Synthesis and composition of cow's milk

Alveoles of the udder.
(c) Sjaastad et al., Scanvetpress

Cow's udder consists of four independent mammary glands, which each have one teat and one opening. Milk is produced in alveoles. Each alveole contains 0,01 ml of milk, which is secreted from the epithelial cells. Once the alveole is full, myoepithelial cells outside the alveole constrict and push the milk into the milk duct, and milk chamber. All in all the udder of an adult cow weighs 10-30 kg, and has 5-20 milk ducts which branch out to smaller milk ducts. The ducts end in lobes, which are filled with alveoles in smaller lobules. The udder needs 400-500 liters of blood to absorb enough nutrients for a kilogram of milk.

Udder development (mammogenesis) begins during the embryonic development. During the first three months of gestation the embryo develops teat channels, milk chambers and initial mammary glands. Actual ducts, teats and fat pads to protexct the udder are formed before the calf is born. From birth until 3 months of age the udder develops isometrically, i.e. as fast as other tissues. Between 3 months and 1 year of age the udder develops allometrically, i.e. faster than other tissues, and then returns to isometric development. Too heavy feeding before puberty decreases the amount of secreting tissue and reduces milk yield.

When the heifer is in gestation, the udder development continues. The ducts grow and increase during the first 3 months, and later the portion of the secreting tissue increases. 10 % of secreting milk ducts develop only after calving, which causes the lactation peak about 2 months after calving. After the peak the number of alveoles begins to decrease. The decrease is faster if the cow gets pregnant again.

Lactogenesis is the actual synthesis of milk, which cab begin once the cow has reached puberty. 0-4 days before calving the amount of progesterone in the body decreases but prolactin and glucocorticoids increase. This signals the body to start producing alpha-lactalbumin, a gene which starts the lactose synthesis in the Golgi apparatus. Lactose changes the osmotic pressure of the udder and thus draws water into the udder. The maintenance of milk production is called galactopoiesis. Prolactin is a hormone which is secreted during milking, and increases the metabolism of the epithelial cells of the milk alveoles. Insulin-like growth factor IGF-1 maintains galactopoiesis and increases milk yield. This is why some countries allow synthetic IGF-1 called BST, Bovine Somatotropin, be injected into the cows to force their metabolism to produce even more milk.

The components of milk are synthetized in different parts of the mammary gland. Lactosis is synthesised in the Golgi apparatus from glucose and VFAs in the blood stream. Lactose is the main component which determines the amount of milk produced. Fats of the milk are synthesized in the cytoplasm from VFAs other fatty acids. Milk protein is synthesised in ribosomes from amino acids absorbed from the blood stream. Minerals and vitamins are secreted into milk directly from the blood stream.

Composition of milk

On average, 87 % of milk is water, 3,25 % is protein and 3,9 % is fat. The rest is a variaety of minerals, trace minerals, vitamins, acids, enzymes and gases. 95 % of the nitrogen in milk is in proteins, the rest is in urea, ammonia and creatine. The composition is affected by several factors:

• Breed: Jersey cows produce milk with higher fat and protein content than ayrshires or Holsteins.
• Phase of lactation: fat and protein content increase towards the end of the lactation. Also right after calving the colostrum is rich in fat and protein,
• Milking technique: Incomplete milking leaves the fatty milk inside the udder. and long intervals (over 16 hrs) between milkings decrease milk yield.
• Udder health: during mastitis there's more Cl and Na in the udder, and less K.
• Somatic cell count: the higher the count, the lower quality the milk has. Excellent milk has less than 250 000 somatic cells in a 3 mth average geometrical calculation.
• Proportions of the VFAs released from the rumen: butyric acid increases fat synthesis, acetic acids maintains it and propionic acid decreases it while increaseing protein synthesis.

Effect of diet on milk production
(c) Babcock Institute e-learning

Feeding is a very important factor in determining the milk yield and the composition of milk. Generall,  high energy intake decreases fat and protein synthesis due to the increase of propionic acid synthesis. However, if the high energy content is due to highly digestable roughage, then fats and proteins in the milk may be increased. Highed energy content generally increases protein synthesis due to increased energy available for the rumen microbes.

• Adding silage or concentrate: increases protein synthesis, decreases fat synthesis
• Adding protein to the food:Same as above, increases the amount of urea in the milk.
• Adding fats to the feed: decreases protein synthesis, may decrease fat synthesis
• Better digestibility of the roughage and limiting the fermentation of the roughage: increases fatty acid and protein synthesis. Protein synthesis is increased more than fatty acid synthesis.
• Using hay or legumes in grass-based roughage decreases protein synthesis. However, especially red clover increases milk yield (~1,4 kg / d)  and protein & fat synthesis, but due to the increased yield the percentage of both fat  and protein in the milk is decreased (-1,5 g/kg and -0,4 g/kg, respectively).

Grazing on a pasture also affects the composition of milk. During grazing season the amount of unsaturated fatty acids in the milk increase due to the composition of grass versus fermented roughage. The relatively high fat content of grass disturbs the biohydrogenation of the rumen, which leads to less fatty acids being saturated. Protein content of milk decreases during grazing. In countries where cows graze only during summer there is clear annual changes in the composition of milk. Both protein and fat content decrease during the summer, and increase again once the cows are moved back indoors.

More information

Dairy Education Series by Professor H. Douglas Goff, Dairy Science and Technology Education Series, University of Guelph, Canada.

Dairy Essentials by Babcock Institute for International Dairy Research and Development, University of Wisconsin-Madison, USA.

(c) Zweber family farms