Plasma and milk metabolomics in primiparous and multiparous Holstein cows under heat stress

Multiparous and primiparous dairy cows face different physiological conditions during lactation because of the growth requirements of primiparous cows. This parity-related difference might also affect their metabolic responses to heat stress (HS). Twelve Holstein cows (42.2±10.6 kg milk/d; 83±28 DIM; primiparous, P, n=6; multiparous, M, n=6) were enrolled to evaluate the effect of parity and HS on the plasma and milk metabolomic profiles. Treatments were heat stress (HS; THI=82), or pair feeding in thermo-neutrality (TN; THI=64) for 14 days in a Latin square design. Blood samples were collected on the last day of treatments and analysed by the Absolute IDQ p400 metabolomics kit (Biocrates, Innsbruck, Austria). ANOVA was used to evaluate treatment (TN, HS) and parity (P, M) effects and their interactions as fixed effect, and the cow as random effect. Raw-P values were adjusted by False discovery rate (FDR) correction. In the plasma, only α-amino adipic acid differed (FDR=0.043) according to parity as it was lower in P, and t4-hydroxyproline tended (FDR=0.057) to be greater in P than M. Thirteen metabolites differed (FDR<0.01) between TN and HS including glycine, phenylalanine, t4-hydroxyproline and 10 lipid species. No significant interaction effects were detected. In the milk, carnitine and 2 acyl-carnitines (C2:0, C5:0-OH) were higher (FDR≤0.025) in P than M cows, and 14 metabolites (glycine, α-amino adipic acid, taurine, 2 acyl-carnitines, 6 phosphatidyl-cholines, 2 sphingomyelins, and 1 triglyceride) differed (FDR≤0.04) between TN and HS, while no metabolite was affected by the interaction. Our results indicate that both the plasma and milk metabolome were affected by the parity, pointing to differences in AA and mitochondrial fatty acid oxidation for energy production.