183. Effects of Low-dose Antimicrobial Feeding on the Ruminant Gastrointestinal Tract Lipid-soluble and Water-soluble Metabolite Profiles over Time.

Abstract Tylosin and monensin are commonly included in high-grain feedlot cattle diets to improve growth performance and feed efficiency. However, increasing concerns regarding antimicrobial resistance highlight the need to develop alternative strategies to reduce reliance on antibiotics. A more mechanistic understanding of how antimicrobial supplementation affects the ruminant gastrointestinal tract (GIT) may help accelerate the development of effective alternatives. Characterizing lipid-soluble metabolites (LSM) and water-soluble metabolites (WSM) may help identify microbial- and host-derived compounds that provide mechanistic insight into the effects of antimicrobial supplementation. Therefore, the objective of this study was to determine how low-dose antimicrobial supplementation influences the LSM and WSM profiles in the ruminant GIT over time. A serial slaughter study was conducted using 72 steers (304.2 ± 36.7 kg initial BW). All steers were fed a high-forage diet pre-study to mimic industry backgrounding conditions, and eight were harvested on day 0 as a baseline. The remaining steers were individually housed, fed a 93% concentrate diet, and beginning on day 1, steers were assigned to one of two dietary treatments: 1) control, without antibiotics, or 2) 75 mg tylosin and 200 mg monensin. Sixteen steers were harvested on day 8, 22, 92, and 172 of the finishing period (n = 8 per treatment per harvest date). At slaughter, samples were obtained from the rumen, jejunum, and cecum. LSM and WSM were extracted by the Bligh & Dyer protocol and then screened by mass spectrometry. Data were analyzed using Principal Component Analysis to evaluate overall LSM and WSM profile changes. Individual LSM and WSM differences were assessed using T-Test in MetaboAnalyst 6.0. In the rumen, LSM profiles differed (P < 0.05) in steers supplemented with Tylosin plus Monensin (T+M) on days 8 and 22, with 178 and 45 altered LSM respectively (P < 0.05). The rumen LSM profile also tended to differ on day 172 (P = 0.089) with 28 affected LSM (P < 0.05). WSM profiles in the rumen also differed (P < 0.05) on days 8 and 172 in T+M steers, with 29 and 23 WSM altered respectively (P < 0.05). In jejunum, the LSM profile tended to differ on day 172 (P = 0.10), with 67 altered LSM (P < 0.05), coinciding with a WSM profile shift on day 172 with 233 altered WSM (P < 0.05). In cecum, LSM profiles were not different (P ≥ 0.05), however, the WSM profile differed on day 172 with 86 WSM affected (P < 0.05). These results suggest that low-dose antimicrobial feeding induces early shifts in rumen LSM and WSM profiles that are partially maintained over time, likely driven by microbial restructuring and modified metabolic pathways. Delayed metabolic shifts in the jejunum and cecum may reflect downstream host-mediated adaptations.