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.
Protein metabolism in the rumen is the result of metabolic activity of ruminal microorganisms. The structure of the protein is a key factor in determining its susceptibility to microbial proteases and, thus, its degradability. Ruminal protein degradation is affected by pH and the predominant species of microbial population. Ruminal proteolytic activity decreases as pH decreases with high-forage dairy cattle-type rations, but not in high-concentrate beef-type rations. Accumulation of amino acid (AA) N after feeding suggests that AA uptake by rumen microorganisms could be the limiting factor of protein degradation in the rumen. In addition, there are several AA, such as Phe, Leu, and Ile, that are synthesized by rumen microorganisms with greater difficulty than other AA. The most common assessment of efficiency of microbial protein synthesis (EMPS) is determination of grams of microbial N per unit of rumen available energy, typically expressed as true organic matter or carbohydrates fermented. However, EMPS is unable to estimate the efficiency at which bacteria capture available N in the rumen. An alternative and complementary measure of microbial protein synthesis is the efficiency of N use (ENU). In contrast to EMPS, ENU is a good measurement for describing efficiency of N capture by ruminal microbes. Using EMPS and ENU, it was concluded that optimum bacterial growth in the rumen occurs when EMPS is 29 g of bacterial N/kg of fermented organic matter, and ENU is 69%, implying that bacteria would require about 1.31 x rumen-available N per unit of bacterial N. Because the distribution of N within bacterial cells changes with rate of fermentation, AA N, rather than total bacterial N should be used to express microbial protein synthesis.
Different doses of 12 plant extracts and 6 secondary plant metabolites were incubated for 24 h in diluted ruminal fluid with a 50:50 forage:concentrate diet. Treatments were: control (no additive), plant extracts (anise oil, cade oil, capsicum oil, cinnamon oil, clove bud oil, dill oil, fenugreek, garlic oil, ginger oil, oregano oil, tea tree oil, and yucca), and secondary plant metabolites (anethol, benzyl salicylate, carvacrol, carvone, cinnamaldehyde, and eugenol). Each treatment was supplied at 3, 30, 300, and 3,000 mg/L of culture fluid. At 3,000 mg/L, most treatments decreased total volatile fatty acid concentration, but cade oil, capsicum oil, dill oil, fenugreek, ginger oil, and yucca had no effect. Different doses of anethol, anise oil, carvone, and tea tree oil decreased the proportion of acetate and propionate, which suggests that these compounds may not be nutritionally beneficial to dairy cattle. Garlic oil (300 and 3,000 mg/L) and benzyl salicylate (300 and 3,000 mg/L) reduced acetate and increased propionate and butyrate proportions, suggesting that methane production was inhibited. At 3,000 mg/L, capsicum oil, carvacrol, carvone, cinnamaldehyde, cinnamon oil, clove bud oil, eugenol, fenugreek, and oregano oil resulted in a 30 to 50% reduction in ammonia N concentration. Careful selection and combination of these extracts may allow the manipulation of rumen microbial fermentation.
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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