181 Essential oils impact rumen kinetics but not post-ruminal digestibility in steers fed high-grain finishing diets.

Abstract We evaluated the effects of essential oils (blend of eugenol, linalol and anethole) and monensin, alone or in combination, on ruminal kinetics and nutrient flow and digestibility. Red Angus steers (n = 4; BW = 435 ± 9 kg) with ruminal and duodenal cannulas were used in a 4 × 4 Latin square design with a 2 × 2 factorial scheme. The factors consist of essential oils (RB, Factor A) and monensin sodium (MON, Factor B), thus resulting in the following treatments: 1) no essential oils or monensin sodium [CON]; 2) RB (14 g/d) and no Mon [RB+]; 3) no RB and fed Mon (400 mg/d) [MON+]; 4) a combination of RB and Mon [RB+MON+]. Feed additives (RB or monensin) were dosed intraruminally daily in gelatin capsules prior to morning feeding. The diet consisted of dry rolled corn (67%), grass hay (10%), DDGS (20%), and vitamin and mineral premix (3%). Each period lasted 28-d, consisting of a 14-d adaptation and 14-d collection period. Steers were dosed intraruminally with chromic oxide (16 g/d) as an indigestible marker from d13 to d20. Total fecal collections were conducted from d16 to d20 of each period. Duodenal collections were taken from d18 to d20, totaling 8 samples per site in 9-h intervals, representing 24-h sampling. Rumen digesta samples were collected on d21 at feeding time (0h) and at 2, 4, 6, 8, 10, and 12h post-feeding and evaluated for concentrations of volatile fatty acids (VFA) and ammonia. Flow and digestibility data were analyzed using the MIXED procedure of SAS. The model included the fixed effects of RB, MON, and their interaction, and the random effects of period and animal; VFA, lactate, and ammonia were analyzed as repeated measures, using animal within period as subject. The RB supplementation resulted in decreased molar proportions of acetate (P = 0.04) and increased propionate (P = 0.05), consequently, the Acetate:Propionate ratio decreased (P = 0.03) in response to RB. Concentrations of butyrate, isobutyrate, valerate, and isovalerate were not affected (P ≥ 0.06) by RB, MON, or their interaction. A RB × MON interaction (P = 0.02) was observed for lactate, however, no difference between treatments was observed when means separation was performed. Ammonia concentrations did not differ between treatments (P ≥ 0.70; 9.07 ± 1.74 mM). Nutrient intake, flow, and digestibility (post-ruminal and total tract) were not affected by treatment (P ≥ 0.16), except for starch. Total tract digestibility of starch was greater (P = 0.05) in response to RB. Microbial N flow was unaffected by supplementation (P ≥ 0.51). In conclusion, nutrient intake, flow, and post-ruminal digestibility were unaffected by the evaluated feed additives; however, RB supplementation increased ruminal concentrations of propionate and decreased Acetate:Propionate ratio.