Nighttime or daytime? How milk melatonin levels relate to differential and somatic cell count in dairy cow milk

This study investigates the impact of winter nighttime versus daytime milking on melatonin concentration in milk and explores its relationship with somatic cell count (SCC) and differential somatic cell count (DSCC). Milk samples were collected from 40 Holstein cows at 12-hour intervals (04:00 AM Night samples and 04:00 PM Day samples) during the winter season (11 h of light and 13 h of darkness). The relationship between melatonin content, SCC and DSCC was assessed. Night milk had significantly higher melatonin levels (15.63 ± 1.90 pg/mL) compared to day milk (6.80 ± 0.75 pg/mL). SCC was significantly lower in night milk compared to day milk. The percentage of polymorphonuclear leukocytes (PMN) + lymphocytes (DSCC) was 66.54% in night milk and 63.36% in Day milk. The percentage of macrophages + epithelial cells was 33.46% in Night milk and 36.64% in Day milk. It has been shown that SCC are lower in Night milk compared to Day milk. This observation confirms a potential beneficial effect of melatonin on cow udder health and indicates that this effect occurs without modify the proportion of PMN + lymphocytes and macrophages + epithelial cells during Night milk and Day milk sampling.