Origin of milk fat

Milk fat is composed of fatty acids originating from two main sources : de novo-synthesised and dietary origin, with the balance supplied by fat released from body reserves.

1. De novo synthesis – fat manufactured in the udder from acetate and butyrate resulting from digestion of fibre in the rumen. This accounts primarily for the short- and medium-chain fatty acids of 4 to 14 carbons and about half of the palmitic acid (C16:0) in milk fat. 
2. Dietary fat – direct supply of fat from the diet accounts for the other half of the C16:0, as well as the C18 (and above) fatty acids in milk fat (e.g. stearic acid, oleic acid, linoleic acid).

A varying proportion of milk fat will arise from mobilisation of body reserves; a higher proportion originates from this source in early lactation when cows are in negative energy balance than in later stages of lactation when cows are in positive energy balance and laying down body fat.

Understanding of these basic sources of fat synthesis highlights the importance of monitoring diet composition and with approximately 50% of milk fat originating from dietary fat, the essentiality of meeting fat requirements in the ration is apparent.