Milk fatty acid profile and BHB as predictors of greenhouse gas emissions in dairy cows affected by ketosis

BACKGROUND AND OBJECTIVES. Metabolic disorders such as ketosis are common in high-producing dairy cows and are associated with alterations in lipid metabolism, milk fatty acid (FA) composition, and changes in greenhouse gas (GHG) emissions. Understanding these relationships can provide insight into metabolic status and environmental impact. This study aimed to evaluate the effects of subclinical ketosis (SCK) on milk FA profiles and to explore the predictive capacity of milk FA and blood β-hydroxybutyrate (BHB) for emissions of methane (CH₄) and carbon dioxide (CO₂). MATERIALS AND METHODS. A total 60 multiparous Holstein-Friesian cows were enrolled between 3 and 28 days in milk. Based on blood BHB concentrations determined at 7, 14, 21, and 28 DIM. Animals were classified as affected by SCK (KET; n = 17; BHB was ≥ 1.0 mmol/L) whereas cows with BHB < 1.0 mmol/L at all time points were considered healthy controls (CTR; n = 43). The CH₄ and CO₂ emissions were quantified individually through an automated in-barn system during voluntary feed intake. Milk FA were determined by gas chromatography following direct transesterification of milk fat. Statistical analysis was performed using linear mixed-effects models. Furthermore, stepwise forward regression was applied to assess the predictive ability of milk FA and BHB for CH₄ and CO₂ emissions, with variables entering the model at p < 0.10. RESULTS. Ketotic cows showed a clear shift in milk FA composition: preformed FA such as stearic acid (C18:0) and oleic acid (C18:1ω9) were increased, reflecting enhanced lipomobilization, whereas de novo FA (butyric to myristic acid; C4:0–C14:0) were reduced, consistent with impaired mammary synthesis. Odd- and branched-chain FA, including pentadecanoic acid (C15:0), pentadecenoic acid (C15:1ω9), heptadecanoic acid (C17:0), and heptadecenoic acid (C17:1ω9), were also affected, suggesting changes in rumen microbial activity. Stepwise regression identified key predictors of gas emissions: BHB (+), C18:1ω9 (+), heptadecenoic acid (C17:1 ω9) (−), and arachidonic acid (C20:4 ω6) (−) (R² = 0.236) for CH₄; C15:1 ω9 (+), C17:0 (−), C17:1 ω9 (−), C18:1 ω9 (−), linoleic acid (C18:2 ω6) (−), eicosadienoic acid (C20:2 ω6) (−), arachidonic acid (C20:4 ω6) (−), and BHB (−) (R² = 0.340) for CO₂. CONCLUSIONS. Ketosis significantly alters milk FA composition, reflecting changes in energy metabolism and rumen microbial activity, and these alterations are associated with variation in CH₄, CO₂, and H₂ emissions. Milk FA profiling, in combination with metabolic indicators such as BHB, provides a promising approach to monitoring cow health and predicting greenhouse gas emissions, offering potential applications for precision nutrition and environmental management in dairy systems.