Abstract :
Energy requirements of animals are most readily expressed in terms of net energy (NE), while the energy yield of feed is, at least
initially, expressed in terms of metabolisable energy (ME). Energy evaluation systems ‘translate’ NE requirements into ME
requirements (ME systems) or assign NE values to feeds (NE systems). Efficiency of ME utilisation is higher for maintenance than
for production and the NE yield of a feed varies, therefore, with ME intake. In addition, energetic efficiency for maintenance and
production is thought to be different for lactating and non-lactating animals and to be affected by diet quality. As a result, there
are currently many national energy evaluation systems that are complex, differ in their approach and are, as a result, difficult to
compare. As ruminants in most production systems are fed ad libitum, this is also the most appropriate intake level at which to
estimate energetic efficiency. Analyses of older as well as more recent data suggest that ad libitum feeding (i) abolishes the
effects of diet quality on energetic efficiency (almost) completely, (ii) abolishes the differences between lactating and non-lactating
animals (almost) entirely and (iii) results in overall energetic efficiencies that are always close to 0.6. The paper argues that there
is now sufficient information to develop an international energy evaluation system for ad libitum fed ruminants. Such a system
should (i) unify ME and NE systems, (ii) avoid the systematic bias and large errors that can be associated with current systems
(iii) be simpler than current systems and (iv) have as a starting point a constant efficiency of ME utilisation, with a value of around
0.6. The remarkably constant efficiency of ME utilisation in ad libitum fed ruminants could be the result of energetic efficiency
as well as feed intake regulation being affected by the same variables or of a direct role of energetic efficiency in feed intake
regulation. Models to predict intake on the basis of the latter hypothesis are already available for non-reproducing ruminants
but remain to be developed for reproducing animals.
