Digestible amino acids for non-ruminant animals: theory and recent challenges

Abstract

Accurate assessment of amino acid requirements of livestock for maintenance and growth and accurate supply of these amino acids in feed is essential to optimise growth and production. During passage through the gut and during absorption, the composition of dietary supplied amino acids is modified compared with that absorbed into the portal circulation. Amino acids are utilised for endogenous secretion and protein synthesis in gut tissue and transamination occurs within the gut tissues. In pigs, each amino acid has its own characteristic of appearance in the portal vein which varies according to the amount of ingested protein. When small rather than larger amounts of amino acids are ingested, the absorbed mixture tends to be poorer in lysine, arginine, serine and proline and richer in histidine. Factors which influence the metabolism of gut tissue may alter the quantity and composition of amino acids absorbed. The gut micro-flora increases protein synthesis in gut tissue; in conventional compared with germ free chicks protein synthesis in gut and liver was increased by 36%. Raw kidney bean lectins produce hyperplasia in gut tissue, and the toxic effect is considerably increased by the presence of a normal gut microbial population. In-feed antibiotics appear to act at least at two levels, either via a nutrient sparing effect, with improvements in digestibility and/or enhanced absorption of amino acids plus a reduction in cell turn-over at the surface of the enterocyte. Hind-gut microbial fermentation influences amino acid digestibility. The lower the digestibility of the protein supplement the more microbial fermentation affects digestibility. Total tract digestibility is therefore not a good estimate of the amino acids absorbed in the small intestine. In intact compared with caecectomized cockerels there was little difference between faecal and ileal digestibility of amino acids for cereals, slight differences for oilseed meals, but significant differences for some animal meals. Lysine digestibility of meat and bone meal in intact and caecetomized birds was 0.88 and 0.82 respectively in meal without heat treatment and 0.58 and 0.45 respectively in heat treated meal. With diets for growing pigs in which meat and bone meal or cottonseed meal partially replaced soya bean meal, formulations based on ileal digestible amino acids produced better performance than those formulated on the basis of total amino acids. Ileal digestibility values are appropriate for estimation of amino acid digestibility of cereals as they account for losses in digestibility but in heat damaged meat meals differences exist between ileal digestibility and availability of lysine, methionine, threonine and tryptophan. Correlations between in vitro methods and in vivo ileal digestibility are quite close although in vitro values tend to be higher; inclusion of the association between amino acids and cell wall carbohydrates raises the variation explained between samples in ileal protein and amino acid digestibility to > 92%. Near-infrared reflectance spectrophotometry holds promise as a tool for predicting ileal digestibility. Use of ileal digestibility values in diet formulation increases the range of ingredients that can be employed, improves the accuracy of formulation and prediction of animal performance. Ileal digestibility measurements represent a good compromise between the requirement for a rapid, economical determination of the digestibility of amino acids in feed and the measurement of availability of amino acids for tissue synthesis.