Net energy evaluation of feeds and determination of net energy requirements for pigs

Feeds for pigs can be attributed different energy values according to, first, the step considered in energy utilization (DE: digestible energy, ME: metabolizable energy and NE: net energy) and, second, the method used for estimation at each step. Reference methods for evaluating DE content are based on in vivo digestibility measurements; indirect estimates of DE values are obtained from in vitro methods or prediction equations based on chemical characteristics. Methods have also been proposed for estimating urinary energy (and gas energy to a smaller extent) in order to calculate ME content from DE value. The NE values originate from energy balance studies (slaughter methods or, more commonly, indirect calorimetry measurements in respiration chambers) and their compilation allows the calculation of NE prediction equations based on digestible nutrient contents or DE or ME contents. Such equations are applicable to both ingredients and compound feeds. They may differ between origins according to the fractionation method of organic matter or assumptions such as the NE requirement for maintenance (or fasting heat production). These measurements represent the bases for establishment of energy values in feeding tables. Results indicate that energy digestibility of feeds is negatively affected by dietary fibre content but this negative effect is attenuated with body weight increase, which suggests that feeds should be attributed DE values according to pig BW; in practice, at least two different DE values, one for growing-finishing pigs and one for mature pigs (reproductive sows), are recommended. The energy digestibility of pig feeds can also be affected by feed processing (pelletting, extrusion, etc.). Efficiency of ME utilization for NE averages 74-75% for conventional pig diets but it is directly dependent on diet chemical composition with efficiencies higher for ME from fat (90%) or starch (82%) than from protein or dietary fibre (60%). The hierarchy between feeds and results of least-cost formulation are then dependent on the energy system with overestimation of protein rich feeds and underestimation of starch and/or fat rich feeds in the DE or ME systems. The NE system provides an energy value which is the closest estimate of the "true" energy value of a feed; it predicts more accurately the performance of the pigs and allows implementing new feeding approaches such as the use of low protein and/or high fat diets. Energy requirements expressed as DE or ME can be transformed to NE requirements if we assume that the average efficiencies of DE or ME for NE are 71 and 74%, respectively. More sophisticated methods including modeling techniques can also be used for evaluating energy requirements.