Metabolic Changes in Serum and Milk of Holstein Cows in Their First to Fourth Parity Revealed by Biochemical Analysis and Untargeted Metabolomics

Simple Summary The body metabolic status of dairy cows is closely related to lactation performance. Their production performance will change with their years of service, and the milk quality also varies among different parities. However, there is still lack of research on the relationship between the metabolic state of Holstein cows and the performance of lactation across multiple parities. This study’s findings uncover the connection between the serum metabolome of Holstein cows and milk production. The results indicate variations in the lactation energy among the Holstein cows at different parities. Blood metabolites emerge as pivotal regulators of the milk yield and quality in the Holstein cows, underscoring the significance of addressing nutritional needs during these stages to enhance lactation performance. It is essential to understand the metabolic changes in Holstein cows at different parities to develop more refined feeding strategies, improve management methods, and strengthen the health of the cows. Abstract The performance of dairy cows is closely tied to the metabolic state, and this performance varies depending on the number of times the cows have given birth. However, there is still a lack of research on the relationship between the metabolic state of Holstein cows and the performance of lactation across multiple parities. In this study, biochemical analyses and metabolomics studies were performed on the serum and milk from Holstein cows of parities 1–4 (H1, N = 10; H2, N = 7; H3, N = 9; H4, N = 9) in mid-lactation (DIM of 141 ± 4 days) to investigate the link between performance and metabolic changes. The results of the milk quality analysis showed that the lactose levels were highest in H1 (p = 0.036). The total protein content in the serum increased with increasing parity (p = 0.013). Additionally, the lipase activity was found to be lowest in H1 (p = 0.022). There was no difference in the composition of the hydrolyzed amino acids in the milk among H1 to H4. However, the free amino acids histidine and glutamate in the serum were lowest in H1 and highest in H3 (p < 0.001), while glycine was higher in H4 (p = 0.031). The metabolomics analysis revealed that 53 and 118 differential metabolites were identified in the milk and serum, respectively. The differential metabolites in the cows’ milk were classified into seven categories based on KEGG. Most of the differential metabolites in the cows’ milk were found to be more abundant in H1, and these metabolites were enriched in two impact pathways. The differential metabolites in the serum could be classified into nine categories and enriched in six metabolic pathways. A total of six shared metabolites were identified in the serum and milk, among which cholesterol and citric acid were closely related to amino acid metabolism in the serum. These findings indicate a significant influence of blood metabolites on the energy and amino acid metabolism during the milk production process in the Holstein cows across 1–4 lactations, and that an in-depth understanding of the metabolic changes that occur in Holstein cows during different lactations is essential for precision farming, and that it is worthwhile to further investigate these key metabolites that have an impact through controlled experiments.