The antimethanogenic efficacy and fate of bromoform and its transformation products in rumen fluid.

Enteric methane emissions from ruminant livestock are a significant source of atmospheric methane. Efforts to address rising atmospheric methane concentrations have led to an expansion of research into mitigating enteric methane production. One of the most effective approaches utilizes bromoform-containing feed supplements, such as the algae Asparagopsis spp., to inhibit methanogenesis in the rumen. Understanding the fate and persistence of bromoform in the rumen is important for developing safe, effective products and feeding strategies. This study conducted a series of in vitro rumen fluid experiments monitoring bromoform, dibromomethane, and bromomethane concentrations, methane production and several biochemical parameters to understand the inhibitory thresholds and degradation processes of these compounds. Analysis of the rumen fluid confirmed bromoform is rapidly dehalogenated. The half-life of bromoform was 26 min, coinciding with the production of dibromomethane accumulating to 22.1% of the initial bromoform amendment, but no bromomethane was detected. Dibromomethane demonstrated a considerably longer half-life of 775 min. In separate dose-response experiments, bromoform, dibromomethane and bromomethane all exhibited anti-methanogenic activity. Bromoform and dibromomethane produced sigmoidal-relationships between concentration and inhibition at approximately 1-2 µM, and yielded similar effective concentration values (EC50s) for antimethanogenic activity. Experiments using Asparagopsis taxiformis algae revealed less accumulation of bromoform and formation of dibromomethane, likely driven by a slower release from the seaweed material. The A. taxiformis dose response was less effective at inhibiting methane per mole of bromoform added compared with direct bromoform additions. These results have significant implications for understanding the dynamics of bromoform-mediated methane inhibition and will aid the development of effective halocarbon additives, feeding strategies, and testing protocols for bromoform and its degradation byproducts.