Energy partitioning in growing pigs: The use of a multivariate model as analternative for the factorial analysis

To quantify the utilization of ME by growing pigs, a factorial analysis met hod is often used in which the ME intake is regressed on protein (PD) and l ipid deposition (LD) rates. The approach has been criticized because there often is a strong correlation between PD and LD, which makes accurate estim ation of model parameters difficult. The current study describes a nonlinea r multivariate analysis procedure in which PD and LD are the result of vari ation in ME intake. The approach requires a hypothesis concerning the parti tioning of ME intake above maintenance between PD and LD. The method was ev aluated using data for growing pigs of different genotypes and sex and weig hing between 20 and 107 kg that were offered a diet close to ad libitum. En ergy, nitrogen, and fat balances were determined at regular intervals over the growing period. The maintenance energy requirement was expressed as a f unction of BW (with group-specific parameters) or as a function of muscle a nd visceral mass. The maintenance energy requirements ranged from 913 to 1, 070 kJ ME/((kg BW)(.60).d) for obese castrates and boars of a synthetic Lin e, respectively. Viscera contributed 1,558 kJ ME/ ((kg tissue)(.70).d) to t he maintenance energy require-ment, whereas muscle contributed only 555 kJ ME/ ((kg tissue)(.70) (.)d). It was assumed that the proportion of ME intak e (above maintenance) designated for PD declined linearly with increasing B W. At 20 kg of BW, 49% of ME intake above maintenance was designated for PD in lean genotypes, whereas this was only 34% in obese genotypes. In genera l, with increasing BW, less energy was designated for PD, but this relation ship depended on genotype and sex. Extremely lean male genotypes maintained a constant partitioning of energy between PD and LD for all BW. The energe tic efficiencies varied (depending on the model used to express the mainten ance requirement) between .58 and .60 for PD and .77 and .82 for LD. Extrap olation of results suggested that animals fed at maintenance energy level w ould still deposit protein at the expense of body lipid. It is argued that this finding requires nonbiological efficiencies of lipid catabolism and pr otein synthesis and illustrates the limitation of the maintenance concept f or growing animals. The multivariate analysis method proposed here circumve nts many of the problems associated with the factorial regression analysis of ME intake on PD and LD. The method can be used to further refine nutriti onal models describing growth in pigs.