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Energy is an abstract term basically describing the capacity to do work. Thereby several forms of energy exist, i. e. electrical energy, mechanical energy, chemical energy, atomic energy. These different forms have in common that they can all be transformed into heat energy. Investigating energy metabolism in animals always involves determining heat energy because organic substances (feed, energy stored in the animal) contain chemical energy that can be transformed into heat. The chemical reactions in an organism apply to the Laws of Thermodynamics known from physics because work and heat are equivalent:
The chemical energy obtained from the feed is transformed into animal products (meat, eggs), products of metabolism and synthesis as well as mechanical work (muscle contractions). Transforming the feed material to 100 percent energy, and gaining the complete energy content is impossible. The partition of food energy in the animal can be illustrated as a “balance diagram” showing the nutritional energy loss involved in energy transformation.
Gross energy (GE), or heat of combustion, is the energy released by burning a sample of feed in excess oxygen (apparatus: bomb calorimeter). Thereby the amount of gross energy is exclusively dependent on the chemical composition of the feed but it cannot help predict the energetic transformation efficiency, viz. gross energy as such is meaningless in animal production, because it does not take into account any losses of energy during ingestion, digestion and metabolism of feed. In fact, 1 kg of starch has about the same gross energy content as one kg of straw even though the energy in the straw cannot be used by pig or poultry due to missing digestive enzymes.
Digestible energy (DE) is the gross energy of feed minus the gross energy of feces. Therefore, this energy system takes into account the digestibility of feed and gives a useful measure of the energy the animal may be able to use. The advantage of digestible energy is that it is easy to determine. The disadvantage is that it does not take into account losses of energy in urine and as combustible gases and during metabolism of the feed. These losses vary among feedstuffs.
The next system is metabolizable energy (ME), which is defined as the digestible energy minus energy excreted in urine and as combustible gases. By taking into account these losses, metabolizable energy gives a better estimate of the energy available to the animal. ME corrects the digestible energy for some of the effects of quality and quantity of protein for example.
Net energy (NE) is defined as metabolizable energy minus the heat increment, which is the heat produced (and thus energy used) during digestion of feed, metabolism of nutrients and excretion of waste. The energy left after these losses is the energy actually used for maintenance and for production (growth, gestation, lactation). That means that net energy is the only system that describes the energy that is actually used by the animal. Net energy is, therefore, the most accurate and unbiased way to date of characterizing the energy content of feed. However, NE is much more difficult to determine and more complex than DE or ME (Moehn et al. 2005, adapted).
The energy requirements of an animal consist of the requirement for maintenance and the requirement for productivity.
The energy required to sustain body tissue is called maintenance energy, viz. it is the amount of feed needed to keep an animal from gaining or losing weight. Thereby maintenance energy is an energy level that does not include energy for any form of production (milk, eggs, wool, fetal growth, fat and protein disposition). Animals unavoidably consume energy in order to maintain basal metabolic function, homeothermy and carry out minimum physical activity i. e. necessary for feed uptake. Again the energy expense is released as heat. At the maintenance level of feeding animals these energy losses are equal to energy uptake with feed, viz. the energy requirements are exactly met and the energy balance is zero.
The amount of energy exceeding maintenance requirements is available for productive purposes. Thereby the heat loss for utilization of ME depends on the individual productive goal. The k-factors described in 1.2.3 characterize these differences by setting the ME available for production in relation to the energy actually retained in the product (NEproduct).