Diphasic allometric growth of body components in white Leghorn pullets fed ad libitum and restricted diets

Growth patterns of crude fat (cfat; ether extract), CP, ash, and water in White Leghorn pullets that ate ad libitum and restricted (feed or lysine) diets were analyzed by mono- and diphasic allometric functions, with plucked empty body mass (EBM) or fat-free EBM (FFEBM) as the independent variates. In general, the diphasic model provided a better fit to the data than the monophasic model, indicating that the allometric slope (beta) changes at a certain level of development. Data demonstrate that, under an ad libitum feeding regimen, cfat growth is proportionally (beta = 1) related to growth of the fat-free body, up to about 500 g FFEBM (pullets aged 8.5 wk). From 500 g FFEBM onwards, the beta changed to about 2, illustrating the late maturation of cfat compared to FFEBM. For feed-restricted groups, slopes and transitions for cfat growth varied from those of the control group, according to changes in dietary supply. Diphasic relationships between each fat-free body component and EBM in each treatment had different slopes for first and second phases (about 1 and 0.70). Differences, however, were mainly due to the diphasic growth pattern of cfat per se, and diminished largely if FFEBM instead of EBM was chosen as the independent variable (about 1 and 0.95). Incorrect statements on body component growth in restricted birds, due to the "masking" role of varying fat deposition rates, can be avoided if relative growth was expressed as a function of the fat-free body instead of BW or EBM. Diphasic growth relationships among the fat-free body components were not affected by feeding regimen, but by age. The beta 1 were about unity for the relationship between each fat-free component and FFEBM, suggesting a constant composition of the fat-free body in the first phase. In the second phase, CP accrued relatively faster than ash and water (beta 2 of 1.10 vs 0.94 and 0.91). Results indicate that the changes in growth pattern of fatty and fat-free tissue, both relative to the fat-free body, had not been revealed if a monophasic allometric model instead of a diphasic had been used.