There is an interdependence between feed evaluation and the requirements of swine and poultry for energy and nutrients. Sound knowledge base of the animal's need for energy and nutrients necessitates all-encompassing information on energy and nutrients available in feed ingredients, which underscores the importance of developing appropriate systems and currency for both. Of central importance for sustainability of animal production is the formulation of diets that more accurately matches energy and nutrients requirements. Thus, it is essential that evaluation of the capacity of feed ingredients to supply energy and nutrients and requirements are expressed in the same currency because of the aforementioned interdependence. Nutrients and energy bioavailability of feed ingredients represent the proportion of ingested dietary nutrients and energy that are absorbed in chemical forms that render them potentially suitable for metabolism and or tissue accretion. There are difficulties and inaccuracies in the direct determination of bioavailability and the use of relative availability. Thus, nutrient and energy digestibility are often used in feed formulation as a more practical way of assessing the quantities of nutrients and energy that are absorbed. This calls for characterization of digestibility of nutrients and energy in feed ingredients used in formulating diets for swine and poultry. Total tract digestible and ileal digestible phosphorus are desirable currencies for pigs and poultry, respectively.These could be obtained by single point or regression assays. Although more laborious, regression method for estimating nutrient digestibility is robust and the determination of basal endogenous losses is not necessary. Digestible or metabolizable energy are still appropriate for feed energy values and energy requirements of swine and poultry. Standardized ileal digestibility of amino acids offers more accurate information for formulating diets than apparent ileal digestibility. An important advantageof using standardized ileal digestibility compared with apparent ileal digestibility is that values for standardized ileal digestibility are more likely to be additive in mixed diets for phosphorus and amino acids.
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