Enteric fermentation represents the largest single source of anthropogenic methane (CH4) emission in the United States, accounting for 30% and 27% of the total CH4 emitted in California (CARB, 2020) and nationwide (US EPA, 2019), respectively. Due to the significant impact of CH4 on climate change and the negative correlation of animal productivity and enteric CH4 production, there is great interest in identifying feed additives that might mitigate CH4 synthesis in the rumen ecosystem. The first aim of this dissertation is to support enteric CH4 mitigation efforts by identifying novel feed additives that have significant potential to reduce enteric CH4 production from ruminant livestock. Red pigmented macroalgae, specifically members from the genus Asparagopsis, have previously been identified as feed additive that possess high potential of anti-methanogenic properties, however, has not yet been tested widely. The second aim of this dissertation was to test the efficacy of Asparagopsis taxiformis (A. taxiformis), a red macroalgae that has shown to reduce enteric CH4 when used in an Australian production environment, using an in vitro rumen simulation technique (RUSITEC) system to quantify the effects on enteric CH4 production and volatile fatty acid (VFA) production under feed regimes specific to California. When supplemented at a 5% organic matter (OM) dosage in vitro, A. taxiformis supplementation of CA specific cattle feed resulted in a 95% reduction in CH4 coupled with an increase in propionate to acetate ratio (Roque et al., 2019a). The third aim focused on the effects of the red macroalgae Asparagopsis armata (A. armata) on enteric CH4 synthesis and animal production parameters when fed to lactating dairy cows. A Latin square design was used with 3 treatment groups with 4 cows within each treatment and three 2-week treatment phases with 1-week washout periods between each treatment phase. Two doses of macroalgae were used (0.5% OM and 1.0% OM) in addition to the control group (0% OM) . Results from this study showed 26% and 67% decrease in CH4 production for 0.5% OM and 1.0% OM treatment groups, respectively. However, dry matter intake (DMI) was reduced by 10 kg/d between the control and 1.0% OM treatments, ultimately representing a reduction in milk yield of approximately 4 kg/d. Due to sorting behaviors observed in the dairy cows, we hypothesized that there was a taste aversion to high levels of macroalgae in the diet (Roque et al., 2019b). The fourth aim tested A. taxiformis macroalgae, similar to what was used in the in vitro study, fed to growing beef steers for 147 days. A randomized block design was used with 3 treatment groups (7 steers in each treatment). All steers were blocked by weight then randomly assigned to one of three treatment groups; control, 0.25% OM, and 0.5% OM. The objectives of this study were to determine the long-term (147 days) efficacy of macroalgae on reducing enteric CH4 and to determine if there were any effects on animal production. Results from this study indicate that A. taxiformis’ ability to reduce enteric CH4 persisted was more effective at reducing enteric CH4 compared to the (Roque et al., 2019b) dairy study and these effects persisted throughout the duration of the 147-day study. Additionally, reductions in DMI were also found in this study however, no changes in average daily weight gain (Roque et al., 2021). To further investigate macroalgae as a methane reducing feed additive, rumen fluid was collected from both in vivo studies and will be used to discover if there are changes happening to the rumen microbiome diversity and activity using metagenomic and metatranscriptomic approaches. The proposed objectives for future research are to observe the role that different rumen microbes play in methanogenesis as well as enteric fermentation in the rumen environment. Additionally, we will compare gene expression profiles of the rumen microbiome to determine which molecular processes are affected in the presence and absence of macroalgae.
