Mitigation of in vitro hydrogen sulfide production using bismuth subsalicylate with and without monensin in beef feedlot diets

The objective of this study was to determine if a sulfur binder, bismuth subsalicylate (BSS), alone or combined with monensin (MON) could decrease the production of H₂S by rumen microbes. In Exp. 1, two 24-h batch culture incubations were conducted using a substrate consisting of 50% corn, 40% distillers grains, 9.75% hay, and 0.25% mineral premix, on a DM basis. Five treatments including BSS concentrations of 0% (control), 0.5%, 1%, 2%, and 4% of DM were assigned in 5 replicates to 120-mL serum bottles containing rumen fluid, buffer, and 0.5 g of dietary substrate. Addition of 2% and 4% BSS decreased (P < 0.05) gas production, whereas all concentrations of BSS reduced (P < 0.05) H₂S production by 18%, 24%, 82%, and 99% for 0.5%, 1%, 2%, and 4% BSS, respectively. Final pH increased (P < 0.05) with 2% and 4% BSS treatments. At 4% of DM, BSS decreased (P < 0.05) total VFA concentration (mM) and propionate (mol/100 mol) but increased acetate (mol/100 mol) and acetate to propionate ratio. Concentration of branched-chain VFA increased (P < 0.05) with the addition of 0.5% BSS, compared with the control. On the basis of these results, addition of BSS (1% of DM) and MON (5 mg/kg) were used to assess their effects on metabolism and H₂S release by rumen microbes in 8 dual flow continuous culture fermenters during two 10-d periods (Exp. 2). Treatments were arranged in a 2 × 2 factorial design. Substrate similar to that used in Exp. 1 was provided at 75 g DM/fermenter daily. Headspace H₂S concentration was reduced (P < 0.05) by 99% with BSS treatment but was not affected (P = 0.21) by MON. An overall increase (P < 0.05) in fermentation pH was found following addition of BSS. Addition of BSS increased (P < 0.05) digestion of NDF and ADF but decreased (P < 0.05) nonfiber carbohydrate digestion and total VFA concentration. Acetate and propionate (mol/100 mol) increased (P < 0.05) with BSS, whereas butyrate (mol/100 mol) and branched-chain VFA (mM) decreased (P < 0.05). Addition of BSS increased (P < 0.05) NH₃-N concentration and NH₃-N outflow but decreased (P < 0.05) microbial N outflow. Results from this study showed no response to monensin addition, but BSS markedly reduced H2S production and altered microbial fermentation during in vitro rumen fluid incubations.