Last updated 08/08/2019

Similar to humans, cows need to be pregnant and give birth for milk production and release to occur. Milk production involves the complex interaction of a number of different hormones, which are set into play during pregnancy.

Typically, during the second and third trimesters of pregnancy various hormones, such as progesterone and oestrogen are produced, which stimulate the growth and development of the milk duct system within the udder.

Oxytocin is another important hormone which allows the secretion of the milk to occur - also known as milk 'let down'. Prolactin (a protein) levels are stimulated when the udder is suckled by the calf or milked by the milking machine and this needs to occur for milk production to continue.

Cows are usually dried off, or milking is stopped, about two months before their next expected calving to allow the udder time to rest and reset itself for the following lactation. A cow’s pregnancy length (gestation) is a little over nine months and generally a cow will calve every 12 months.

Dairy cows are selectively bred to produce high levels of milk and this is far more than a calf could normally drink. The volume of milk produced by a cow will depend upon various factors, such as her level of nutrition, breed, genetics and age. For her to continue to produce milk once she has calved, she needs to be milked each day.