From just five industries in the United Kingdom — brewing, distilling, milling, sugar extraction and potato processing — at least 2.7 × 106 mega joules (MJ) of metabolizable energy (ME) and 4 × 105 tonnes of crude protein (CP) are available annually to livestock farming as by-products. This is equivalent to 1.6 × 106 tonnes of barley and 4.7 × 105 tonnes of soya bean meal, although in some cases nutrient density may differ somewhat from that found in barley or soya.

A large proportion of the by-products available is already used in animal feeds, either djrectly by the farmer or through inclusion in compound feeds which are then used as components of balanced rations.

The materials available are potentially alternative feedstuffs to conventional forages or concentrates. As such they will only form part of a balanced ration and it is in this context that their relative value and usefulness can be judged. In many investigations there has been a tendency to consider particular by-products in isolation and as a consequence any nutrient imbalance has been highlighted to the detriment of the material as an alternative feed. Very few straight feedstuffs contain ratios of nutrients balanced for particular levels of animal production and invariably rations for livestock consist of blends of different materials. Whilst extremes of nutrient imbalance may be identified in individual by-products they are, none the less, wholly suited to blending with other by-products or feeds of contrasting nutrient content in order to produce a completely balanced ration.