The partial efficiency of use of metabolisable energy for growth in ruminants
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Published information on animal bioenergetics, and energy partitioning and utilization is vast and contains broad scientific knowledge but lacks proper integration. The main objective of this paper is to discuss the principal factors affecting the partial efficiency of use of metabolisable energy (ME) for growth (kg). The development of the partial efficiencies of use of ME to net energy (NE) for maintenance and growth has dramatically increased our understanding of energy partition for growing cattle in the last century. Maintenance and growth and their partial efficiencies of use of ME are intrinsically connected and interdependent, suggesting they should be analyzed together and cannot be studied apart. There are several factors that can affect kg; the key ones are dietary (energy concentration and end product volatile fatty acid - VFA - profile) and composition of the gain. We hypothesized that dietary factors provide the initial base of the energetic efficiency given the proportion of VFA and maybe glucose availability, but animal genetics and stage of maturity ultimately control the proportion of fat and protein deposited. Therefore, the maximization of the efficiency of use of ME can be achieved if diets are accurately formulated to meet the desired composition of the gain under specific production scenarios. Asynchrony between these two factors (and maybe others) is likely to decrease the energy efficiency and be limited by the least efficient one. A more integrated system of maintenance and growth nutritional energetics is needed. The inability to completely separate maintenance from growth requirements for different levels of energy intake likely leads to inaccurate assignments of energy requirements, causing errors in the prediction of kg. Efforts should be focused on developing methods to integrate dietary and animal factors to more accurately predict kgand specific factors should be determined for diverse breed types at similar stages of maturity. Some apparent contradictions reported in the literature can be resolved if a curvilinear relationship is assumed between tissue energy and energy intake.
