Enteric methane emission by ruminants fed forages is highly variable and
depends on forage composition, intake and digestibility. In addition, plant secondary
compounds such as tannins have antimethanogenic properties; however, the mitigating
effect of tannins on CH4 is inconsistent. A meta-analysis approach was used to compare
the effects of different forages, supplemented or not with tannins, fed to ruminants on
CH4 emission. Tannin content (TAN, condensed or hydrolysable), averaged 35.6 (sd
=53.0) g/kg DM and varied from 0 to 199 g/kg DM, for 19 experiments and 53
treatments. Methane production, expressed per kg of digestible OMI (g/kg DOMI)
significantly decreased when feeding level (FL, calculated as DM intake % liveweight)
increased and when NDF content of the forage decreased. Moreover, tannins content
(g/kg DM) decreasing effect on methane emission was significant. The effect of tannins
for mitigating CH4 emission is in agreement with previous studies, but in the present
study the impact of tannins was lower, probably due to accounting for decreasing effect
of FL and increasing effect of NDF effects in the equation and to small relations between
these factors.