Asparagopsis oil products are of interest due to the stabilizing effects of the Asparagopsis-derived antimethanogenic bioactive compound bromoform (CHBr 3 ). The objective of this in vitro series is to characterize antimethanogenic efficacy of freeze-dried Asparagopsis (FD-Asp) and Asparagopsis oil (Asp-Oil) and compare relative antimethanogenic response over time at multiple levels of CHBr 3 delivery. Relative methane (CH 4 ) emissions (mL/g) are based on in vitro apparent feed digested dry matter (IVDDM) after 24, 48, and 72 h of fermentation. CHBr 3 contained in FD-Asp was included at 95, 191, and 286 mg/kg, and CHBr 3 contained in Asp-Oil was included at 78, 117, and 175 mg/kg, to produce the Low, Mid, and High inclusions, respectively. Low FD-Asp had no significant impact on CH 4 emissions, Mid FD-Asp demonstrated 91%, 44%, and 37% reductions, and the High FD-Asp demonstrated complete inhibition of CH 4 , after 24, 48, and 72 h of fermentation, respectively. Comparatively, Low Asp-Oil demonstrated a 46%, 28%, and 18% CH 4 reduction, Mid Asp-Oil resulted in 99%, 92%, and 73% reductions, and the High Asp-Oil demonstrated complete inhibition of CH 4 after 24, 48, and 72 h of fermentation, respectively. IVDDM and total volatile fatty acid (tVFA) production were not changed by the inclusion of FD-Asp and Asp-Oil. The results from this study show that Asparagopsis is not only a compelling CH 4 mitigating feed supplement but is also able to be delivered in edible oil forms which How to cite this paper:

The idea of delivering bromoform from Asparagopsis using edible oil has gained momentum recently due to the improved processing time and that it is already a feed that many livestock producers use. The stability of bromoform in oil compared to freeze-dried product is still not well understood. To fill this gap, a systematic study was carried out to determine the effects of storage temperatures (40 °C, 25 °C, 4 °C and -20 °C), fluorescent light and exposure to open air, on the retention of bromoform in freeze-dried Asparagopsis (FD-Asp) and Asparagopsis oil (Asp-Oil) over 24-week period. In the absence of fluorescent light, Asp-Oil was a more effective way to preserve bromoform compared to FD-Asp due to either no change or higher Asp-Oil bromoform content (storage temperature dependent) after 24-week storage. Under the same conditions, FD-Asp bromoform content decreased by 74% at 40 °C, 53% at 25 °C, 6% at 4 °C, and no change of FD-Asp bromoform content at -20 °C. The presence of fluorescent light negatively affected Asp-Oil bromoform content at both 25 °C and 40 °C while the effect was insignificant on FD-Asp. The exposure of Asp-Oil to open air resulted in the decrease of bromoform content to below quantification limit (0.18 mg g−1) on week 8 for 40 °C sample and on week 16 for 25 °C sample. This study provides empirical evidence on the stabilising effect of oil in preserving bromoform extracted from Asparagopsis, confirming it is a more attractive medium to deliver bromoform compared to the freeze-dried powder form.