7 August 2012

2 minute read

Methane emission rates during milking vary considerably for individual dairy cows under commercial conditions. And although much of this variation is probably due to feed intake, consistent differences are predicted between cows at equal live weights and milk yields when fed on the same diet.

Just two of many interesting findings from a study, carried out by scientists at the University of Nottingham, to examine the sources of variation in methane emissions by individual cows and to compare breath measurements with daily methane output measured in respiration chambers.

Total methane emissions at farm or national level are the product of both the number of cows and emissions per cow. “It is difficult to measure emissions by individual cows on farms using conventional methods, so we developed a technique based on continuous breath analysis during milking,” explained Phil Garnsworthy, who led the trial.

Methane concentrations in breath were measured for 219 Holstein cows, with an average annual milk yield of 10,000 litres, from 66,734 individual robotic milkings during a five-month period at the Nottingham University Dairy Centre.

Breath was sampled by a methane analyser via a PVC pipe with an inlet near the cow’s nose. Methane emission rate during milking (MERm), in milligrammes per minute, was calculated for each milking as mean peak integral multiplied by peak frequency. Daily methane output was recorded for between three and seven days and regressed against mean MERm for the 10 days preceding transfer.

“And we found that there was considerable variation in MERm among cows. Between-cow variation was 32 times greater than within-cow variation,” said Professor Garnsworthy. “Much of the between-cow variation could be explained by live weight and milk yield. But even after adjusting for these factors, a significant amount of between-cow variation was still unexplained.”

Changes in live weight and milk yield explained significant proportions of the within-cow variation. It is likely that live weight and milk yield act as proxies for dry matter intake, which is the main driver of methane emissions.

“Comparison of breath measurements with respiration-chamber measurements confirms that MERm is a reliable indicator of total daily methane emissions.

“Variability among individuals has implications for methane inventories and monitoring mitigation strategies, and also raises the possibility of selecting cows for low emissions,” added Professor Garnsworthy.

Full details: Garnsworthy PC, Craigon J, Hernandez-Medrano JH and Saunders N: “Variation in methane emissions measured during milking for individual dairy cows under commercial conditions.”

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