The effect of thymol and thyme oil feed supplementation on growth performance, serum antioxidant levels, and cecal Salmonella population in broilers

To search for a potential feed additive as an antibiotic alternative for poultry production, 2 essential oils, thyme and cinnamon bark oil, were evaluated along with their respective components, as well as the common phenolic acid, caffeic acid, by in vitro and in vivo experiments. Caffeic acid, thymol, and thyme oil were better antioxidants than carvacrol, cinnamaldehyde, and cinnamon bark oil when using an oxygen radical absorbance capacity assay. When an antimicrobial assay with a growth indicator was used, minimum inhibitory concentrations against Salmonella enterica serovar Typhimurium were determined to be 2.0, 0.4, 0.78, 0.84, 1.54, and 1.83 mg/mL for caffeic acid, thymol, carvacrol, cinnamaldehyde, cinnamon bark oil, and thyme oil, respectively. Lower minimum inhibitory concentrations were recorded for Salmonella enterica serovars Kentucky, Enteritidis Nal(r), and Senftenberg. No cytotoxic effects were found when testing these compounds on chicken embryos. Thymol was an effective antioxidant and antimicrobial agent based on the 2 assays, so it was used as a feed additive in 3 feed trials. After a 28-d feeding trial, dietary thymol did not affect broiler growth performance and feed conversion efficiency compared with the control. However, supplementation affected the serum antioxidant status. Although the addition of thymol showed no significant inhibition against cecal Salmonella for chicks inoculated with 10(4) and 10(8) cfu/mL of naladixic acid-resistant Salmonella, the highest concentration of thymol (4 g/kg) demonstrated some protective effects for specific-pathogen-free chicks against Salmonella Enteritidis Nalr, preventing further mortality.