Influence of monensin and lauric acid distillate or palm oil on in vitro fermentation kinetics and metabolites produced using forage and high concentrate substrates

Two in vitro experiments were conducted to evaluate the interactions of monensin with lauric acid distillate (LAD) or palm oil (PO) in forage and concentrate diets on 24 h in vitro rumen fermentation characteristics. Two Holstein steers, each surgically fitted with a ruminal cannula, consuming 0.50 alfalfa cubes and 0.50 cracked corn-based concentrate at 1.75 × NEm requirements were used as rumen fluid donors. For both experiments in vitro gas production was measured in a completely random design with a 3 × 2 × 3 factorial treatment structure. Factors were diet [control (no substrate), forage (alfalfa), and high concentrate (corn)], 3 μg/ml of monensin (±), and LAD (0, .05 and 0.10 of the substrate, Exp. 1) or PO (0, .05 and 0.10 of the substrate, Exp. 2). Gas production was affected by diet (P < 0.001), and decreased (P < 0.001) for monensin addition in both experiments. Gas production had a diet × monensin interaction (P < 0.01). Lauric acid distillate and PO addition did not influence gas production. Degradation rate also responded with a diet × monensin interaction (P < 0.01) and lag time had only a diet effect (P = 0.001) in both in vitro experiments. Both the degradation rate and lag time were not affected by a monensin × fat (LAD or PO) interaction. Monensin addition reduced total volatile fatty acid concentration across all 3 substrates (control, alfalfa and high concentrate). Monensin addition decreased acetate:propionate (P < 0.001) and increased the propionate proportion (P < 0.001). Butyrate proportion had a diet × monensin interaction (P < 0.01) as butyrate proportion was increased with monensin for the alfalfa but decreased for the concentrate substrate. Ammonia concentration was reduced (P < 0.01) by monensin on all substrates and was slightly higher (P < 0.01) for the alfalfa substrate compared with control and concentrate. Lauric acid distillate or PO addition did not influence fermentation metabolites in the present study. In addition, a monensin by LAD or PO interaction was not observed for the metabolites. Methane production and the methane proportion to total gas production had a diet × monensin interaction (P < 0.01). The methane proportion was decreased quadratically (P = 0.028) with LAD addition, whereas PO addition linearly increased (P = 0.014) the methane proportion; both of these effects were small. Meanwhile, monensin by LAD or PO interactions were not observed for methane concentration and proportion. In conclusion, the combinations of monensin and LAD or PO in forage and high concentrate diets had no interactions on in vitro fermentation kinetics and end product production.