The cost of feed is the most important cost in pig production (60-70%) and the energy component represents the greatest proportion (more than 80%) of that cost. Protein and minerals supply in feed are less expensive but crucial for performance of animals and levels of wastes and potential pollutions in the environment. Therefore, it is important to precisely estimate the nutritional value of feeds, either for least-cost formulation purposes (ingredients) or for adapting feed supply to requirements of animals (complete feeds). In addition, new and alternative feed ingredients, and new technologies (enzymes, for instance) are used and their impacts must be evaluated.
Not all gross energy (GE) of a feed is available for meeting the requirements of animals since variable proportions of GE are lost in excreta (feces and urine), as fermentation gases (methane, hydrogen) and as heat (or heat increment; HI).
The digestible energy (DE) content of a feed is equal to its GE content minus fecal energy losses, the latter ones being related to the levels of dietary fiber in the feed. The metabolizable (ME) content of a feed corresponds to the difference between the DE content and energy losses in urine and gases. Urinary energy is directly dependent on dietary nitrogen content. Methane production is very small and negligible in growing pigs, but it should be considered in adult pigs.
Energy digestibility (DE/GE) increases with body weight (BW) increases in pigs, with the highest difference between adult pigs (reproductive sows) and young growing pigs. The change is more accentuated for high dietary fiber feeds. The practical consequence of these variations is that DE or ME values should vary with BW or the physiological stage in pigs.
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