Excessive postpartum body condition score loss coincides with skeletal muscle proteolysis in dairy cows: molecular and biochemical evidence.

The transition period (- 21 to + 21 days relative to calving) in dairy cows is characterized by profound metabolic adaptations that often result in negative energy balance (NEB) and mobilization of body reserves. This study evaluated the association of postpartum body condition score (BCS) loss on the expression of skeletal muscle genes involved in protein metabolism and related biomarkers in blood. Two hundred high-yielding multiparous Holstein cows (≥ 30 kg milk/day) without postpartum disorders were grouped according to BCS loss between calving and + 42 days in milk (DIM): 0.00, - 0.25, - 0.50, and - 0.75 units. Muscle biopsies (n = 40) were collected on days 21 and 42 postpartum for quantification of mRNA abundance of ubiquitin-proteasome pathway genes (UBE2G1, UBE2G2, UBA1, MuRF1, Atrogin-1) and mTOR signaling genes (mTOR, EIF3K, 4EBP1). Cows losing - 0.50 or - 0.75 BCS units exhibited significantly higher expression of UBE2G1, UBE2G2, UBA1, MuRF1, and Atrogin-1, and lower expression of 4EBP1, compared with cows with no or mild BCS loss (P < 0.05). mTOR and EIF3K expression were unaffected. Cows with greater postpartum BCS loss (0.50 and 0.75 units) had significantly higher (P < 0.05) plasma concentrations of creatinine and 3-methylhistidine compared with cows that lost 0 to 0.25 BCS units. These increases were proportional to the extent of BCS loss, with the highest concentrations observed in the 0.75-unit group. These findings indicate that moderate to severe BCS loss is associated with activation of proteolytic pathways and suppression of key regulators of protein synthesis in skeletal muscle, underscoring the importance of maintaining optimal BCS during the transition period to preserve muscle integrity and metabolic health.