The Effect of Photoperiod on the Profile of Prolactin, Leptin, Insulin and the Content of Bioactive Substances in Sheep Milk During the Rearing Period of Lambs

Photoperiod and seasonality influence reproduction and lactation in sheep, but their effects on milk hormones, milk composition, and lamb growth are not fully understood. This study assessed the effect of season on milk prolactin, leptin, and insulin concentrations, milk chemical composition, lactation performance, and lamb growth in Polish Mountain ewes. Forty ewes were divided into the following two groups: short-day (lambing in December, n = 20) and long-day (lambing in May, n = 20). Milk samples were collected on days 5, 15, 25, 35, and 45 of lactation. Ewes in the long-day photoperiod had higher milk yield (p < 0.01) and higher prolactin and insulin concentrations (p < 0.01), whereas leptin concentrations did not differ seasonally. Milk from short-day ewes was characterized by higher dry matter and fat content (p < 0.01) and higher protein and lactose content (p < 0.05). Lambs from the long-day group achieved higher mean daily gain (p < 0.01). These results indicate that photoperiod influences lactation performance, milk composition, and offspring growth through seasonal hormonal and metabolic mechanisms, suggesting that appropriate lambing timing and day length manipulation can improve milk production efficiency and lamb growth in practical sheep production systems.