Feeding plants containing elevated levels of polyphenols may reduce ruminal CH 4 emissions, but at the expense of nutrient utilisation. There might, however, be non-additive effects when combining high-phenolic plants with well-digestible, high-nutrient feeds. To test whether non-additive effects exist, the leaves of Carica papaya (high in dietary quality, low in polyphenols), Clidemia hirta (high in hydrolysable tannins), Swietenia mahagoni (high in condensed tannins) and Eugenia aquea (high in non-tannin phenolics) were tested alone and in all possible mixtures (n 15 treatments). An amount of 200 mg DM of samples was incubated in vitro (24 h; 39 o C) with buffered rumen fluid using the Hohenheim gas test apparatus. After the incubation, total gas production, CH 4 concentration and fermentation profiles were determined. The levels of absolute CH 4 , and CH 4 :SCFA and CH 4 :total gas ratios were lower (P, 0·05) when incubating a combination of C. papaya and any high-phenolic plants (C. hirta, S. mahagoni and E. aquea) than when incubating C. papaya alone. Additionally, mixtures resulted in non-additive effects for all CH 4 -related parameters of the order of 2 -15 % deviation from the expected value (P,0·01). This means that, by combining these plants, CH 4 in relation to the fermentative capacity was lower than that predicted when assuming the linearity of the effects. Similar non-additive effects of combining C. papaya with the other plants were found for NH 3 concentrations but not for SCFA concentrations. In conclusion, using mixtures of high-quality plants and high-phenolic plants could be one approach to CH 4 mitigation; however, this awaits in vivo confirmation.Key words: Ruminants: Methanogenesis: Phenolic compounds: Forage Various investigations are currently under way to identify and test means for mitigating CH 4 emissions that originate from ruminants due to the activity of methanogenic archaea during feed fermentation (1) . As a product of fermentative digestion, CH 4 emission levels depend considerably on the quantity and composition of feeds consumed (2) . The potential of mitigating CH 4 emissions by the extracts of phenolic compounds, which are synthesised in the intermediary metabolism of plants, has been demonstrated experimentally (3 -5) . Also, the direct inclusion of plants containing phenolics in ruminant diets reduced CH 4 emissions compared with control diets, both in vitro (6) and in vivo (7,8) .A major drawback in implementing diets with doses of phenolics aimed at reducing CH 4 emissions is often a decline in the digestibility of the feed and therewith the productivity of the animals (9) , even at dosages where toxic side effects are excluded. As a consequence, there are often no or only small declines in CH 4 per unit of digested feed and, therefore, food produced. Analysing a larger dataset by principal components analysis illustrated that plants with high forage quality are arranged opposite to those with a high CH 4 -mitigating potential (10) . This indicates that achiev...
