Efficiency of monolaurin in mitigating ruminal methanogenesis and modifying C-isotope fractionation when incubating diets composed of either C3 or C4 plants in a rumen simulation technique (Rusitec) system

Mitigation of methanogenesis in ruminants has been an important goal for several decades. Free lauric acid, known to suppress ruminal methanogenesis, has a low palatability; therefore, in the present study the aim was to evaluate the mitigation efficacy of its esterified form (monolaurin). Further, C-13-isotope abundance (delta C-13) and C-13-C-12 fractionation during methanogenesis and fermentation were determined to evaluate possible microbial C-isotope preferences. Using the rumen simulation technique, four basal diets, characterised either by the C-3 plants grass (hay) and wheat (straw and grain), or the C-4 plant (C-13 excess compared with C-3 plants) maize (straw and grain), and a mixture of the latter two, were incubated with and without monolaurin (50 g/kg dietary DM). Added to hay, monolaurin did not significantly affect methanogenesis. When added to the other diets (P<0.05 for the wheat-based diet) methane formation was lowered. Monolaurin decreased fibre disappearance (least effect with the hay diet), acetate: propionate ratio, and protozoal counts. Feed residues and SCFA showed the same delta C-13 as the diets. Methane was depleted in C-13 while CO2 was enriched in C-13 compared with the diets. Monolaurin addition resulted in C-13 depletion of CO2 and enrichment in CH4 (the latter only in the hay diet). In conclusion, monolaurin proved to effectively decrease methanogenesis in the straw-grain diets although this effect might partly be explained by the concomitantly reduced fibre disappearance. The influence on C-13-isotope abundance and fractionation supports the hypothesis that ruminal microbes seem to differentiate to some extent between C-isotopes during methanogenesis and fermentation.