Overview of nutritional strategies to lower enteric methane emissions in ruminants
ABSTRACT Since ruminants are capable of utilizing fibrous feeds not digested by mono-gastrics, they represent a valuable natural resource for meeting future increases in global food supply. Ruminants have both local (nitrogen and phosphorus pollutions) and global (greenhouse gases, GHG) environmental footprints. It is estimated that the livestock sector is responsible for 18% of global anthropogenic GHG emissions. Losses of methane represent 30 to 50% of total GHG from livestock production, with the contribution from ruminants accounting for about 80%. Due to the concerns of increases in GHG emissions into the environment and potential effects on global warming, there is a need to develop strategies to lower methane emissions from ruminants as part of an overall requirement to improve the sustainability of ruminant food production systems. Methane is produced as a by-product of anaerobic fermentation in the reticulo-rumen, largely due to the activity of methanogenic archaea. Recent research has focused on the potential of novel feed ingredients (probiotics, ionophores, acetogen-based inoculants, bacteriocins, organic acids and plant extracts) or vaccines to lower hydrogen production and/or increase the transfer and utilization of metabolic hydrogen in the production of end-products other than methane in the rumen. Research to date has provided evidence that dietary supplements of plant or marine oils, oilseeds, specific fatty acids and condensed tannins, as well as defaunation, increases in production level or decreases in the proportion of forage in the diet may lower enteric methane production. Even though dietary lipid supplements can be used to lower methane output, in high amounts a decrease in intake and milk production can be expected. While further investigations have demonstrated the efficacy of specific agents on methanogenesis in vitro, the effects have not been substantiated in vivo. Altering the ratio of H2 /non-H2 producing fibrolytic bacteria to lower methanogenesis without altering fibre digestion has been demonstrated under experimental conditions. Furthermore, non-H2 producing communities have been characterized in the digesta of certain ruminant species. In contrast, stimulating acetogenesis by inoculation with rumen acetogens or non-rumen acetogens have met with limited success in vitro and in vivo. Research has also concentrated on stimulating the ultilisation of metabolic hydrogen by sulphate reducing bacteria, but there remains concern over the toxicity of H2S in the host ruminant. Investigations of nitrate reducing bacteria which produce more NH3 and less toxic nitrite, have indicated promising results. Increasing the number of capnophilic bacteria which use CO2 and H2 to produce organic acids, succinic acid in particular, may decrease methane production. In isolation, several approaches have been shown to decrease enteric methane emissions, but often part of the changes observed are related to lowered organic matter digestion in the rumen. However, lowering methane production per unit product over the lifetime of an animal should be regarded as the central goal to decrease GHG from ruminant livestock systems. This highlights the need for integrated solutions to improve digestive efficiency, as well as fertility and health. In conclusion, any prospective solution to lower on-farm GHG emissions must be practical, cost effective and have no adverse effect on the profitability of ruminant meat and milk production. Recent research has indicated significant potential, but none of the strategies tested thus far satisfy all of the necessary criteria for immediate implementation.