The essential oils are secondary metabolites of plants that have beneficial actions, as antimicrobial activity on rumen microbial metabolism and the gut of ruminants. The objective is to evaluate the use of a blend of essential oils via milk-replacer or concentrated, with regard to improvements in overall performance and ocurrence of diarrhea, as well as evaluate the effect of oils on microbial population and rumen fermentation and intestinal microbiota . Forty Holstein calves 1 to 4 days of age will be fed 6L of liquid diet per day, divided into two meals (7h and 18h), will have free access to water and starter concentrate, formulated in accordance with the recommendations of the NRC (2001). The animals will be supplemented with a mixture of essential oils (Activio, Grasp), consisting of: cinnamon essential oil, rosemary, oregano and pepper extract, via milk-replacer and starter. Animals will be distributed in a randomized block, in the following treatments: 1) No supplement with essential oils, 2) dose of 200 mg / kg of essential oils in milk replacer, 3) dose of 400 mg / kg of essential oils in milk replacer; and 4) dose 150 mg / kg of essential oils in the starter.From the second week of life of animals will be sampled blood two hours after the morning feeding, ruminal fluid (50 ml) and feces directly from the rectum. Plasma samples will be analyzed for glucose and ²-hydroxybutyrate concentrations. Rumen contents samples will be used to determine the concentrations of short chain fatty acids, ammonia-N and quantification of cellulolytic bacteria, and protozoa through amylolytic bacterial counts. The feces will be diluted in sterile distilled water and after serial dilution, distributed in specific culture media for the quantification of BAL (lactic acid bacteria), coliform bacillus and also through a bacterial CFU counts.
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