Calcium salts of long-chain fatty acids (CSFA) from linseed oil have the potential to reduce methane (CH4) production from ruminants; however, there is little information on the effect of supplementary CSFA on rumen microbiome as well as CH4 production. The aim of the present study was to evaluate the effects of supplementary CSFA on ruminal fermentation, digestibility, CH4 production, and rumen microbiome in vitro. We compared five treatments: three CSFA concentrations—0% (CON), 2.25% (FAL) and 4.50% (FAH) on a dry matter (DM) basis—15 mM of fumarate (FUM), and 20 mg/kg DM of monensin (MON). The results showed that the proportions of propionate in FAL, FAH, FUM, and MON were increased, compared with CON (P < 0.05). Although DM and neutral detergent fiber expressed exclusive of residual ash (NDFom) digestibility decreased in FAL and FAH compared to those in CON (P < 0.05), DM digestibility-adjusted CH4 production in FAL and FAH was reduced by 38.2% and 63.0%, respectively, compared with that in CON (P < 0.05). The genera Ruminobacter, Succinivibrio, Succiniclasticum, Streptococcus, Selenomonas.1, and Megasphaera, which are related to propionate production, were increased (P < 0.05), while Methanobrevibacter and protozoa counts, which are associated with CH4 production, were decreased in FAH, compared with CON (P < 0.05). The results suggested that the inclusion of CSFA significantly changed the rumen microbiome, leading to the acceleration of propionate production and the reduction of CH4 production. In conclusion, although further in vivo study is needed to evaluate the reduction effect on rumen CH4 production, CSFA may be a promising candidate for reduction of CH4 emission from ruminants.

We investigated the effects of supplementary calcium salt of fatty acid (CSFA) from linseed oil with different oil adsorbents on in vitro gas production and rumen fermentation characteristics in barley substrate condition. A non-supplementation treatment (CONT) and treatments of six products, CSFA without oil adsorbent (2.1 fatty acid/ Ca molar ratio) and CSFAs with silica gel, zeolite, bentonite, diatomite, and vermiculite (2.8 fatty acid/Ca molar ratio), were prepared. The supplementary 2% and 4% CSFA with silica gel (+S) in the substrate reduced CH 4 production 56% and 79%, respectively, compared with that in CONT (p < 0.01). The products, except for +S, did not decrease CH 4 production. The dry matter (DM) disappearance in CSFAs with oil adsorbents was lower than that in the CSFA without oil adsorbent (74.8%-77.3% vs. 79.3%, p < 0.01), and crude protein (CP) disappearance in +S supplementation was lower than that of the other products (53.5% vs. 57.2%-59.1%, p < 0.01). The +S supplementation decreased acetate proportion and increased propionate proportion (p < 0.01). Our study indicated that although the disappearance of DM and CP might decrease, using silica gel as an oil adsorbent of linseed oil calcium salt with high fatty acid/Ca molar ratio has the potential to mitigate CH 4 emissions from ruminants. K E Y W O R D S calcium salt of fatty acid, in vitro fermentation, methane mitigation, oil adsorbent, silica gel 1 | INTRODUCTION Strategies to decrease global greenhouse gas (GHG) emissions have been developed to address environmental issues. Methane (CH 4 ) is an important source of GHG and has 28 times the global warming potential of CO 2 calculated on a 100-year time frame (Intergovernmental Panel on Climate Change [IPCC], 2013). Methane emissions from ruminants account for 24.0% of the total CH 4 emissions source (United Nations Framework Convention on Climate Change[UNFCCC], 2015), and 2%-12% of gross energy intake is also lost by CH 4 emissions in ruminants (Johnson & Johnson, 1995). Therefore, inhibition of CH 4 emission from enteric fermentation of ruminants can contribute not only to preventing global warming but also to improving feed efficiency.The effectiveness of feeding lipids on CH 4 reduction differs from that of lipid sources or the amount of lipid supply (Martin et al., 2008;Martin et al., 2016). For example, canola and linseed oils, which are rich in monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs), respectively, reduce CH 4 emissions in lactating dairy cows (Martin et al., 2010) and beef cattle (Beauchemin &