Metabolic status and performance at the onset of lactation in dairy cows are associated with circulating serotonin

Twelve Holstein dairy cows were used to study the impact of physiological differences in serum serotonin (5-HT) concentrations on the metabolic status and milk production at the onset of lactation. Blood and milk samples were collected at the first 6 morning and evening milkings, starting within 4 h of calving. Sampling was continued at the evening milkings on d 5, 8, 10, and 14. Cows (6 cows/group) were retrospectively divided into low serum 5-HT (LSS) and high serum 5-HT (HSS) groups according to their calculated areas under the curve for the entire experimental period. Concentrations of 5-HT, free fatty acids (FFA), beta-hydroxybutyrate (BHB), glucose, calcium, IGF-1 and growth hormone (GH) were measured in blood. Milk was analyzed for fat, protein, lactose and 5-HT concentrations. Serum 5-HT concentrations were higher in HSS compared to LSS (P<0.001). The ECM was lower in HSS compared to LSS (P<0.001). The HSS group tended to produce less colostrum (P=0.06) and have decreased milk yield (P=0.07), specifically during the first 5 milkings. Glucose in HSS decreased less than in LSS and was higher at the 2nd and 3rd milking compared with LSS (P<0.05). IGF-1 was elevated in HSS compared to LSS throughout the experiment (P<0.001), whereas GH was concomitantly lower (P<0.05). Total circulating calcium concentrations tended to be elevated in HSS compared with LSS beginning at the 3rd milking (P=0.07). Milk 5-HT decreased overall during the experimental period with LSS maintaining higher 5-HT concentrations compared to HSS until d 14 of lactation (P<0.05). In conclusion, cows with high serum 5-HT concentrations experienced a reduced metabolic load at the onset of lactation, concomitantly lower milk yield, and improved glucose and calcium homeostasis. Higher glucose concentrations in HSS indicate a greater availability of glucose for other physiological functions.