Effects of Essential Oils Alone or in Combination with Monensin in an In Vitro Model of Ruminal Acidosis

The objective of this study was to evaluate the effect of essential oils and/or monensin on the concentration of volatile fatty acids (VFA), ammonia, and pH in a ruminal acidosis in vitro model. Red Angus steers (n = 4; BW = 435 ± 9 kg) with ruminal and duodenal cannulas were allocated in a 4 × 4 Latin square design. Treatments were 1) Fed no essential oils (RB) or monensin sodium (MON:CON); 2) Fed RB at a rate of 14 g daily with no Mon (RB+); 3) Fed no RB and fed Mon at 400 mg daily (MON+); 4) Fed RB and Mon [RB+MON+] at the same levels of inclusion. The basal diet (TMR) consisted of dry rolled corn (67%), grass hay (10%), DDGS (20%), and vitamin and mineral premix (3%). Feed additives (RB or MON) were dosed in the rumen through the cannulas in gelatin capsules to guarantee the steers were receiving the targeted dose. Each period lasted 28-d, consisting of a 14-d adaptation and 14-d collection period, where in-vitro ruminal acidosis challenges were performed on d 14 and 26. Briefly, rumen fluid from each steer was placed into three 250-ml glass flasks (30 mL of rumen fluid per flask) containing McDougall buffer (120 ml per flask), thus resulting in a triplicate sample per treatment (rumen fluid:buffer ratio of 1:4 per flask). In addition to the buffer, each flask contained 7.5 g of substrate (reconstituted TMR). Flasks were incubated for 36 h at 39 ºC, subsamples (4 mL) were taken every 4 hours and pH was measured. Samples were stored at -20 ºC for further analysis of VFA and ammonia. Data were analyzed as repeated measures using the Glimmix procedure of SAS with fixed effects of treatment, day, time, and their interactions. Animal and period were considered random effects. Significance was set at P ≤ 0.05. A treatment × day interaction (P ≤ 0.001) was observed for concentrations of all VFA. Acetate decreased from d 14 to d 26 for CON, with similar levels observed for the other treatments. Concentrations of propionate increased from d 14 to d 26 for CON and RBMON and decreased for MON. Butyrate concentrations decreased from d 14 to d 26 for RB+ and RB+MON+ treatments. Isobutyrate increased from d 14 to d 26 for MON and decreased for RB+MON+. Concentrations of valerate decreased from d 14 to d 26 for MON, RB+, and RB+MON+, while the opposite was observed for CON. Isovalerate concentrations decreased from d 14 to d 26 for CON and increased for MON. Acetate:Propionate ratio remained similar between d 14 and d 26 for MON, RB, and RBMON, and decreased for CON from d 14 to d 26. Sampling time affected concentrations of all VFA, ammonia, and pH (P < 0.001). In vitro ruminal pH decreased (P < 0.001) from d 14 to d 26, and as expected pH decreased (P < 0.001) from 0 to 36h. No differences in pH were observed between treatments, with an average of 5.69. Discrepancies between the present in vitro findings and those reported in a parallel in vivo study suggest that the absence of a treatment effect may be attributed to limitations inherent to the in vitro model, such as gas accumulation, rapid pH decline, and the absence of ruminal wall interactions.