| Abstrakt: |
A meta-analysis was conducted to determine NE and net protein requirements of growing bulls, steers, and heifers of Nellore purebred and Nellore × Bos taurus crossbreds. A database of 16 comparative slaughter studies (n = 389 animals) was gathered to provide enough information to develop equations to predict the requirements of NEm, NEg, and net protein for maintenance (NPm) and growth (NPg). The data were analyzed using a random coefficients model, considering studies as random effects, and sex and castrate status (bulls, steers, and heifers; n = 262, 103, and 24, respectively) and breeds as fixed effects. There were no differences in NErn requirements among sex and castrate status (P = 0.73) and breeds (P = 0.82). The combined data indicated a NEm requirement of 75 kcal/kg0.75 of empty BW ~EBW) with a partial efficiency of use of ME for NEm of 0.67. The NEg requirement was different (P = 0.009) among sex and castrate status and tended (P = 0.06) to be different among breeds. The equation for NEg requirement for bulls was 0.0514 × EBW0.75 × EWG1.070 for steers, it was 0.0700 × EBW0.75 × EWG1.070 and for heifers, it was 0.0771 × EBW0.75 × EWG1.070, where EWG = EBW gain (kg/d). The partial efficiency of use of ME for NEg was not different among sex and castrate status (P = 0.33) and breeds (P = 0.20) and averaged 0.44. There were no differences in NPm requirement among sex and castrate status (P = 0.59) and breeds (P = 0.92); the overall NPm requirement was 1.74 g of NP·kg0.75 of EBW·d-1. The overall MP requirement for maintenance was 2.59 g of MP·kg0.75 EBW·d-1. The NPg requirement (g/d) was not different among sex and castrate status (P = 0.59) and breeds (P = 0.14); the overall equation was EWG × [217 - (12.8 × RE/EWG)], where RE = retained energy (Mcal/d). The percentage of RE deposited as protein (%REp) decreased exponentially as the content of RE in the gain (REc, Mcal/kg of EWG) increased. Because no study effect was observed, we pooled the data across studies and the overall equation to predict %REp was 10.1 + 167e(-0.66×REc). Our results do not support the hypothesis that bulls have greater NEm requirements than steers and heifers. Likewise, no significant differences in the NPm requirements among bulls, steers, and heifers were detected. Nonetheless, the NEg requirement of steers was greater than bulls and less than heifers. Even though the %REp was negatively correlated with the concentration of energy in the EWG, our findings indicated no differences in NPg requirement among bulls, steers, and heifers. [ABSTRACT FROM AUTHOR] |
