Partitioning of retained energy in broilers and birds with intermediate growth rate.

An experiment was conducted to study energy retained (TER) as fat (TERF) and protein (TERP) in 3 strains of birds with different growth rate; commercial broilers, Barred Plymouth Rock, and Leghorns. Birds were fed ad libitum a diet providing 3,100 kcal of AMEn/kg and 20% CP from 0 to 42 d. Body composition, TER, TERF, and TERP were determined at 0, 7, 10, 15, 19, 23, 28, 33, 37, and 42 d of age. The TER, TERF, and TERP were derived from whole body analyses. Linear and nonlinear models (quadratic, allometric, and Gompertz equation) were used as a means to characterize observed patterns of energy deposition. The TER, TERF, and TERP increased quadratically (P < 0.001) over time in all 3 strains of birds. Over 42 d, broilers deposited a constant proportion (50%) of body energy as fat and protein (P < 0.001). When applying the Gompertz equation to relate empty BW (EBW) to time, the estimated value for EBW at maturity of the broilers was unrealistically high (11.1 kg) and estimated poorly (SE 5.5 kg). Quadratic equations may be used as an alternative for Gompertz equations to represent growth of EBW, TER, TERF, or TERP vs. time in chickens between 0 and 42 d of age. Within the BW ranges that were evaluated in this study, allometric functions or Gompertz equations can be used to relate TERF and TERP to EBW, but model parameters differ between bird strains. Based on the Gompertz equation and in broilers, the maximum rate of TERF and TERP was reached at 1.16 and 1.22 kg of EBW, respectively, and then declines slowly as BW increases. Quantifying and partitioning TER as TERF and TERP as major components of ME requirements can be used to establish models that have economic consequences to the broiler industry.