Effects of the circadian rhythm on milk composition in dairy cows: Does day-milk differ from night-milk?

ABSTRACT Metabolism in most organisms can show variations between the day and night. These variations may also affect the composition of products derived from livestock. The aim of the present study was to investigate the difference in composition between the day-milk and night-milk of dairy cows. Ten multiparous Holstein cows (milk yield = 25.2 ± 5.00 kg/d) were randomly selected during mid lactation. Milk samples were collected at 0500 h (“night-milk”) and 1500 h (“day-milk”) and analyzed to determine their composition. Mid-infrared spectroscopy was used to analyze macronutrient content of milk. Metabolomics and lipidomics were used to detect and analyze small molecules and fatty acids, respectively. An automatic biochemical analyzer and ELISA kits were used to determine biochemical indicators, as well as antioxidant and immune parameters in the milk. Though milk fat, protein, lactose, and total milk solids were not different between day-milk and night-milk, small molecules, metabolites and lipids, and hormones and cytokines differed between day-milk and night-milk. Regarding biochemical and immune-related indicators, the concentrations of malondialdehyde, HSP70, and HSP90 in night-milk were lower than that in day-milk. However, interferon-γ levels were higher in night-milk. Additionally, night-milk was naturally rich in melatonin. Lipidomics analyses showed that the levels of some lipids in night-milk were higher than those in day-milk. Metabolomics analyses identified 36 different metabolites between day-milk and night-milk. Higher concentrations of N-acetyl- d -glucosamine, cis-aconitate, and d -sorbitol were observed in day-milk. However, the other 33 metabolites analyzed, including carbohydrates, lipids, AA, and aromatic compounds, showed lower concentrations in day-milk than in night-milk. The present findings show that the composition of night-milk differs considerably from that of day-milk. Notable changes in the circadian rhythm also altered milk composition. These results provide evidence to support the strategic use and classification of day-milk and night-milk.