Effect of Age on Energy Requirement for Maintenance and Growth of Dorper and Hu Crossbred F1 Ewes Weighing 20 to 50 kg

This research aimed to define the energy requirement of Dorper and Hu Hybrid $F_1$ ewes 20 to 50 kg of body weight, furthermore to study energy requirement changes with age and evaluate the effect of age on energy requirement parameters. In comparative slaughter trial, thirty animals were divided into three dry matter intake treatments (ad libitum, n = 18; low restricted, n = 6; high restricted, n = 6), and were all slaughtered as baseline, intermediate, and final slaughter groups, to calculate body chemical components and energy retained. In digestibility trial, twelve ewes were housed in individual metabolic cages and randomly assigned to three feeding treatments in accordance with the design of a comparative slaughter trial, to evaluate dietary energetic values at different feed intake levels. The combined data indicated that, with increasing age, the net energy requirement for maintenance ($NE_m$) decreased from $260.62{\pm}13.21$ to $250.61{\pm}11.79kJ/kg^{0.75}$ of shrunk body weight (SBW)/d, and metabolizable energy requirement for maintenance (MEm) decreased from $401.99{\pm}20.31$ to $371.23{\pm}17.47kJ/kg^{0.75}$ of SBW/d. Partial efficiency of ME utilization for maintenance ($k_m$, 0.65 vs 0.68) and growth ($k_g$, 0.42 vs 0.41) did not differ (p>0.05) due to age; At the similar condition of average daily gain, net energy requirements for growth ($NE_g$) and metabolizable energy requirements for growth ($ME_g$) for ewes during late fattening period were 23% and 25% greater than corresponding values of ewes during early fattening period. In conclusion, the effect of age upon energy requirement parameters in the present study were similar in tendency with previous recommendations, values of energy requirement for growth ($NE_g$ and $ME_g$) for Dorper and Hu crossbred female lambs ranged between the NRC (2007) recommendation for early and later maturating growing sheep.