Effects of a range of effective inclusion levels of Asparagopsis armata steeped in oil on enteric methane emissions of dairy cows

Asparagopsis armata steeped in edible oil (Asp -Oil) to stabilise its primary antimethanogenic compound, bromoform, has emerged as a potent enteric methane mitigant for ruminant livestock. The aim of this 41 -day study was to evaluate the response of feeding a range of effective inclusion levels of a canola oil based Asp -Oil providing increasing dietary concentrations of bromoform to lactating dairy cows in a supplemental feed concentrate twice daily at milking on i) enteric methane emissions, milk yield (MY) and dry matter intake (DMI), and ii) bromoform, dibromomethane or bromine concentrations in milk, urine, faeces, blood and breath. Thirty lactating, multiparous, Holstein -Friesian (42 +/- 12.9 days in milk) were randomly allocated to one of five treatments; ASP0 (0 mg bromoform/d), ASP1 (132 mg bromoform/d), ASP2 (267 mg bromoform/d), ASP3 (409 mg bromoform/d) and ASP4 (467 mg bromoform/d). The Asp -Oils were mixed with the concentrate supplement and offered twice daily during milking and all cows received equal dietary canola oil. The basal diet consisted of vetch hay ad libitum and intakes were measured daily via electronic monitoring of individual cows at the feed bins. Methane emissions were measured using the modified sulphur hexafluoride (SF6) tracer technique on days 36-41. Feeding Asp -Oil showed no effect on total DMI but concentrate DMI and MY decreased linearly with increased concentration of bromoform in the Asp -Oil. There was a linear decrease in methane production (MeP; g CH4/d), methane yield (MeY; g CH4/kg DMI) and methane intensity (MeI; g CH4/kg energy corrected milk) with increased bromoform concentration in the Asp -Oil. Milk bromoform was detected in each of the Asp -Oil treatment groups, however accounted for less than 0.07% of total bromoform intake and is more than 14 times less than the acceptable daily intake limits for human consumption. It is concluded that feeding lactating dairy cows a concentrate including Asp -Oil with increasing concentrations of bromoform showed a linear response on methane emissions with reductions of up to 38% in MeP.