The relationship between methane emission and daytime-dependent fecal archaeol concentration in lactating dairy cows fed two different diets

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&thinsp;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 consecutiverespiration trials with four primiparous and six multiparous lactatingHolstein cows were performed. In the first trial (T1) at <span class="inline-formula">100±3</span>&thinsp;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 <span class="inline-formula">135±3</span>&thinsp;d IM, cowsreceived a diet lower in starch and fat. Individual MEM (g&thinsp;d<span class="inline-formula">⁻¹</span>) was measuredfor 24&thinsp;h. Fecal samples were taken at 06:30, 10:00, and 14:30&thinsp;LT and analyzed for fArch using Soxhlet lipid extraction and GC–MS. Cows produced less methane (364&thinsp;g&thinsp;<span class="inline-formula">CH₄</span>&thinsp;d<span class="inline-formula">⁻¹</span>) during T1 and had significantly lower fArch concentrations (37.1&thinsp;<span class="inline-formula">µ</span>g&thinsp;g<span class="inline-formula">⁻¹</span> dry matter; DM) compared to T2 (392&thinsp;g&thinsp;<span class="inline-formula">CH₄</span>&thinsp;d<span class="inline-formula">⁻¹</span> and 47.6&thinsp;<span class="inline-formula">µ</span>g&thinsp;g<span class="inline-formula">⁻¹</span> DM). A significant positive relationshipbetween fArch (<span class="inline-formula">µ</span>g&thinsp;g<span class="inline-formula">⁻¹</span> fecal DM) and MEM, expressed on a dry matter intake (DMI) basis (g&thinsp;kg<span class="inline-formula">⁻¹</span> DMI), was found (<span class="inline-formula">R²=0.53</span>, <span class="inline-formula">n=20</span>). Among samples collected over the day, those collected at 10:00&thinsp;LT provided the best coefficient of determination for MEM (<span class="inline-formula">R²=0.23</span>). 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&thinsp;<span class="inline-formula">:</span>&thinsp;fArch ratios are required.