Conclusions
Methane emissions by livestock are very significant contributors to anthropogenic emissions of GHGs in many countries and the onus is increasingly on the farming industry to find ways of reducing these. A number of strategies are currently being investigated and developed. One obstacle that hinders prospects for discovering efficient tools to mitigate methane emissions is our incomplete knowledge of the ecology and nature of the enigmatic Archaea that produce the methane. The identification of the full spectrum and diversity of methane-producing Archaea in the rumen is a sine qua non for the future success of some mitigation strategies, notably vaccination and enzyme inhibition. Nevertheless, the complete sequencing of the genome of methanogens is yielding a tremendous amount of important information which will indicate suitable targets in the various strategies.
All strategies have different profiles of feasibility, cost and possible uptake by end-users. Approaches are at different stages of development, and the fact that many options are being considered enhances the prospects that one avenue will eventually lead to the creation of a successful methane mitigation tool. Strategies must however suit particular classes of livestock. For example, improved forages and feed supplementation might be appropriate for intensively managed dairy cows, although this would be need to be balanced by the production of GHGs associated with the cultivation or manufacture of other feeds and the overall cost. Fewer options exist for sheep and beef cattle that are more extensively grazed. In countries such as New Zealand some approaches (e.g., vaccination) are likely to be more feasible than others; feed supplementation is unlikely to form a user-friendly approach, although production of selected pastures may prove to be beneficial. The breeding of animals for low methane emissions is worth exploring, although the possibility that negative traits will be selected in the process will have to be seriously examined.