Protein and lipid accretion in body components of growing pigs : effects of body weight and nutrient intake

In pig production, optimization of the conversion of animal feeding-stuffs into body components, especially lean meat, requires knowledge of the response relationships between nutrient intake and animal performance. In this study, the separate effects of protein and energy intake on rate and composition of body gain have been determined, in pigs with a high genetic capacity for lean tissue gain, from 20 to 45 kg. In addition, the response in body gain to energy intake has been investigated from 20 to 45 kg and from 45 to 85 kg, and the effects of body weight and previous nutrition on this response were examined. The relationship between protein intake and protein accretion was described well with a linear-plateau model, reflecting a protein and an energy dependent phase in protein deposition. The amino acid pattern of body protein was influenced by protein and energy intake. The optimal lysinelenergy ratio was not significantly affected by the level of feed intake. Protein and lipid accretion responded linearly to energy intake, both from 20 to 45 kg and from 45 to 85 kg. The increase in protein gain per unit increase in energy intake, decreased with increasing body weight. The ratio between lipid and protein deposition, and consequently body lipid content, responded curvilinearly to energy intake and increased with increasing body weight. The percentage of lean tissue decreased curvilinearly with increasing energy intake. Pigs which were restricted in energy intake from 20 to 45 kg, gained faster from 45 to 85 kg. However, this gain was largely explained by an increase in digestive tract contents and in organ gain. The percentage lean tissue at 85 kg was higher in these previously restricted pigs, but this was the result of their higher lean percentage at 45 kg, and not of a compensatory gain from 45 to 85 kg. Consequences of these results for defining an optimal feeding strategy have been discussed.