Effects of butyrate precursors on electron relocation when methanogenesis is inhibited in ruminal mixed cultures

Aims:  Inhibition of ruminal methanogenesis often causes accumulation of H2, formate and ethanol, which are not energy substrates for ruminants. It was hypothesized that the addition of butyrate precursors would avoid the formation of these products and relocate electrons into butyrate. Methods and Results:  In four ruminal 24-h incubations, two inhibitors of methanogenesis, each at three different initial concentrations (0, 2 or 4 mmol l−1 for propynoic acid, and 0, 4 or 8 mmol l−1 for ethyl 2-butynoate), were combined with two butyrate precursors at two different initial concentrations (0 or 4 mmol l−1 for crotonic acid or 3-butenoic acid). Ground lucerne hay was the substrate. Propynoic acid at 4 mmol l−1 decreased CH4 formation by more than two-thirds. Ethyl 2-butynoate at 8 mmol l−1 suppressed methanogenesis by more than 90%. Butyrate precursors generally did not decrease the accumulation of H2 and formate or ethanol production. Conclusions:  Butyrate precursors were ineffective as electron acceptors because they were not completely converted to butyrate and were also metabolized through other pathways. Significance and Impact of the Study:  Effectiveness of butyrate precursors may be improved by adding them to the fermentation continuously or by enhancing the kinetics of their conversion into butyrate.