The relationship between methane emission and daytime-dependent fecal archaeol concentration in lactating dairy cows...

Archaeol is a cell membrane lipid of methanogenic archaea excreted in feces and is therefore a potential biomarker for individual methane emission (MEM). The aims of this study were to examine the potential of the fecal archaeol concentration (fArch) to be a proxy for MEM prediction in cows fed different diets and determine if the time of fecal collection affected the archaeol concentration. Thus, we investigated (i) the variation of the fArch concentration in spot samples of feces taken thrice within 8 h during respiration chamber measurements and (ii) the effect of two diets differing in nutrient composition and net energy content on the relationship between fArch and MEM in lactating cows. Two consecutive respiration trials with four primiparous and six multiparous lactating Holstein cows were performed. In the first trial (T1) at $$  d in milk (IM), a diet moderate in starch and fat content was fed for ad libitum intake, whereas in the second trial (T2) at $$  d IM, cows received a diet lower in starch and fat. Individual MEM (g d $$ ) was measured for 24 h. Fecal samples were taken at 06:30, 10:00, and 14:30 LT and analyzed for fArch using Soxhlet lipid extraction and GC-MS. Cows produced less methane (364 g  $$  d $$ ) during T1 and had significantly lower fArch concentrations (37.1  $$ g g $$ dry matter; DM) compared to T2 (392 g  $$  d $$ and 47.6  $$ g g $$  DM). A significant positive relationship between fArch ( $$ g g $$ fecal DM) and MEM, expressed on a dry matter intake (DMI) basis (g kg $$  DMI), was found ( $$ , $$ ). Among samples collected over the day, those collected at 10:00 LT provided the best coefficient of determination for MEM ( $$ ). In conclusion, fArch offers some potential in serving as a proxy for innovative breeding schemes to lower enteric methane when fecal samples are taken at a certain time of the day, but more data on the sources of variation of the MEM  $$  fArch ratios are required.