Trudeau, Michaela2022-02-152022-02-152021-12https://hdl.handle.net/11299/226432University of Minnesota Ph.D. dissertation. December 2021. Major: Animal Sciences. Advisors: Gerald Shurson, Pedro Urriola. 1 computer file (PDF); xi, 172 pages.The mechanisms of action for antibiotic growth promotion are poorly understood, making it difficult to select effective alternatives capable of providing similar responses. The objective of this thesis was to identify mechanism of action when feeding growth promoting levels of antibiotics and antibiotic alternatives. The first trial identified tylosin- responsive metabolites in growing pigs. This experiment identified that feeding tylosin increased the concentration of the secondary bile acids hyodeoxycholic acid and lithocholic acid and multiple bacteria in the Clostridia family. These findings suggest the mechanism of growth promotion involves an alteration in bile acid metabolism. The second experiment evaluated the growth performance, metabolic responses, and changes in intestinal microbiome composition of nursery pigs fed a variety of feed additives and antibiotic controls. Only the antibiotic group had improved growth performance over the negative control. Although none of the feed additives tested had an effect on alpha or beta microbiome diversity, supplementing multiple herbal blends increased the relative abundance of cecal bacteria in the genus Lactobacillus. In addition, the effect of experiment location had a greater influence on the microbiome than the effect of dietary treatment. Overall, these experiments identified that antibiotic growth promotion could use a mechanism involving bile acid metabolism, but this was not consistent when a different antibiotic was used. In addition, the proposed antibiotic alternatives had a minimal impact on animal growth, microbiome, and metabolism, but experiment location significantly impacted these parameters and this effect should be further evaluated.enThesis or Dissertation