Metabolic and Physiological Associations with LMD-Based Enteric CH4 Concentration in Early Lactation Dairy Cows: A Multivariate Approach with Parity-Specific Insights

Abstract

Understanding cow-level factors associated with enteric methane (CH4) is important for precision-based methane mitigation in dairy systems. This study evaluated associations between laser methane detector (LMD)-based CH4 concentration, relative CH4 concentration-based indices, biochemical markers, sensor-derived variables, and milk traits in clinically healthy Holstein cows, with an emphasis on parity. Ninety-one cows within 100 days in milk (DIM) were examined: 46 primiparous and 45 multiparous cows. Methane concentration was measured using a portable LMD. Data were evaluated using parity comparisons, principal component analysis (PCA), DIM-adjusted multiple linear regression with parity interaction terms, and DIM-adjusted partial correlation analyses with Benjamini–Hochberg false discovery rate (FDR) correction. Mean CH4 concentration did not differ between primiparous and multiparous cows (370.83 vs. 361.16 ppm; p = 0.765), despite higher estimated group-level dry matter intake (DMI; +11.2%) and milk yield (+23.1%) in multiparous cows. The milk-yield-adjusted CH4 concentration index was higher in primiparous cows (11.26 vs. 8.91 ppm CH4/kg milk; p = 0.022). In the primary PCA-based regression model, the energy metabolism/lipid mobilisation component showed a positive but non-significant association with CH4 concentration (p = 0.077), while the overall model’s explanatory capacity was limited (R2 = 0.105; adjusted R2 = 0.018). Exploratory DIM-adjusted subgroup analyses identified associations involving non-esterified fatty acids (NEFA), triglycerides, serum iron, and milk lactose that remained significant after FDR correction. However, formal predictor × parity interaction analyses did not confirm effect modification by parity. These findings suggest that LMD-based CH4 concentration is best interpreted within a broader metabolic, behavioural, and production context and warrants validation in larger longitudinal studies.