Residual feed intake and reproductive traits of growing Purunã bulls

Residual feed intake (RFI) and its relationship with reproductive traits was evaluated in growing bulls. Fifty-two growing Purunã bulls (11 mo initial age) were fed ad libitum in individual feedlot pens for 112 d. The animals were ranked for RFI and assigned to 3 feed efficiency groups: efficient (low RFI), intermediate (medium RFI), and inefficient (high RFI). Initial and final BW and ADG did not differ (P> 0.10) among the efficiency groups and were mean values of 254.6 (SD 44), 373.0 (SD 62), and 1.06 kg (SD 0.25), respectively. Mean values of 7.12 ± 0.28, 7.78 ± 0.28, and 8.04 ± 0.28 kg/d for DMI and −0.38 ± 0.04, −0.02 ± 0.04, and 0.51 ± 0.04 kg of DM/d for RFI were observed in the efficient, intermediate, and inefficient groups, respectively. Crude protein and ME intake were strongly correlated (r= 0.74, P< 0.001 for both variables) with RFI and were lower (P< 0.10) in the efficient group (13.6 ± 0.2 g DM/kg BW0.75∙d and 0.252 ± 0.003 Mcal/kg BW0.75∙d, respectively) and greater (P< 0.10) in the inefficient group (15.3 ± 0.2 g DM/kg BW0.75∙d and 0.282 ± 0.003 Mcal/kg BW0.75∙d, respectively). Testicular measures (width, length, volume, ultrasonogram pixel intensity, and scrotum perimeter) and serum testosterone were not correlated with RFI (P> 0.10) but showed moderate to strong correlations with initial BW (rranged from 0.36 to 0.51, P≤ 0.01), final BW (rranged from 0.36 to 0.64, P≤ 0.01), and ADG (rranged from 0.29 to 0.53, P< 0.05). All the reproductive traits (testicular measurements and concentrations of serum testosterone) showed similar values among the efficiency groups. During the growth phase, feed intake and protein and energy requirements were decreased in more efficient bulls in terms of RFI. However, both groups maintained similar BW and ADG. Regardless of their RFI classification, Purunã bulls had the same reproductive traits during the growth phase.