Feed efficiency and its correlations with carcass traits measured by ultrasound in Nellore bulls

Forty-six Nellore bulls were individually fed for 84 days and evaluated for residual feed intake (RFI) and feed conversion ratio (FCR) with the objective to correlate RFI1 (usual), RFI2 (adjusted for final rump fat thickness) and FCR with carcass composition traits measured by ultrasound and to verify differences between RFI groups (classified by standard deviation) in performance and carcass traits. Carcass traits were evaluated by ultrasound at the beginning and at the end of the test and the gain over the test period was calculated for ribeye area (REA), backfat thickness on the 12-13th ribs (BFT) and rump fat thickness on the Biceps femoris muscle (RFT). The RFI1 was positively correlated (P < 0.05) with the final RFT (r = 0.34) and with the gain in RFT (r = 0.36) and also there were differences (P < 0.05) in these traits and also in dry matter intake (DMI) between groups, in which the most efficient animals had lower values of DMI, final RFT, and gain in RFT. For the RFI2, there were no differences (P > 0.05) between groups and neither were there significant correlations between those with the carcass traits, only for FCR, feed efficiency and DMI. FCR was correlated with the gain in REA and BFT (r = − 0.43 and r = − 0.31, respectively) and with the initial BFT (r = 0.31). The positive correlation between residual feed intake and subcutaneous fat of Nellore bulls' may cause some concern, because with the selection of more efficient individuals (with negative RFI), carcasses which are too lean will be produced. Adjustments in the estimation of RFI including carcass traits may override this undesirable effect.