Understanding methane-reducing tannins in enteric fermentation using grape marc as a model tannin source
Methane from ruminant animals contributes approximately 10% of Australia’s GHG emissions. There is potential to reduce these emissions by supplementing livestock feed with tannins or tannin-containing feed, which have been shown to reduce the production of methane. Grape marc has been suggested as a possible source of tannin able to be used for this purpose. This project applied the AWRI’s existing expertise in tannin chemistry to gain a thorough understanding of the tannin in grape marc, and apply this knowledge to achieve reduced methane emissions and productivity improvements in the livestock industry. Funded by the DA through its ‘Filling the Research Gap’ program, this project was managed by Meat and Livestock Australia as part of the National Livestock Methane Program.
Grape marc fermentation and outcomes
Laboratory-based experiments were conducted to simulate the fermentation that occurs in an animal’s rumen, which converts feed material into energy that can be used by the animal. Different forages supplemented with different rates of grape marc were assessed for their impact on fermentation. High proportions of grape marc (50 and 75%) generally resulted in reductions in the amount of material that fermented (measured through gas and fatty acid production), suggesting they would cause reductions in animal performance (e.g. milk yield, weight gain or wool growth) if used as animal feed. Forages with lower rates of grape marc inclusion (25% or lower) allowed for a greater extent of fermentation in most cases. Hay-based ferments behaved differently, likely because of hay’s low energy and protein content. Supplementation of hay with high rates of grape marc increased energy and protein content and hence improved the amount of material that fermented. For each forage investigated, the optimum type of grape marc to be included was different, although all that proved beneficial possessed high tannin concentrations and lower fat contents. Subsequent experiments highlighted small, extractable tannin molecules with certain structural characteristics as the most potent in reducing methane emissions without inhibiting the overall fermentation performance.
Results from the previously reported experiment supplementing cattle feed with grape marc showed reductions in animal performance (milk yield) that overshadowed any anti-methanogenic impact of grape marc tannin. However, the different feeds used in this experiment had different energy content, so the reduction in milk yield from the feeds containing the lower energy grape marc were not unexpected.
In a more recent experiment using sheep, two distinct grape marc parcels were supplemented at 10, 20 and 30% of the total feed ration in such a way as to achieve diets of equivalent energy contents to that of the control. The results showed that grape marc can be added to a ruminant diet without negative impacts on animal performance (in this case, weight gain). The experimental section of this project was completed and a final scientific milestone report was submitted in May 2015.
The overall findings from this work are that tannin from grape marc has the potential to modulate ruminant digestion and reduce methane emissions, but the feeding scenarios need to be adapted to the low energy content of the grape marc. Maintenance feeding during the summer-autumn feed gap when livestock energy requirements are low is likely to be the most suitable time to incorporate grape marc into animal feed. This is also a time when other feed sources are scarce.