The supply of essential amino acids  i.e. which cannot be synthetized by the animal is a determining factor for the growth of the animal. In fact, after being digested and metabolized, the dietary amino acids will be used to synthetize the animal’s own body proteins. A supply below the requirement for a single essential amino acid will directly limit protein synthesis and therefore gains in weight gain. In pig and poultry feeds mainly based on corn, wheat and soybean meal formulas, these first limiting amino acids usually include lysine, threonine, methionine and tryptophan. Taking into account the essential amino acid composition of each raw material entering the feed’s composition in order to determine the dietary profile makes it possible to better adjust supplies to the animal requirement.

 

 

However, the amino acids supplied through dietary proteins are not entirely absorbed by the animal's digestive tract. In fact, the amount of each amino acid absorbed depends directly on the digestion of dietary protein. The result of these digestive processes mainly depends on the type of raw material used - the variety, the structure of the proteins, the content in dietary fibre, etc. and on the amino acid under consideration. Knowledge of the digestibility of each single amino acid, i.e. the difference between the ingested amount and the excreted amount divided by the ingested amount, is therefore essential for the accurate adjustment of the dietary supply of amino acids.

 

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The notion of digestibility has gradually developed on both a methodological and a conceptual level. Digestibility, which was initially measured at the fecal level, is now measured at the ileal level, due to changes caused by the microflora of the large intestine. On the one hand, undigested amino acids on the ileal level can be catabolized by the microflora of the large intestine or used for the synthesis of microbial protein. On the other hand, some amino acids are synthetized de novo by bacteria without being absorbed by the animal; these eventually end up in the feces. Thus, the proportion of nitrogen of microbial origin at the fecal level may represent 65% to 90% of total fecal nitrogen. Ileal digestibility therefore represents the absorption of amino acids in the digestive tract far better than fecal digestibility. However, the profile of amino acids at the end of the small intestine is not the only result of the absorption of dietary amino acids. The digestive process is also accompanied by secretions of digestive juices, mucins, immunoglobulins and cellular desquamations, whose protein elements blend with the proteins of dietary origin in the intestinal lumen. These secretions are partially digested and reabsorbed, but some of them, representing between 10 and 80% of the nitrogen present in the ileal digesta, escape this re-absorption. This fraction of endogenous origin can be measured by feeding the animal with a protein-free diet. Although this technique only enables basal endogenous or non-specific losses to be measured, it makes it possible to propose corrected apparent ileal digestibility coefficients which are called true or standardized, meaning corrected from the basal endogenous losses (see also Standardised digestible amino acids EvaPig®).

 

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The main tables which have been published and are currently available for the formulation give information about standardized digestibility coefficients (AmiPig, 2000, INRA & AFZ, 2002) in order to formulate the content of amino acids in feeds as closely as possible to their real availability for the animal.

 

	Tables of composition and nutritive value of feed materials: pigs, poultry, cattle, sheep, goats, rabbits, horses, fish Sauvant D., Perez J.-M., Tran G. Eds. ISBN 9076998418 2004, 304 p. INRA Editions Versailles For more information please visit INRA and AFZ websites: