Ideal amino acid profiles as a basis for feed protein evaluation

The most important single factor affecting the efficiency of protein utilization is the profile of digestible essential amino acids entering the small intestine. Assuming a constant ideal amino acid profile of absorbed protein the requirements of all amino acids can be calculated when the requirement of one individual amino acid has been determined. In non-ruminants, the supply will be greatly influenced by the diet, but less so in ruminants due to the rumen microbial degradation of feed protein and synthesis of microbial protein with high biological value. However, the high-producing dairy cow requires a significant amount of rumen escape protein. Thus, the profile of the undegraded feed protein could influence the profile of the amino acids entering the small intestine to a point at which individual amino acids become limiting. For non-ruminants, lysine is usually the first limiting amino acid in feedstuffs. This has led to the general practice of expressing the requirements for all other essential amino acids relative to lysine. Alternatively, the amino acid requirements can be expressed relative to N. Then, dietary N required for synthesis of other N-compounds is also included. Moreover, the requirements of all amino acids can be calculated when the N requirement has been determined. However, this method cannot be used for ruminants, because the N requirement of rumen microbes may be higher than the requirement of the animal. In the future, computerised animal models based on scientifically correct feed evaluation systems offer the opportunity for securing optimal feeding of farm animals in relation to their ideal amino acid profile for all different purposes of production.  2000 Published by Elsevier Science B.V. All rights reserved.