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Piglets (n = 128) weaned at 21 days of age were used in a 35-day seeder model to evaluate the effects of dietary additives differing in active ingredients, chemical, and physical formulation, and dose on Salmonella colonization and shedding and intestinal microbial populations. Treatments were a negative control (basal diet), the positive control (challenged, basal diet), and six treatments similar to the positive control but supplemented with the following active ingredients (dose excluding essential oils or natural extracts): triglycerides with butyric acid (1.30 g kg-1); formic and citric acids and essential oils (2.44 g kg-1); coated formic, coated sorbic, and benzoic acids (2.70 g kg-1); salts of formic, sorbic, acetic, and propionic acids, their free acids, and natural extracts (2.92 g kg-1); triglycerides with caproic and caprylic acids and coated oregano oil (1.80 g kg-1); and caproic, caprylic, lauric, and lactic acids (1.91 g kg-1). On day 6, half the piglets (seeder pigs) in each group were orally challenged with a Salmonella Typhimurium nalidixic acid-resistant strain (4 x 109 and 1.2 x 109 log CFU per pig in replicate experiments 1 and 2, respectively). Two days later, they were transferred to pens with an equal number of contact pigs. Salmonella shedding was determined 2 days after challenge exposure and then on a weekly basis. On day 34 or 35, piglets were euthanized to sample tonsils, ileocecal lymph nodes, and ileal and cecal digesta contents. The two additives, both containing short-chain fatty acids and one of them also containing benzoic acid and the other one also containing essential oils, and supplemented at more than 2.70 g kg-1, showed evidence of reducing Salmonella fecal shedding and numbers of coliforms and Salmonella in cecal digesta. However, colonization of tonsils and ileocecal lymph nodes by Salmonella was not affected. Supplementing butyric acid and medium-chain fatty acids at the applied dose failed to inhibit Salmonella contamination in the current experimental se [ABSTRACT FROM AUTHOR]