Date Published: October 28, 2015
Publisher: Public Library of Science
Author(s): Jolien B. Veneman, Stefan Muetzel, Kenton J. Hart, Catherine L. Faulkner, Jon M. Moorby, Hink B. Perdok, Charles J. Newbold, Jose Luis Balcazar.
It has been suggested that the rumen microbiome and rumen function might be disrupted if methane production in the rumen is decreased. Furthermore concerns have been voiced that geography and management might influence the underlying microbial population and hence the response of the rumen to mitigation strategies. Here we report the effect of the dietary additives: linseed oil and nitrate on methane emissions, rumen fermentation, and the rumen microbiome in two experiments from New Zealand (Dairy 1) and the UK (Dairy 2). Dairy 1 was a randomized block design with 18 multiparous lactating cows. Dairy 2 was a complete replicated 3 x 3 Latin Square using 6 rumen cannulated, lactating dairy cows. Treatments consisted of a control total mixed ration (TMR), supplementation with linseed oil (4% of feed DM) and supplementation with nitrate (2% of feed DM) in both experiments. Methane emissions were measured in open circuit respiration chambers and rumen samples were analyzed for rumen fermentation parameters and microbial population structure using qPCR and next generation sequencing (NGS). Supplementation with nitrate, but not linseed oil, decreased methane yield (g/kg DMI; P<0.02) and increased hydrogen (P<0.03) emissions in both experiments. Furthermore, the effect of nitrate on gaseous emissions was accompanied by an increased rumen acetate to propionate ratio and consistent changes in the rumen microbial populations including a decreased abundance of the main genus Prevotella and a decrease in archaeal mcrA (log10 copies/ g rumen DM content). These results demonstrate that methane emissions can be significantly decreased with nitrate supplementation with only minor, but consistent, effects on the rumen microbial population and its function, with no evidence that the response to dietary additives differed due to geography and different underlying microbial populations.
Livestock are estimated to be responsible for 14.5% of the total greenhouse gas (GHG) emission from anthropogenic sources , with methane resulting from enteric fermentation the second largest source of anthropogenic GHG, representing 39% of the livestock sector emissions . Numerous studies have investigated the potential to decrease methane from enteric fermentation in ruminants using dietary strategies or dietary additives . However, relatively few studies have considered the wider consequences of these interventions on the functioning of the rumen microbial ecosystem . It has been suggested that rumen function will be disrupted if rumen methane production is inhibited without the provision of alternative hydrogen sinks . Here we report on the effect of two dietary additives (nitrate and linseed oil) selected based on a previous meta-analysis as persistent and potentially practical methane mitigation additives  on rumen fermentation, methane and hydrogen emissions and the rumen microbiome. Furthermore, given the concerns that geography and management might influence the underlying microbial population and hence the response to the additive , the additives were tested in two matched experiments in New Zealand and in the UK.
Methane yield was higher in Dairy 2 than Dairy 1, but both were in the range typically observed in dairy cows [23, 24]. Linseed oil did not affect methane emissions in either experiment. This contrasts with studies in the literature showing a decrease in methane emissions by around 3.6 to 5.6% for each 10 g/kg DM increase in crude fat content [23, 25–27]. Linseed oil is thought to decrease methane emissions as the PUFA present in linseed oil might be toxic to certain cellulolytic bacteria and rumen protozoa which supply hydrogen to the methanogenic archaea . However, no effect was observed on the main cellulolytic genera such as Fibrobacter or Ruminococcus and neither did linseed oil decrease protozoa 18S copy numbers. Overall, the absence of a reduction in methane emissions with linseed oil coincides with an absence of a response in the rumen microbial ecosystem as a whole. The effects of linseed oil on animal performance, particularly on milk composition were, however, pronounced in both experiments. Linseed oil caused a reduction in milk fat percentage in agreement with previous studies (see meta-analysis of ). It is believed this is caused by the biohydrogenation of PUFA into specific FA intermediates, in particular trans-10, cis-12 conjugated linoleic acid which inhibit the de novo synthesis of FA in the mammary gland, causing a reduction in saturated FA and total milk fat .
This study has demonstrated that significant decreases in rumen methane emissions can be achieved without drastic effects on either the rumen microbial population or its function. Furthermore, we found no evidence to support concerns that the response to dietary additives in different geographical locations will differ due to different underlying rumen microbial populations.