Benefits of the red seaweed Asparagopsis taxiformis as an ingredient to manage methane (CH 4 ) emissions from the red meat and dairy industries continue to evolve. Asparagopsis has been demonstrated to eliminate enteric CH 4 emissions in vitro and reduce it greater than 80% in animals. Variability in animal studies is suspected to be associated with variable inclusion and proportions of grass and grain in the diet. This in vitro study aimed to elucidate effects of gradient grass to grain proportions in the fermentation using five steps from 100% Rhodes grass (RG) to 100% barley grain (BG). Gradient inclusion of Asparagopsis was in six steps of Control with no inclusion (C), Low (L), Low-Medium (LM), Medium (M), Medium-High (MH), and High (H) levels tested in three fermentation durations (24 h, 48 h, 72 h). There was significant effect of RG/BG and inclusion of Asparagopsis such that CH 4 production decreased with increasing Asparagopsis independent of RG/BG; however, there was enhanced reduction at greater proportions of BG. Thus, the level of Asparagopsis required to completely inhibit CH 4 production in vitro was decreased with decreasing RG/BG. Increasing the duration of fermentation had greatest effect on CH 4 at C, L, and LM levels of Asparagopsis independent of RG/BG, although magnitude of CH 4 production was greater for higher proportions of BG for the C and L levels. Digestibility of in vitro substrate increased with fermentation duration and increasing BG; however, there was no change associated with inclusion levels of Asparagopsis.

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: