Potential of antibiotic alternatives to impact nursery pig growth performance, immune status, intestinal morphology, physiology, and microbial populations under both conventional and <i>Salmonella</i> challenge environments
The purpose of the studies in chapters II, III and IV were to evaluate the efficacy of direct fed microbials (DFMs) and organic acids as potential alternatives to growth promoting antibiotics in weanling pigs, under conventional and Salmonella challenge environments. The study in chapter II was designed to investigate the effects of administration of a bolus of DFMs at weaning with or without in-feed DFMs on weanling pig growth performance and gut health. The study indicated that DFM inclusion in weanling pig diets, in particular, continuous inclusion coupled with oral treatment at weaning, may beneficially enhance the gastrointestinal environment contributing to better overall gut health of the weanling pig. In chapter III, a study was conducted to examine the predominance of each of five probiotic Lactobacillus and Pediococcus strains in the ileum of weaned pigs compared to strain distribution in the feces. In addition, the immunomodulatory potential of this five-strain probiotic combination was investigated. It was concluded that bacteriocin production may offer a competitive advantage over other probiotics in a mixture, allowing for such strains to dominate both in the ileal lumen and to bind more effectively to the ileal wall. The probiotic mixture also demonstrated some potential to modulate host immunity. In chapter IV, a study was conducted to evaluate the potential of a water-delivered Enterococcus and Bacillus combination, or a propionic acid based blend of organic acid as a substitute for antibiotics following a Salmonella challenge in weanling pigs. Following a Salmonella enterica var Typhimurium challenge, both DFM and organic acid administered through drinking water enhanced growth performance and gastrointestinal histology. Salmonella presence in ileal contents was reduced by DFM treatment, and active glucose transport increased, while water acidification reduced induced intestinal chloride ion secretion. Current findings suggest that under a bacterial challenge, DFMs performed similar, or in some cases, superior to in-feed antibiotics, while in healthy pigs, DFMs also demonstrated the potential to enhance intestinal health. Therefore, in a conventional environment, and in particular, during a Salmonella challenge, DFMs should be considered as a viable antibiotic alternative in weanling pig diets.