PSI-8 Evaluating the impact of residual feed intake on feeding behavior, growth performance, breeding soundness and carcass traits of beef bulls.

Abstract Residual Feed Intake (RFI) is a widely recognized metric for evaluating feed efficiency. It is a desirable tool for selection, as it represents the inherent differences in genetic merit related to metabolic processes that affect energetic efficiency. While RFI serves as a reliable metric for selecting feed-efficient animals, its influence on bull fertility and reproductive performance remains a critical consideration for its adoption as a selection tool in breeding programs. Beef breeders suspect that more efficient bulls may be less active in multi-sire mating groups, resulting in reduced reproductive performance and fertility on pasture during the breeding season. Reports on the impact of RFI on bull fertility are mixed, with some reporting no negative effect on sperm quality or breeding soundness evaluation, while others have reported reduced sperm motility but an increased number of progenies per sire. The objectives of the study were to evaluate the impact of RFI on the feeding behavior, growth, and reproductive performance of young beef bulls. A total of 17 junior (8-10 months) bulls (Angus, Simmental, and Simmental × Angus) were evaluated for growth and reproductive performance over 84 days at the bull test station of Southern Illinois University, Carbondale, IL. The bulls were fed a total mixed ration consisting of 25% grass hay, 50% cracked shelled corn, 21% distiller’s dried grains with solubles, and 4% vitamin-mineral supplement (DM basis). Performance parameters, including dry matter intake (DMI), average daily gain (ADG), gain:feed (G:F), and RFI, were evaluated. Of the bulls evaluated, 4 bulls had -RFI while 13 had +RFI. Bulls were ultrasounded for carcass traits and evaluated for breeding soundness. Data were analyzed using the mixed model procedure of SAS (version 9.4), with Pearson correlation coefficients computed using the Proc Corr procedure of SAS to assess the relationship between RFI and growth performance, carcass traits, and breeding soundness. Bulls did not vary in DMI, ADG, or G:F. However, RFI bulls visited the feed bunks less frequently (28.8 vs. 34.3 visits/day; P > 0.05). The longissimus thoracis area (cm2) was numerically greater (93.9 vs. 84.4; P = 0.12) for -RFI bulls. Ultrasound evaluation indicated a numerically greater backfat thickness (0.75 vs. 0.56 cm) for –RFI bulls, while Pearson correlation analysis revealed a weak negative correlation (P = 0.07) between RFI and backfat thickness. These findings suggest that selecting bulls for feed efficiency may not compromise growth performance or breeding soundness while potentially improving carcass characteristics. Further evaluation of bulls during and after the breeding season will provide detailed information on the impact of RFI on reproductive performance and post-breeding recovery of young beef bulls for beef producers to make informed purchase decisions for sustainable beef cattle production.