Enteric methane production in response to direct ruminal infusion of synthetic bromoform in cattle.

Methane inhibitors in cattle diets are a promising strategy to reduce enteric methane (CH4) emissions. Bromoform (CHBr3) is effective but not well understood in terms of mechanism, dose response, and side effects such as reduced feed intake. Studies using CHBr3-containing seaweed are inadequate to isolate the effect of CHBr3 from other factors. This study aimed to determine the dose-dependent effects of synthetic CHBr3, delivered via ruminal infusion, on DMI, enteric gas emissions, temporal patterns of gas emissions, and ruminal fermentation in cattle. Four nonlactating, nonpregnant, rumen-fistulated Holstein-Friesian cows (12 yr of age; 781 ± 33 kg BW; mean ± SD) were randomly assigned to 1 of 4 treatment sequences in a 4 × 4 Latin square design. Treatments consisted of 0 (CNTL), 59 (LOW), 192 (MED), or 592 (HIGH) mg CHBr3/d infused into the rumen as an aqueous solution via the rumen fistula. The daily CHBr3 dose was divided into 3 equal infusions every 8 h over a 24-h period (0545, 1345, and 2145 h) for 14 consecutive d during experimental periods, followed by a 7-d washout period. Cows were fed grass hay ad libitum, and additional concentrate was supplied through a GreenFeed (C-Lock Inc.), which was also used for enteric gas measurements. Total DMI did not respond linearly or quadratically to increasing doses of CHBr3 but was numerically 6.2 kg/d lower for HIGH compared with CNTL. Both CH4 production (g/d) and yield (g/kg DMI) decreased quadratically with increasing doses of CHBr3. Conversely, hydrogen (H2) production (g/d) and yield (g/kg DMI) increased quadratically with increasing dose of CHBr3. Total VFA concentration did not respond linearly or quadratically to increasing doses of CHBr3 but was numerically 27.8 mM lower for HIGH compared with CNTL. Molar proportions of propionate, butyrate, valerate, and branched-chain VFA increased quadratically, and the molar proportion of acetate decreased quadratically with increasing dose of CHBr3. Separate 4-parameter logistic functions were fitted to CH4 and H2 production (g/d) over the infusion and washout periods, using data from the HIGH treatment. The decrease in CH4 production and increase in H2 production stabilized after 56 and 49 h of CHBr3 infusions, respectively (defined as the time at which 95% of the asymptotic change in gas production was attained). After cessation of the CHBr3 infusions, the increase in CH4 production and decrease in H2 production stabilized after 117 and 99 h, respectively. In conclusion, synthetic CHBr3 supplementation reduced enteric CH4 production in a nonlinear, dose-dependent manner, with minor effects at 59 and 192 mg CHBr3/d doses, but ∼90% reduction at 592 mg CHBr3/d. The HIGH treatment numerically reduced DMI, indicating a potential trade-off between efficacy and feed intake. Temporal modeling showed that the CH4-suppressing effect of CHBr3 infusion at 592 mg/d emerged within 2 to 3 d and reversed more gradually over 4 to 5 d after the last infusion. These findings highlight the importance of both dose and temporal dynamics in evaluating CHBr3 for CH4 mitigation in ruminants.