The nutritive value of compound feeds for ruminants has been classically described in terms of starch equivalent for energy and digestible crude protein for the protein fraction. Over the last ten years there has been a move to describe energy in terms of metabolisable energy and protein in terms of rumen degradable (RDP) and rumen undegradable protein (UDP). Both of these descriptions involve measurements on animals in order to derive values for individual ingredients to be used in compound feeds. The assumption is then made that these individual ingredient values are additive when the ingredients are used in the compound.

Metabolisable energy is measured in animals kept in calorimetric chambers or metabolic crates, in the latter case energy loss in the form of methane is assumed to be constant. However, data from the Rowett indicates that this is not the case, losses varying from 1.1-2.5 MJ/kg DM (Table 1). This inaccuracy can have a dramatic economic affect in feed formulation for 1MJ difference is worth £7/tonne of ingredient.