Variation in nutritionally relevant components in retail Jersey and Guernsey whole milk

► Quality characteristics of retail Jersey and Guernsey milk and standard retail milk were compared. ► Seasonal and yearly changes in milk quality were compared at four sampling points over 2 years. ► Some of the less desirable nutritional traits of Jersey and Guernsey milk were reduced in summer. ► n-3 PUFA in Jersey and Guernsey milk increased more from winter to summer than in standard milk. ► Interactions between breed and season were greater for the fatty acid than protein profiles of milk.