Observational study on the associations between milk yield, composition and coagulation properties with blood biomarkers of health in Holstein cows

The considerable increase in the production capacity of individual cows, due to both the selective breeding and innovations in the dairy sector, has posed challenges to management practices in terms of maintaining the nutritional and metabolic health statuses of dairy cows. This observational study aimed to investigate the associations between milk yield, composition, and technological traits with a set of 21 blood biomarkers, comprising those related to energy metabolism, liver function/hepatic damage, oxidative stress and inflammation/innate immunity in a population of 1,369 high-yielding Holstein-Friesian dairy cows. The milk traits investigated in this study were: 4 production traits (i.e., milk yield, fat yield, protein yield, daily milk energy output [dMEO]), 5 traits related to milk composition (i.e., percentages of fat, protein, casein, and lactose, and urea), 11 milk technological traits (5 milk coagulation properties [MCP], and 6 curd firming traits [CFt]). All milk traits (i.e., production, composition and technological traits) were analyzed according to a linear mixed model which took into consideration the days in milk, the parity order and the blood metabolites (tested one at the time) as a fixed effect and the herd/date of sampling as random effect. Our findings revealed that milk yield and dMEO are positively and linearly associated with total cholesterol, nonesterified fatty acids (NEFA), urea, aspartate aminotransferase (AST), γ-glutamyl transferase (GGT), total bilirubin, albumin, and ferric-reducing antioxidant power (FRAP), whereas they are negatively associated with glucose, creatinine, alkaline phosphatase, total reactive oxygen metabolites (ROMt), and proinflammatory proteins (i.e., ceruloplasmin, haptoglobin, and myeloperoxidase). Regarding composition traits, the protein percentage is negatively associated with NEFA and β-hydroxybutyrate (BHB), while the fat percentage is associated positively with BHB, and negatively with paraoxonase. Moreover, we found that the lactose percentage increased with increasing cholesterol, and albumin, and decreased with increasing ceruloplasmin, haptoglobin, and myeloperoxidase. Milk urea increased with an increase in cholesterol, blood urea, NEFA, and BHB, and decreased with an increase in proinflammatory proteins. Finally, no association was found between MCP and CFt and the blood metabolites. In conclusion, this study shows that variations in blood metabolites have strong associations with milk productivity traits, the lactose percentage and milk urea, but no relationships with technological traits of milk. Specifically, increasing levels of proinflammatory and oxidative stress metabolites, such as ceruloplasmin, haptoglobin, myeloperoxidase, and ROMt, were shown to be associated with reductions in milk yield, daily milk energy output (dMEO), lactose percentage and milk urea. These results highlight the close connection between the metabolic/innate immunity status and production performance, which is not limited to specific clinical diseases nor to the transition phase but manifests throughout the entire lactation. These outcomes put emphasis on the importance of identifying cows with subacute inflammatory and oxidative stress with the aim of reducing metabolic impairments and avoiding milk fluctuations.