Utilization of metabolizable energy by young broilers and birds of intermediate growth rate.

An experiment was conducted to study the utilization of ME by 3 strains of birds with different growth rates: commercial broilers, Plymouth Barred Rocks, and Leghorns. Birds were fed a diet providing 3,100 kcal of ME/kg and 20% CP from 0 to 42 d. ME and nitrogen balance were determined from 10 to 15, 23 to 28, and 37 to 42 d. Energy retained (ER) as fat (ERF) and protein (ERP) were derived from body analyses for each period and group of birds. ERF and ERP were obtained as the difference in values at the end of each period and their respective predicted values based on BW at the beginning of the period. ER, ERF, and ERP per unit metabolic BW were significantly different for broilers vs. the other 2 strains in all periods (P < 0.001). Heat production and ER increased significantly (P < 0.01) for all groups. There was a significant decrease in HP (P < 0.001) over time with no change in ER, ERF, or ERP over time. Age appeared to have no significant effect on ERF/ME regardless of strain. Estimation of maintenance requirements as a function of BW0.75 vs. BW0.60 showed that requirements for broilers per kilogram of BW0.75 are 8% lower compared with estimates based on BW0.60. It seems that energy requirements for maintenance has been underestimated for younger or smaller birds and implies greater apparent efficiency for growth. Because ME intake for maintenance requirements represents a large portion of the ME intake, the results from this experiment could be considered in calculation of energy requirements for growing broilers.