Effect of octadeca carbon fatty acids on microbial fermentation, methanogenesis and microbial flora in vitro

This study was carried out to investigate effects of type and level of octadeca carbon fatty acids (C18-fatty acids) on rumen fermentation, methane emission and populations of ruminal microbes using an in vitro gas production technique. Four types of C18-fatty acids, stearic, oleic, linoleic and linolenic acids, were added at levels of 0, 35 and 70 g/kg of substrate dry matter, respectively. Methane emission was determined by gas chromatography. Total deoxyribonucleic acid (DNA) was extracted from ruminal microbes, and populations of rumen microbes were determined by a real-time quantitative polymerase chain reaction. Populations of total rumen methanogens, protozoa, fungi, Ruminococcus flavefaciens and Fibrobacter succinogenes were expressed as a proportion of total rumen bacterial 16S ribosomal deoxyribonucleic acid (rDNA). The 24 h gas production and methane emission were decreased by type, level and their interaction (P<0.001). Compared to the control, C18-fatty acids had little effect on pH, ammonia N and total volatile fatty acids. However, the fermentation patterns were changed, reflected by lower proportion of acetate and higher propionate with increasing levels and unsaturation of C18-fatty acids (P<0.001). Methanogens and protozoa populations relative to total bacterial 16S rDNA were decreased by linoleic and linolenic acids, with the linolenic acid addition being most efficient. However, addition of these unsaturated C18-fatty acids also inhibited growth of fibrolytic microbes including fungi, F. succinogenes and R. flavefaciens. From the present study, it is inferred that there is an effect of unsaturated C18-fatty acids in suppressing methanogenesis, probably mediated by direct action against the rumen microbes involved in methane formation.