Relationship of residual feed intake with semen parameters and testicular ultrasound of Nellore bulls

Residual feed intake (RFI) is one of the main tools used to identify feed efficiency. However, there is no consensus on the true impact of RFI on male reproductive traits in cattle. The study aimed to evaluate characteristics of the testicular parenchyma and vascular parameters of the pampiniform plexus obtained by ultrasound, semen quality parameters, and sperm freezability in Nellore bulls classified based on RFI. Twenty-seven bulls (21.82±0.88 months of age) evaluated for feed efficiency were sampled for the study, including 15 with low RFI (-0.592±0.09 kg dry matter/day) and 12 with high RFI (0.792±0.10 kg dry matter/day). In ultrasound and Doppler assessment, the most efficient animals (low RFI) showed higher pulsatility and resistive indexes, as well as a tendency (P=0.061) towards greater heterogeneity of the testicular parenchyma (0.625±0.032 vs. 0.508±0.032; 1.012±0.072 vs. 0.802±0.072 and 12.9±0.96 vs. 10.2±0.96, respectively, for low vs. high RFI). However, these animals tended (P=0.0652) to have lower peak diastolic velocity (5.19±0.50 for low RFI vs. 6.54±0.50 for high RFI). Analysis of fresh semen showed a lower percentage of minor defects in low RFI animals (2.67±1.19%) compared to high RFI animals (8.10±1.19%), without differences in the other parameters in fresh or thawed semen and after thermoresistance testing. Evaluation of flow cytometry parameters showed a higher quality of mitochondrial respiration in semen samples of low RFI animals (22.04±2.50%) compared to high RFI animals (12.29±2.71%). In conclusion, although RFI exerts an effect on the Doppler parameters of the pampiniform plexus, it was not sufficient to affect the quality of fresh or thawed semen.