A common feature of nutritional models developed in the last three decades is that estimates of energy and protein are generally made independent of each other, even though interactions between them are recognized.

Most of the energy systems have adopted metabolizable energy (ME) or net energy (NE) as the unit for energy evaluation. Accurate measurement of the ME content of a food requires in vivo digestibility measurements using cattle or sheep, with methane losses either measured or predicted. This approach is clearly unsuitable for routine food evaluation and most systems predict the ME content from chemical analyses, using regression equations derived from calorimetric measurements.

The new generation of protein systems recognize the need to consider, separately, the energy and nitrogen requirements of the rumen micro-organisms and the protein requirements of the host animal. Despite this common base, systems differ markedly in estimates of energy supply to rumen bacteria, the degree to which dietary protein is degraded in the rumen, microbial protein yield and the digestibility of the microbial and undegraded dietary protein.

Although empirically based, current protein systems contain dynamic elements relating to rates of fermentation and protein degradation. In most cases, data have been derived using the in situ nylon bag technique, although this has major limitations. Recent developments in the gas production technique suggest that this approach has the potential for describing fermentation and degradation rates of individual foods.

To be effective, energy and protein systems need to describe foods in units that are both easily understood by non-specialists and can be measured using procedures that are rapid, reliable and cost effective. Near infra-red reflectance spectroscopy (NIRS) can be both rapid and cost effective and there is evidence that it may be able to predict not only proximate analyses but also dynamic characteristics of foods.