[Variations of nitrogen retention in the growing pig: effects on requirements]

The first part of this paper deals with the distribution of the protein fraction (total protein, amino acids) in the tissues and body compartments in the growing pig, according to age, growth potential (sex, genotype) and nutritional factors (feeding level). Overall protein retention is studied in the second part of the paper. The following points have been considered: changes in the protein retention curve during growth; variations according to the type of pig and nutrient supply; particular aspects of amino acid deposition with a relative constancy in the amino acid composition of deposited protein during the major part of growth. In the third part, the following consequences of variations in protein retention on protein requirement for growth have been analyzed: --relatively constant ratios between the requirements for individual essential amino acids and non-essential nitrogen (using lysine as a reference); --relation between protein (amino acid) requirement and level of protein deposition; --variations in the efficiency of utilization of dietary amino acids for growth, according to muscle growth potential and production objectives (final slaughter weight, type of pig and feeding program). The benefit of taking into consideration the components of apparent protein retention (synthesis and degradation) is discussed with a view to better adapting protein feeding to growth potential.