Methane (CH 4 ) emissions by dairy cows vary with feed intake and diet composition. Even when fed on the same diet at the same intake, however, variation between cows in CH 4 emissions can be substantial. The extent of variation in CH 4 emissions among dairy cows on commercial farms is unknown, but developments in methodology now permit quantification of CH 4 emissions by individual cows under commercial conditions. The aim of this research was to assess variation among cows in emissions of eructed CH 4 during milking on commercial dairy farms. Enteric CH 4 emissions from 1964 individual cows across 21 farms were measured for at least 7 days/cow using CH 4 analysers at robotic milking stations. Cows were predominantly of Holstein Friesian breed and remained on the same feeding systems during sampling. Effects of explanatory variables on average CH 4 emissions per individual cow were assessed by fitting a linear mixed model. Significant effects were found for week of lactation, daily milk yield and farm. The effect of milk yield on CH 4 emissions varied among farms. Considerable variation in CH 4 emissions was observed among cows after adjusting for fixed and random effects, with the CV ranging from 22% to 67% within farms. This study confirms that enteric CH 4 emissions vary among cows on commercial farms, suggesting that there is considerable scope for selecting individual cows and management systems with reduced emissions.

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This study investigated the effects of feeding system on diurnal enteric methane (CH 4 ) emissions from individual cows on commercial farms. Data were obtained from 830 cows across 12 farms, and data collated included production records, CH 4 measurements (in the breath of cows using CH 4 analysers at robotic milking stations for at least seven days) and diet composition. Cows received either a partial mixed ration (PMR) or a PMR with grazing. A linear mixed model was used to describe variation in CH 4 emissions per individual cow and assess the effect of feeding system. Methane emissions followed a consistent diurnal pattern across both feeding systems, with emissions lowest between 05:00 and 08:59, and with a peak concentration between 17:00 and 20:59. No overall difference in emissions was found between feeding systems studied; however, differences were found in the diurnal pattern of CH 4 emissions between feeding systems. The response in emissions to increasing dry matter intake was higher for cows fed PMR with grazing. This study showed that repeated spot measurements of CH 4 emissions whilst cows are milked can be used to assess the effects of feeding system and potentially benchmark farms on level of emissions.