Limited effects of a commercial direct-fed microbial on weaning pig performance and gastrointestinal microbiology

Objectives: To determine the effects of a direct-fed microbial (DFM) and a specific regimen of antibiotic administration (subtherapeutic dosages) on fecal Escherichia coli concentrations, protection against Salmonella and rotavirus infections, intestinal volatile fatty acid (VFA) concentrations, and growth of nursery pigs. Methods: Parameters were compared in groups of pigs fed the DFM, the antibiotics, or a control diet under field conditions and after experimental challenge with Salmonella enterica serovar Typhimurium and rotavirus. Results: In the field study, average daily gain of antibiotic-fed pigs was larger than that of DFM-fed pigs. Other growth-performance parameters, fecal Escherichia coli concentrations, and prevalence of Salmonella serovars were similar among treatment groups. Under experimental conditions, total fecal coliform concentration was significantly lower in the antibiotic-fed group than in the two other groups. Total VFA concentration in the DFM group was significantly higher than that in the antibiotic-fed group. Prevalence of Salmonella serovars and rotavirus following challenge was similar in all groups. Implications: Under the conditions of this study, this DFM does not enhance growth of nursery pigs or protect against Salmonella or rotavirus infection. Effectiveness of a DFM should not be assumed solely on the basis of the genera of bacteria included. Each strain of bacteria in a DFM should be validated for effectiveness. Additional details concerning the mechanisms by which DFMs and subtherapeutic dosages of antibiotics modulate the ecological balance of bacterial flora in the gastrointestinal tract are required to understand how the beneficial effects associated with certain feed additives are mediated.