Growing pig nutrition : integration of concepts of ideal protein, digestive availability of amino-acids and net energy

Abstract

Net energy values of feeds can be calculated from their digestible energy content according to the same equation regardless of the pig physiological status. This involves changes in the hierarchy between feedstuffs, undergrading high protein and high fiber ones and upgrading high starch and high fat ones. Theoretical ideal protein, defined as protein of biological value 1, is the same concept as net energy for nitrogen supply and requirement. The experimental approach of this protein, through suppression of excess amino acids, suggests large potential improvements in the efficiency of digestible nitrogen utilisation. This efficiency measures the metabolic availability of the limiting amino acid, which is little related to dietary ingredients. Feedstuffs are better qualified by the digestive availability of their amino acids which can be measured by their ileal digestibility. The additivity of digestible amino acid supplies from dietary ingredients suppose a correction of each apparent digestibility value for nonspecific endogenous losses. The resulting true digestibility values can be predicted from the concentration of specific substances in each feedstuff. However, in the "true" undigestible nitrogen or amino acid content of a feedstuff, the quantity and the amino acid cost of the specific endogenous fraction are unknown. Minimum limiting amino acid supply (lysine) per unit of net energy is always independent of energy intake in pigs less than 55 kg live weight, while it increases with the level of restriction in heavier barrows. Minimal supplies of other essential amino acids can be worked out from experimental determinations of ideal ratios to lysine. Imbalance by excess of essential amino acids can be prevented by correction (tryptophan/large neutral aminoacids) or limitation (threonine) of their supply in the diet. Furthermore, sparing mechanisms of essential (threonine) by nonessential amino acids could change their optimum ratios to total nitrogen.