The current knowledge about the biological basis of between-animal variation in methane (CH₄) production is still limited. The aim of this study was to evaluate between-animal variability and repeatability animal factors expected to affect CH₄ emissions using a meta-analytical approach. The data set were collected from 35 studies conducted in Nordic countries involving 126 individual cows and 566 cow/period observations. All studies were conducted either using Latin square or change over designs. Diets comprised principally grass silage and cereal grain concentrates with the mean forage: concentrate ratio of 60:40 on DM basis. CH₄ emissions per mol VFA were calculated based on VFA stoichiometry (Wolin, 1960). The variance components analysis was conducted with MIXED procedure of SAS. Repeatability (R) was calculated as R = δ²_cow / (δ²_cow + δ²_Resid). Of animal variables affecting CH₄ emissions DMI showed the greatest variation and repeatability (Table 1), whereas both OMD and CH₄VFA displayed small variability. Efficiency of microbial N synthesis (ENS) and passage rate of iNDF were more variable and repeatable than CH₄VFA. Increased feed intake was negatively (P < 0.001) associated with reduced OMD and CH₄VFA and positively (P < 0.01) with EMPS (H₂ sink). Reduced CH₄/kg DM with increased DMI can be attributed to these factors. Efficiency of MPS decreased (P < 0.001) with improved OMD that can result from increased digesta retention time in the rumen. It is concluded that between-animal CV in rumen fermentation pattern cannot account observed variation in CH₄. Between-animal differences in passage rate can contribute to variation in CH₄ as it influences digestibility and partitioning of fermented substrate between VFA and microbial cells. Table 1. Between cow variability of variables possibly related to CH₄ production

Item	DMI1	OMD	ENS	iNDF-kp	Acet	Prop	CH4VFA
Mean	19.4	738	23.8	0.023	648	182	356
CV
Total	14.0	4.6	18.1	21.9	8.1	13.1	5.0
Cow	8.7	1.50	6.5	8.4	1.25	2.52	0.96
Cow1		1.39	6.2	8.6	1.25	2.35	0.97
Repeatability	0.71	0.37	0.40	0.39	0.42	0.11	0.10

¹DMI as covariate.