At each of the four chamber periods CH4 production was measured for 2× 48 h in four 17 m3 open-circuit respiration chambers. The cows were housed individually and the chambers were covered with transparent polycarbonate and placed in a square so that pairs of cows faced each other. After the first 48 h the cows changed chambers diagonally in order to balance out any potential differences in background levels of CH4 and carbon dioxide (CO2) and any possible effect of chamber, as only one mean background was recorded.