A mechanistic model to represent the mode of action of 3-nitrooxypropanol in the rumen of dairy cattle: Impact on methanogenesis and rumen fermentation.

The feed additive 3-nitrooxypropanol (3-NOP) has been proven effective in reducing enteric methane (CH4) emissions. Its effectiveness depends on dosage level and characteristics of the basal diet, but the biological causes driving this variability are not completely understood. Our aim was to develop a mechanistic model to represent the mode of action of 3-NOP in the rumen considering the kinetics of methyl-coenzyme M reductase (MCR) inhibition, alternative H2 utilization pathways (i.e., reductive acetogenesis and ethanol formation), and methanogen population responses to 3-NOP, to enable accurate predictions of CH4 mitigation efficacy while providing mechanistic insight into the factors governing 3-NOP effectiveness. The model was built as an extension of a previous mechanistic model that simulates the production of VFA, hydrogen (H2), and CH4 in the rumen in response to dietary changes. The extension introduces variables for 3-NOP, the enzyme MCR which is inhibited by 3-NOP and the regulation of methanogenesis, and use of alternative H2 sinks, i.e., ethanol production and acetogenesis. A database was created including 54 treatments from 22 literature studies that tested the addition of 3-NOP in dairy cows. The model predicted the inhibition of CH4 production due to 3-NOP with RMSE of 7.3%-units relative to the control diet, and the associated increases in emission of H2 with RMSE of 0.81% (corresponding to the log-transformed increase of H2) and shifts in rumen VFA molar proportions (decreased acetate with 2.5% RMSE, increased propionate with 3.4% RMSE and butyrate 6.7% RMSE) (all relative to the control diet). Key to these predictions were: simulated saturation kinetics of MCR inactivation by 3-NOP, adaptive methanogen responses after addition of 3-NOP, including increased growth efficiency and reduced methanogen passage rate, and alternative H2 sinks through ethanol production and reductive acetogenesis, which proved to be essential to avoid severe overestimation of H2 emissions. By representing these multiple mechanisms together, the model successfully simulated variation in 3-NOP effectiveness at the same dose level. This variation was partially associated with DMI, with simulated inhibition of CH4 production ranging from -19% at low DMI to -40% at high DMI on the same diet and 3-NOP dose (80 mg/kg DM), and by diet composition, with 3-NOP achieving greater CH4 inhibition on high concentrate diets (-42%) compared with roughage-only diets (-26%) at the same DMI level and 3-NOP dose (80 mg/kg DM). The model contributes to quantitative understanding of 3-NOP mode of action in the rumen including the role of alternative H2 pathways in regulating fermentation responses, and provides mechanistic insight into variation in its efficacy across feeding conditions.