Microorganisms in the rumen degrade nutrients to produce volatile fatty acids and synthesize microbial protein as an energy and protein supply for the ruminant, respectively. However, this fermentation process has energy (losses of methane) and protein (losses of ammonia N) inefficiencies that may limit production performance and contribute to the release of pollutants to the environment. Antibiotic ionophores have been very successful in reducing these energy and protein losses in the rumen, but the use of antibiotics in animal feeds is facing reduced social acceptance, and their use has been banned in the European Union since January 2006. For this reason, scientists have become interested in evaluating other alternatives to control specific microbial populations to modulate rumen fermentation. Essential oils can interact with microbial cell membranes and inhibit the growth of some gram-positive and gram-negative bacteria. As a result of such inhibition, the addition of some plant extracts to the rumen results in an inhibition of deamination and methanogenesis, resulting in lower ammonia N, methane, and acetate, and in higher propionate and butyrate concentrations. Results have indicated that garlic oil, cinnamaldehyde (the main active component of cinnamon oil), eugenol (the main active component of the clove bud), capsaicin (the active component of hot peppers), and anise oil, among others, may increase propionate production, reduce acetate or methane production, and modify proteolysis, peptidolysis, or deamination in the rumen. However, the effects of some of these essential oils are pH and diet dependent, and their use may be beneficial only under specific conditions and production systems. For example, capsaicin appears to have small effects in high-forage diets, whereas the changes observed in high-concentrate diets (increases in dry matter intake and total VFA, and reduction in the acetateto-propionate ratio and ammonia N concentration) may be beneficial. Because plant extracts may act at different levels in the carbohydrate and protein degradation pathways, their careful selection and combination may provide a useful tool to manipulate rumen microbial fermentation effectively. However, additional research is required to establish the optimal dose in vivo in units of the active component, to consider the potential adaptation of microbial populations to their activities, to examine the presence of residues in the products (milk or meat), and to demonstrate improvements in animal performance.

Two experiments were conducted to determine the effects of several essential oil active compounds on rumen microbial fermentation. In the first experiment, 4 doses (5, 50, 500, and 5,000 mg/L) of 5 essential oil compounds were evaluated using in vitro 24-h batch culture of rumen fluid with a 60:40 forage:concentrate diet (18% crude protein; 30% neutral detergent fiber). Treatments were control (CON), eugenol (EUG), guaiacol, limonene, thymol (THY), and vanillin. After 24 h, the pH was determined, and samples were collected to analyze ammonia N and volatile fatty acids (VFA). The highest dose of all compounds decreased total VFA concentration and increased the final pH. Eugenol at 5 mg/L tended to reduce the proportion of acetate and the acetate to propionate ratio, at 50 and 500 mg/L tended to reduce ammonia N concentration, and at 500 mg/L reduced the proportion of propionate and branched-chain VFA concentration, without affecting total VFA concentration. All other treatments had minor effects or changes occurred only after total VFA concentration decreased. In the second experiment, 8 dual-flow continuous culture fermenters (1,320 mL) were used in 3 replicated periods (6 d of adaptation and 3 d of sampling) to study the effects of THY and EUG on rumen microbial fermentation. Fermenters were fed 95 g/d of DM of a 60:40 forage:concentrate diet (18% crude protein; 30% neutral detergent fiber). Treatments were CON, 10 mg/L of monensin (positive control), and 5, 50, or 500 mg/L of THY and EUG, and were randomly assigned to fermenters within periods. During the last 3 d of each period, samples were taken at 0, 2, 4, and 6 h after the morning feeding and analyzed for peptides, amino acids, and ammonia N concentrations, and total and individual VFA concentrations. Monensin changed the VFA profile as expected, but inhibited nutrient digestion. Eugenol and THY decreased total VFA concentration and changed the VFA profile, and only 5 mg/L of THY tended to reduce the proportion of acetate, increased the proportion of butyrate, and increased the large peptides N concentration without decreasing total VFA concentration. Most of these essential oil compounds demonstrated their antimicrobial activity by decreasing total VFA concentration at high doses. However, EUG in batch fermentation and 5 mg/L of THY in continuous culture modified the VFA profile without decreasing total VFA concentration, and EUG in batch fermentation decreased ammonia N concentration.

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