How is energy used by animals

Fat is the highest energy nutrient available in the diet with a gross energy (GE) of 39.3 MJ/kg compared to that of carbohydrates 18 MJ/kg and protein 23.4 MJ/kg (approximate values). However, this energy is only available to the animal if the nutrient is digestible, as the indigestible component will pass out of the animal in the faeces and make no energy contribution to the animal. Similarly, other losses of energy occur until the metabolisable energy (ME) and ultimately net energy (NE) concentrations of an ingredient can be determined (Figure 1).

Figure 1      Breakdown of energy descriptors for animals

The primary factor influencing the energy value of a feed ingredient is digestibility which differs between different ingredients and nutrient sources. In the case of fat, minimal energy is lost in urine or methane gas, so the ME and digestible energy (DE) are essentially the same. The efficiency of conversion of ME to NE is not 100% efficient resulting in loss of energy as heat during mastication of feed and digestion and metabolism of nutrients. Fat is a very simple nutrient which is not fermented in the rumen and is more efficiently metabolised than other nutrient sources such that NE value is higher than other ingredients.