PSXIII-25 The influence of bromoform dose on in vitro rumen fermentation characteristics in rumen fluid obtained from cannulated steers fed a medium quality grass hay.

Abstract This study aimed to evaluate the effect of bromoform (CHBr3) dose on in vitro rumen fermentation characteristics and the accumulation of H2, CHBr3, and dibromomethane (CH2Br2) in solution and the gas cap. Six cannulated steers (658.4 ± 10 kg BW; ~4.0 years old) were maintained on a medium-quality grass hay diet (6.5% CP). Approximately 50 ml of rumen fluid was combined with a modified McDougall’s buffer in 1:1 proportion and served as the in vitro inoculum. The in vitro treatments consisted of six bromoform doses (0, 1, 10, 100, 1000, 10,000 µg of CHBr3 per sample), with five replicates per dose at each of 13 incubations time points (0, 0.25, 0.5, 1, 2, 3, 4, 5, 6, 12, 24, 48, and 72 h). The medium-quality grass hay diet was ground through a 2 mm screen, and 0.5 g was added to each in vitro tube as substrate. Statistical analyses were performed using the MIXED procedure of SAS 9.3 for randomized block design with significance considered at P ≤ 0.05. The 10,000 µg of CHBr3 treatment inhibited fermentation at all time points and was removed. Dose was a significant (P < 0.001) source of variation for 10 fermentation parameters: as the CHBr3 dose increased, the molar proportion of volatile fatty acids (VFAs) acetate, isobutyrate, and butyrate decreased, and the molar proportion of propionate, isovalerate, and valerate increased. Furthermore, the acetate:propionate ratio and the amount of N2 and CH4 produced per gram of dry matter digested (DMD) decreased as the CHBr3 dose increased. Total VFAs concentrations and DMD were not impacted by CHBr3 dose. In the gas cap, there was a dose by incubation time interaction (P < 0.05) for cumulative CH4 and H2 production; as the dose of CHBr3 increased, CH4 decreased and H2 increased in the gas cap in a dose-dependent manner. In solution, there was a dose x incubation time interaction (P < 0.01) for CHBr3 and CH2Br2 concentrations; for all doses, the CHBr3 concentration dropped below detection limits within 3 h post incubation. Conversely, CH2Br2 concentrations in vitro tubes containing 1,000 µg of CHBr3 increased in solution beginning 1 h post incubation and plateaued by hour 5. The concentration of CH2Br2 in vitro tubes containing 100 µg of CHBr3 increased slightly after 5 h. For all other doses, CH2Br2 concentrations in the solution were below the detection limits of 20 ppb. This study demonstrates that a dose of CHBr3 can impact in vitro rumen fermentation characteristics by reducing CH4, increasing H2, and converting CHBr3 to CH2Br2 within 3 hours.