – 4.684
NEg = 1.42ME - 0.0416ME2 + 0.0007ME3 – 6.904
Where: ME = metabolisable energy content of dry matter, and all energy values are expressed in MJ/kg.
Net energy values for lactation (NEl) for the US system have been calculated from TDN, digestible energy by the following equation, where df (discount factor) is correction factor related to the fibre content of fedstuffs :
In the future, as energy systems are modified to incorporate new findings they are likely to become even more complex. As the need for simplicity of calculation is diminished by the increasing availability of computers, energy systems are becoming parts of much larger mathematical models of nutrient requirements that are capable of dealing simultaneously with energy, amino acids, vitamins and minerals (and with the interactions between them). Complex systems tend to obscure the principles upon which they are based. Students of animal nutrition should therefore pay particular attention to the principles of energy metabolism outlined in the previous chapter and at the same time should familiarise themselves with the energy systems currently used in their own countries.
The use of DCP for evaluating food proteins for ruminants has been largely abandoned. This resulted from a growing awareness of the extensive degradative and synthetic activities of the microorganisms of the rumen. Rumen microorganisms are responsible for providing the major part of the energy requirements of the host animal by transforming dietary carbohydrates to acetate, propionate and butyrate. In order to do this and to exploit the energy potential of the food fully, they must grow and multiply and this involves large-scale synthesis of microbial protein. The nitrogen for this is obtained, in the form of amino acids, peptides and ammonia, by breakdown of the nitrogen fraction of the food. Bacteria acting on the structural carbohydrate (SC) fraction of the diet use only ammonia, whereas those acting on the non-structural fraction (NSC) derive about 65 per cent of their nitrogen from amino acids and peptides, and the remainder from ammonia.
The microbial protein passes from the rumen, is digested in the small intestine, and so makes a contribution to satisfying the nitrogen require¬ments of the host animal. The magnitude of this contribution depends upon the speed and extent of microbial breakdown of the dietary nitrogen fraction, upon the efficiency of the transformation of the degraded material into microbial protein (nitrogenous compounds), the digestibility of the microbial protein and the biological value of the latter.
The degradative and synthetic processes taking place in the rumen are of major importance in the nitrogen economy of the host animal since they determine the nature of the amino acid mix made available for protein syn¬thesis at tissue level. Satisfying the demands of the rumen microorganisms for readily available nitrogen is a major function of the diet and to this end a certain proportion of the nitrogen fraction must be degradable by the rumen microorganisms.
Current systems for the evaluation of food protein for ruminant animals involve determinations of the degradability 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 acids. The methods used to determine these components of the system are described next, after which their use in the systems will be illustrated.