Dairy Science Information

Farm – milk production

Milk production and biosynthesis

Milk is synthesised in the udder, in clusters of tiny sacs called alveoli that are lined with secretory cells. These cells draw raw materials from the cow's blood – it takes on the order of several hundred litres of blood passing through the udder to make a single litre of milk – and assemble them into the components of milk.

The main components and how they are made

• Lactose – the milk sugar, built in the secretory cells from glucose and galactose. Because lactose draws water in by osmosis, the amount synthesised largely sets the volume of milk produced.
• Milk fat – triglycerides assembled from fatty acids. Short-chain fatty acids are made within the udder from acetate and beta-hydroxybutyrate (products of rumen fermentation), while longer-chain fatty acids are drawn from the blood. Fat is the most variable component, and the most diet-sensitive.
• Milk proteins – chiefly the caseins, plus whey proteins, synthesised in the secretory cells from amino acids taken from the blood.
• Minerals and vitamins – including calcium, phosphorus and the milk salts, secreted along with the other components.

What affects milk yield and composition

• Stage of lactation – yield and composition change across the lactation cycle.
• Nutrition – feed and forage strongly influence fat and protein, especially the fat content.
• Breed and genetics – set the baseline for yield and the fat-to-protein balance.
• Season and environment – temperature and grazing affect intake and composition.
• Udder health – mastitis raises somatic cell count and alters composition.

Milk production: frequently asked questions

Where is milk made in the cow?

Milk is synthesised in the mammary gland, in small sacs called alveoli lined with secretory cells that draw raw materials from the cow's blood and assemble them into milk.

What determines how much milk a cow produces?

Lactose synthesis is the main driver of volume, because lactose pulls water into the milk by osmosis. Overall yield is then shaped by breed, stage of lactation, nutrition, season and udder health.

Why does milk fat content vary so much?

Milk fat is the most diet-sensitive component. It is built partly from rumen fermentation products and partly from blood fatty acids, so changes in feed and forage have a marked effect on the fat percentage.

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