| Summary: | The aim of this study was to investigate variability in enteric CH<sub>4</sub> emission rate and emissions per unit of milk across lactations among dairy cows on commercial farms in the UK. A total of 105,701 CH<sub>4</sub> spot measurements were obtained from 2206 mostly Holstein-Friesian cows on 18 dairy farms using robotic milking stations. Eleven farms fed a partial mixed ration (PMR) and 7 farms fed a PMR with grazing. Methane concentrations (ppm) were measured using an infrared CH<sub>4</sub> analyser at 1s intervals in breath samples taken during milking. Signal processing was used to detect CH<sub>4</sub> eructation peaks, with maximum peak amplitude being used to derive CH<sub>4</sub> emission rate (g/min) during each milking. A multiple-experiment meta-analysis model was used to assess effects of farm, week of lactation, parity, diet, and dry matter intake (DMI) on average CH<sub>4</sub> emissions (expressed in g/min and g/kg milk) per individual cow. Estimated mean enteric CH<sub>4</sub> emissions across the 18 farms was 0.38 (s.e. 0.01) g/min, ranging from 0.2 to 0.6 g/min, and 25.6 (s.e. 0.5) g/kg milk, ranging from 15 to 42 g/kg milk. Estimated dry matter intake was positively correlated with emission rate, which was higher in grazing cows, and negatively correlated with emissions per kg milk and was most significant in PMR-fed cows. Mean CH<sub>4</sub> emission rate increased over the first 9 weeks of lactation and then was steady until week 70. Older cows were associated with lower emissions per minute and per kg milk. Rank correlation for CH<sub>4</sub> emissions among weeks of lactation was generally high. We conclude that CH<sub>4</sub> emissions appear to change across and within lactations, but ranking of a herd remains consistent, which is useful for obtaining CH<sub>4</sub> spot measurements.
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