A meta-analysis of 3-nitrooxypropanol effects on methane production and yield in beef cattle

Beef cattle are a major source of enteric methane (CH 4 ) emissions, a potent greenhouse gas. The feed additive 3-nitrooxypropanol ( 3-NOP ) has been shown to reduce CH 4 emissions by inhibiting methyl-coenzyme M reductase, an enzyme critical to methanogenesis in archaea. This study aimed to quantify the effects of 3-NOP on CH 4 production (g/d) and yield (g/kg DM intake; DMI ) in beef cattle and to evaluate how diet composition influences the mitigation response. A systematic literature review identified 17 eligible in vivo studies (16 peer-reviewed and 1 preprint), yielding 45 treatment means. Treatment effects were expressed as absolute and relative mean differences versus control groups. Predictor variables included 3-NOP dose, 3-NOP dose 2 , DMI, dietary concentration of NDF, CP, starch, fat, and OM, roughage proportion, BW, and dietary inclusion of monensin (yes/no). Four types of models were developed, all including the intercept and 3-NOP dose as fixed predictors, differing as follows: (model 1) optional inclusion of 3-NOP dose 2 when P < 0.10; (model 2) model 1 plus pre-inclusion of NDF concentration; (model 3) pre-inclusion of NDF concentration plus additional predictors (pairwise r ≤ 0.5) that significantly improved model accuracy ( P < 0.10); and (model 4) additional predictors selected under the same criteria as model 3, without pre-inclusion of NDF concentration. For models 3 and 4, a maximum of 5 predictors were considered and evaluated using leave-one-out cross-validation. Across studies, 3-NOP doses ranged from 32 to 338 mg/kg of DM. On average, 3-NOP reduced CH 4 production by 49.9 ± 28.61 g/d (36.2 ± 24.43) and CH 4 yield by 5.3 ± 3.61 g/kg DMI (33.2 ± 25.54%). The best models were selected based on biological interpretability, statistical significance, and predictive accuracy (as measured by RMSE) and included 3-NOP dose, dietary NDF concentration, fat, DMI, and BW as significant predictors (fat only for relative mean difference in CH 4 yield and DMI and BW only for absolute CH 4 production). Mitigation efficacy increased with higher DMI and declined with increasing NDF concentration and BW. Absolute reductions of 53.4 g/d and 5.96 g/kg of DMI, and relative reductions of 37.9% in CH 4 production and 35.5% in CH 4 yield were predicted when moderators were at their mean value (3-NOP dose = 134.4 mg/kg of DM; NDF concentration = 32.8% of DM; DMI of 8.6 kg/d; BW = 429 kg; fat concentration = 3.8% of DM). These results support the effectiveness of 3-NOP in mitigating enteric CH 4 emission in beef cattle and provide quantitative models to be used in assessment tools and greenhouse gas inventory methodology.