The basis of least-cost formulation is to rank raw materials according to both price and nutritive value and then to find the cheapest compromise meeting all constraints. Only feed formulation on basis of digestible amino acids (AA) will provide AA in an amount sufficient for the requested performance of an animal (growth, production of eggs, fertility). Additionally, the use of all raw materials in diet formulation is enabled, even raw materials of a lower AA digestibility because balancing according to animal’s requirement is possible (e.g., by supplementing AA). For the determination of digestibility different methods were developed. Impact factors such as a combined AA excretion of urine and feces, metabolization of indigested AA by intestinal microbes and their excretion, excretion of endogenous AA, and the variation of AA digestibility with age are considered by the concept of standardized ileal digestibility (SID). For determination of SID, apparent ileal AA digestibility is measured in growing broilers is standardized for a table value of endogenous AA losses. The concept has been introduced in feed formulation for broilers in the early 2000s. A first data set with SID coefficients for 20 raw materials plus combined values for peas and beans has been published in 2004 (Lemme et al.). A scan of recent literature as well as own research delivered data allowing for an extension of the number of raw materials to 24 backed by 36 references. The number of data backing coefficients for a raw material varies between 1 (soy protein concentrate and soybean full fat) and 70 (rapeseed/canola meal). Also the number of data backing single AA coefficients with a raw material varies between 0 (mostly for cysteine, tryptophan, and nonessential amino acids) and 70 (essential amino acids except tryptophan). For an improvement of a table for SID coefficients future research should concentrate on less investigated raw materials and consider always the determination of SID for cysteine, tryptophan and nonessential amino acids.

Key Words: amino acid, standardized ileal digestibility, broiler