Methane emissions among individual dairy cows during milking quantified by eructation peaks or ratio with carbon dioxide.

The aims of this study were to compare methods for examining measurements of CH₄ and CO₂ emissions of dairy cows during milking and to assess repeatability and variation of CH₄ emissions among individual dairy cows. Measurements of CH₄ and CO₂ emissions from 36 cows were collected in 3 consecutive feeding periods. In the first period, cows were fed a commercial partial mixed ration (PMR) containing 69% forage. In the second and third periods, the same 36 cows were fed a high-forage PMR ration containing 75% forage, with either a high grass silage or high maize silage content. Emissions of CH₄ during each milking were examined using 2 methods. First, peaks in CH₄ concentration due to eructations during milking were quantified. Second, ratios of CH₄ and CO₂ average concentrations during milking were calculated. A linear mixed model was used to assess differences between PMR. Variation in CH₄ emissions was observed among cows after adjusting for effects of lactation number, week of lactation, diet, individual cow, and feeding period, with coefficients of variation estimated from variance components ranging from 11 to 14% across diets and methods of quantifying emissions. No significant difference was detected between the 3 PMR in CH₄ emissions estimated by either method. Emissions of CH₄ calculated from eructation peaks or as CH₄ to CO₂ ratio were positively associated with forage dry matter intake. Ranking of cows according to CH₄ emissions on different diets was correlated for both methods, although rank correlations and repeatability were greater for CH₄ concentration from eructation peaks than for CH₄-to-CO₂ ratio. We conclude that quantifying enteric CH₄ emissions either using eructation peaks in concentration or as CH₄-to-CO₂ ratio can provide highly repeatable phenotypes for ranking cows on CH₄ output.