Effect of day length and exogenous melatonin on chemical composition of sheep milk

Abstract. Changes in day length are a major factor in the productivity of farm animals showing reproductive seasonality. Parameters of milk production in sheep and goats are dependent on melatonin and prolactin concentrations. Changes in the prolactin secretion in lactating sheep have an effect on the amount of milk produced, synthesis of milk proteins, fat and immunoglobulins (i.e. milk composition). In the end the quality and commercial value of milk is determined as well. The aim of this study was therefore to find out the effect of day length and exogenous melatonin on the chemical composition of milk and the level of fatty acids. Subjects were 60 Polish Longwool Sheep. Animals were randomly assigned to three groups, Group I (n=20) – ewes raised under natural day length; Group II (n=20) ewes raised under natural day length and were implanted with melatonin; Group III (n=20) – ewes exposed to an artificially short photoperiod (16D:8L). Sheep were milked twice daily using the Alfa Laval Agri milking machine. Composite milk samples were collected every 28 days to determine chemical composition and fatty acid content. The results obtained showed that the administration of exogenous melatonin and the simulation of a short-day photoperiod during the summer period had significant effects on the milk levels of solids, protein, fat and lactose, and on the fatty acid content of sheep milk.