Indications for a lower methane yield from digested fibre in ruminants digesting fibre more efficiently

It is assumed that the absolute amount of methane (CH₄) produced on a given diet increases proportionately (i.e., in a linear manner) with the amount of digested fibre. Therefore, the CH₄ yield per unit of digested fibre is considered constant for a given diet. This conceptually matches findings of lower digestibility in low-CH₄ emitting animals, and of lower CH₄ yield at higher intake levels due to shorter digesta passage and hence reduced digestibility. Irrespective of these observations, this general assumption was challenged by findings in one study where CH₄ yield per unit of digested fibre had unexpectedly declined in individuals digesting the fibre provided by the same diet more efficiently. To investigate this finding in more detail, we collated a dataset from 16 studies with cattle and sheep with a total of 61 forage-based diet groups consisting of at least five animals each (472 animals in total). We assessed whether there was a linear relationship between the daily CH₄ emission and the amount of digested fibre, both within the same and across the different diet groups. Across diets, CH₄ emissions did not increase linearly with the amount of digested neutral or acid detergent fibre in either species. Within diet groups, the majority of cases also showed evidence for less-than-linear increase of CH₄ emissions with increasing amount of digested neutral or acid detergent fibre, even though the 95% confidence intervals could not rule out a linear relationship in many cases. Reasons why this phenomenon was not described earlier may include that the great individual variation associated with an accumulation of errors in the variables concerned often prevented statistical significance in individual studies. Although the findings across diets concerning the variation in CH₄ yield per unit of digested fibre do not exclude some diet-specific effects, the within-diet assessment clearly points towards individual animal effects in microbial fibre digestion in a way that CH₄ production is proportionately lower when fibre is digested more efficiently. Mechanistically, animals with a more efficient fibre digestion might produce volatile fatty acids at a higher rate and have a locally lower ruminal pH, favouring microbiota of propionate-producing pathways. The presence of animal-individual differences in CH₄ yield per unit of digested fibre with varying efficiency of fibre fermentation should be confirmed in a specific experiment where also the reasons for such a