Modelling the relation between energy intake and protein and lipid deposition in growing pigs

When modelling the effect of a changing nutrient supply to growing animals, it is important to distinguish the individual response curve of an animal from the change in this response that may occur during growth. A data analysis model is proposed where, for an individual animal, the relation between protein deposition (PD) and metabolizable energy (ME) intake above maintenance (MEp) is curvilinear, so that PD intersects the origin and reaches ifs maximum at the maximum protein deposition rate (PDmax). An increase of MEp beyond that required to attain PDmax would not change PD. The MEp not used for protein synthesis can be used for lipid deposition (LD). The relation between PD and LD on the one hand and ME on the other hand can then be described as a function of the maintenance energy requirement (MEm), PDmax, the level of ME required to attain PDmax (F; as a multiple of MEm) and the energetic efficiencies of PD (k(p)) and LD (k(f)). Of these statistics, only k(p) and k(f) were assumed to be independent of body weight (BW), age or genotype. Variation in PDmax was described as a Gompertz function (of age) whereas variation in F was assumed a linear function of BW. Maintenance energy requirement was expressed as a power function of BW. To evaluate the model, 145 nitrogen and energy (indirect calorimetry) balances were obtained from three types of pigs (Large White castrated males (cLW) and Pietrain X Large White castrated males (cPPX) and males (bPPX)) ranging in BW between 45 and 100 kg and housed under thermoneutral conditions. Animals were allotted to one of four energy levels ranging from 0.70 to 1.00 of ad libitum intake. The MEm was not different between genotypes (849 kJ/kg BW(0.60)) whereas the k(p) and k(f) were 0.56 and 0.75, respectively For castrated animals on ad libitum intake, PDmax started limiting PD at approximately 130 days of age (78 and 86 kg BW for cLW and cPPX, respectively). Before this age and for bPPX, PD was limited by MEp. In bPPX, the difference between PD and PDmax was small (less than proportionately 0.05). The F did not change with BW for bPPX (2.85 X MEm) whereas for the other genotypes, it decreased linearly from 4.47 at 45 kg to 2.00 at 100 kg of BW. Due to ifs nature, the model allows estimation of PDmax even when energy is restricting PD.