Seasonal and stage of lactation effects on milk fat composition in northern Victoria

Effect of herd nutrition, time of year and season of calving on milk fat composition and physical properties were examined on irrigated commercial dairy farms in northern Victoria that made use of split-calving and a diverse range of feeding systems. Twenty-four farms were included in the study, and from each farm, morning and evening milk samples were collected from 16 cows that calved in autumn and 16 cows that calved in spring. There were no significant effects of season of calving on the concentration of fatty acids or phospholipids in milk fat, but there were interactions between season of calving and time of year (P<0.001). These differences could be attributed to changes in energy balance and body condition with stage of lactation. The phospholipids comprising mainly phosphotidylcholine (PC), phosphtidylethanolamine (PE) and sphingomyelin (SP) also varied, with PC and PE being highest in late lactation and SP lowest during peak and mid lactation for both calving groups. Milk fat colour and the concentrations of free fatty acids were more influenced by factors associated with time of year rather than stage of lactation. Milk fat colour in particular showed strong seasonal variation being distinctly lighter in summer-early autumn when compared with rest of the year. Increasing the amount of concentrates fed was associated with decreases in short-chain fatty acid concentration and increases in the solid fat content of milk fat. Variations in nutritional management practices had only small (non-significant) effects on fat composition.