Dose response to eugenol supplementation in growing beef cattle: Ruminal fermentation and intestinal digestion

Eugenol (EUG; 4-allyl-2-methoxyphenol; C10H12O2) is a phenolic compound with wide-spectrum antimicrobial activity against gram-positive and gram-negative bacteria. This study was conducted to determine whether EUG could be used as a rumen modifier in growing beef heifers fed high concentrate diets. The experiment was designed as a 4 × 4 Latin square using four ruminally and duodenally cannulated beef heifers with four treatments being: control (no EUG added); 400 mg/d EUG (low); 800 mg/d EUG (medium); and 1600 mg/d EUG (high), and four 21 d periods. The diets consisted of 150 g/kg barley silage, 800 g/kg dry-rolled barley grain, and 50 g/kg supplement (dry matter [DM] basis). Feed intake, rumen pH and fermentation characteristics, site and extent of digestion, microbial crude protein (CP) synthesis, blood metabolites and acute phase protein status were measured. Intake of DM averaged 9.6 kg/d and was not affected by EUG supplementation. Degradabilities of organic matter (OM) and starch in the rumen and the digestibilities in the total digestive tract were not affected. However, ruminal degradability of neutral detergent fibre linearly decreased and ruminal degradability of CP tended (P=0.09) to linearly decrease with increasing dose of EUG. Flows (g/d) of microbial N to the duodenum tended (P=0.08) to quadratically increase with EUG supplementation even though the amount of ruminal fermented OM did not differ. Rumen pH, concentration of total volatile fatty acids (VFA), and protozoa counts were not affected, whereas concentration of acetate linearly declined, and the molar proportion of propionate (P=0.09), and ratio of acetate to propionate, tended (P<0.10) to linearly decrease with increasing EUG supplementation. Plasma concentrations of non-esterified fatty acids, triglyceride, glucose and urea N were not changed by feeding EUG. Acute phase proteins including serum amyloid A and lipopolysaccharide in plasma were not affected by EUG, although plasma haptoglobin tended (P<0.07) to linearly decrease with increasing EUG. Results indicate that supplementation of a high concentrate diet with EUG tended to provide more protein by tending to increase microbial CP production and lowering CP degradation in the rumen. Positive changes in ruminal N dynamics (i.e., more rumen undegraded CP flowing to the small intestine) and VFA pattern of fermentation (i.e., lower acetate to propionate ratio) were small, but may be enough to improve the growth rate of cattle. Overall, results indicate that EUG may have limited potential to improve the growth rate of cattle fed high concentrate diets.