Data collected during the measurement period only were used for statistical analyses. Data on gaseous emissions from the last 72 h of each measurement period were averaged per period before analysis. Data measured on individual animals (milk production, milk composition, blood parameters, and DMI) were subjected to repeated-measures ANOVA including block and treatment as fixed factors. Data collected during the 4 experimental periods were treated as repeated measures per animal or pair of animals. Data collected on pairs of cows (gaseous exchange and diet digestibility parameters) were subjected to repeated-measures ANOVA with treatment as fixed factor. Significance of effects was declared at P < 0.05. The statistical program Genstat (11th ed., Lawes Agricultural Trust, Rothamsted, UK) was used to analyze the data.