Evaluation of a Postbiotic on Salmonella enterica Prevalence, Serotype Diversity, and Antimicrobial Resistance in the Subiliac Lymph Nodes of Cull Dairy Cattle

This study aimed to determine whether the farm-level use of a Saccharomyces cerevisiae-based postbiotic was associated with Salmonella prevalence and concentration, serotype diversity, and antimicrobial resistance in the subiliac lymph nodes (LN) of cull dairy cows. In collaboration with two commercial processing plants in the Southwestern (SW) and Northeastern (NE) regions of the U.S., cull dairy cattle lots processed in the same week from dairy farms that fed or not the postbiotic were sampled. Up to 20 LN were collected from dairy cattle from each supplier farm at least once every season. Samples were analyzed for Salmonella by culture and quantitative PCR methods, and isolates were subjected to serotype identification and antimicrobial susceptibility testing. Although a numerically lower prevalence was observed, the preharvest administration of the postbiotic was not significantly associated with Salmonella prevalence in cull dairy cattle. However, Salmonella prevalence significantly varied by region; the SW region showed a higher prevalence than the NE region. Whereas dominant Salmonella serotypes included Montevideo, Mbandaka, Muenster, Cerro, Meleagridis, and Anatum, the probability of isolating a dominant serotype did not significantly vary by feed additive status (FAS) or region, but varied by season. Up to 34 isolates (out of 391) exhibited resistance to each antimicrobial, with the highest number of isolates exhibiting resistance to streptomycin and ciprofloxacin; however, this did not significantly vary by FAS, season, or region. The precise reasons for the lack of effectiveness of the postbiotic in reducing Salmonella burden are unknown; however, varied administration durations of the postbiotic due to culling at different lactation cycle stages or during dry periods may have hindered its impact, especially if dairy cows were culled early. Other factors may include dietary components, farm management practices, and external environmental influences.