EFFECTS OF RFI CLASSIFICATION ON GROWTH EFFICIENCY, FEEDING BEHAVIOR PATTERNS, AND SEMEN QUALITY TRAITS IN GROWING BULLS

he objectives of this study were to evaluate the effects of residual feed intake (RFI) classification on growth efficiency, feeding behavior patterns, carcass ultrasound and fertility traits in growing bulls. In study 1, feed intake and feeding behavior traits were measured in 395 Beefmaster bulls (3 trials) using the GrowSafe System. For each trial, bulls were sorted by RFI were classified into low, medium, and high RFI groups based on ± 0.5 SD from mean RFI. Low-RFI bulls consumed 17% less (P < 0.001) feed compared to high-RFI bulls even though ADG and BW were similar. Low-RFI bulls had fewer (P < 0.005) bunk visit (BV) events, shorter BV and meal events, shorter head down (HD) durations, a higher HD duration per meal duration, and took 27 min longer (P < 0.005) to approach the feed bunk following feed delivery (Time to bunk; TTB) compared to high-RFI bulls. Although day-to-day variation in DMI was not affected by RFI classification, low-RFI bulls had less (P < 0.05) day-to-day variation in BV and HD duration, but more (P < 0.01) variation in maximal NFI and TTB compared to high-RFI bulls. In study 2, performance, feed intake, feeding behavior, carcass ultrasound, and fertility traits were measured in 625 Angus, SimAngus and Simmental bulls (8 trials). Low-RFI bulls consumed 20% less (P < 0.01) feed compared to high-RFI bulls. LowRFI bulls had fewer (P < 0.005) BV events, shorter BV and meal events, shorter HD durations, and a reduced HD duration per meal duration. There was less (P < 0.01) daily variation in feed intake in low-RFI vs high-RFI bulls. Additionally, there was less (P < 0.05) day-to-day variation in BV and HD duration in low-RFI vs high-RFI bulls. Backfat (BF) depth was 10% less (P < 0.01) in low-RFI bulls compared to high-RFI bulls, even though ADG and BW were similar. Low-RFI bulls had fewer (P < 0.005) bunk visit (BV) events, shorter BV and meal events, shorter head down (HD) durations, a higher HD duration per meal duration, and took 27 min longer (P < 0.005) to approach the feed bunk following feed delivery (Time to bunk; TTB) compared to high-RFI bulls. Although day-to-day variation in DMI was not affected by RFI classification, low-RFI bulls had less (P < 0.05) day-to-day variation in BV and HD duration, but more (P < 0.01) variation in maximal NFI and TTB compared to high-RFI bulls. In study 2, performance, feed intake, feeding behavior, carcass ultrasound, and fertility traits were measured in 625 Angus, SimAngus and Simmental bulls (8 trials). Low-RFI bulls consumed 20% less (P < 0.01) feed compared to high-RFI bulls. LowRFI bulls had fewer (P < 0.005) BV events, shorter BV and meal events, shorter HD durations, and a reduced HD duration per meal duration. There was less (P < 0.01) daily variation in feed intake in low-RFI vs high-RFI bulls. Additionally, there was less (P < 0.05) day-to-day variation in BV and HD duration in low-RFI vs high-RFI bulls. Backfat (BF) depth was 10% less (P < 0.01) in low-RFI bulls compared to high-RFI bulls, however loin muscle area (LMA) and intramuscular fat (IMF) were not affected by RFI classification. There were no significant differences observed in semen quality traits as assessed by motility and morphology between bulls with divergent RFI phenotypes. Additionally, scrotal circumference was not affected by RFI classification. Results from these studies demonstrate that growing bulls with divergent phenotypes for RFI have distinctively different feeding behavior patterns. In general, bulls with low-RFI phenotypes have fewer BV and meal events that are shorter in duration then high-RFI bulls. Additionally, low-RFI bulls appear to have less daily variation in feed intake and feeding behavior traits then high-RFI bulls. Furthermore, results from these studies indicate that semen quality traits and scrotal circumference was not associated with RFI, suggesting that selection for RFI would not be negatively associated with fertility traits in growing bulls