Variations in methane emissions associated with rumen-accessible fatty acids, dietary characteristics, and ruminal...

This meta-analysis aimed to quantitatively summarize variations in methane (CH₄) emissions in relation to dietary rumen-accessible fatty acids (RAFA), dietary composition, animal characteristics, and ruminal fermentation in lactating dairy cows. A database was collated from 48 peer-reviewed publications containing 180 treatment means, which reported CH₄ emissions in lactating dairy cows fed additional lipids. Dietary RAFA included C12:0, C14:0, C16:0, C16:1, C18:0, cis-C18:1, trans-C18:1, C18:2, and C18:3. Mixed-effects regression models were developed, where study was included as random effect in the model, and regressions were weighted by the inverse of the standard error of the observed mean. Models were compared using the concordance correlation coefficient, root estimated variance related to study σ^_s and error σ^_e, corrected Akaike information criterion values, and root mean square error observations standardized ratio. The daily CH₄ production (g), yield (g/kg DMI), and intensity (g/kg ECM) were linearly reduced by 26, 0.92, and 0.90 per 10 g/kg increase of total dietary RAFA. Meanwhile, the mitigating effect of some fatty acids, including C14:0, C16:0, C16:1, C18:0, C18:1, and C18:2 on CH₄ production and yield diminished at higher inclusion levels, indicating quadratic dose-response. In contrast, other FA such as C12:0 and C18:3 showed a linear reduction effect across the dose range in the datasets. Models with individual RAFA showed that CH₄ emissions were primarily affected by C12:0, C14:0, C16:0, cis-C18:1, C18:2, and C18:3. Dietary starch and rumen available ether extract were negatively associated with both CH₄ production (g/d) and CH₄ yield (g/kg DMI). The DMI and NDF digestibility were among animal variables that could best explain variation in CH₄ emissions. Rumen variables such as total VFA concentration and molar proportions of acetate and butyrate were positively related to CH₄ production, whereas molar proportions of propionate were negatively correlated with CH₄ yield and intensity (g/kg ECM). Model comparisons revealed that RAFA alone did not fully explain the observed variation in CH₄ emissions. The models for CH₄ production revealed a greater explanatory power for models including animal variables such as DMI and NDF digestibility, which may suggest a greater indirect effect of fat supplementation on CH₄ production. Models for CH₄ yield and intensity showed an improved explanatory power when ruminal variables were included, indicating the importance of considering these factors in future models for predicting CH₄ emissions. Furthermore, interaction effects were observed, with CH₄ emissions being more strongly inhibited by medium-chain fatty acids (C12:0, C14:0, and C16:0) in diets with starch level >20% DM or NDF level ≤35% DM. Our findings highlight the importance of considering the complex interactions among dietary FA, dietary composition (e.g., NDF and starch content), and rumen fermentation dynamics to develop effective strategies using lipid to mitigate CH₄ emission in dairy cows.