How Cow's Make Milk

September 06, 2019

How Cow's Make Milk


Before a cow can start producing milk, she must have delivered a calf. Cows usually produce one
calf each year.

A cow starts to produce milk when her first calf is born, which typically happens when the cow is
about two years old. Pregnancy lasts nine months and the cow is usually able to fall pregnant
again about 100 days after her calf is born. This annual cycle ensures that calves are born at the
best time of year.

She continues to produce milk for the first seven months of pregnancy. The farmer stops milking
her two months prior to the birth so she can devote all her energy to producing her new calf.


Cows belong to a group of animals called ruminants, which have four stomach compartments that
play different roles in digesting food and making milk. Other ruminants include goats, sheep,
giraffes and camels.
To produce milk, cows need to eat a variety of grasses, clover and bulky fodder, plus food that’s
rich in protein and energy.

The four stomach compartments are:

The cow half-chews the grass before swallowing it into her first stomach – the rumen – which can
hold about 100 litres of chewed grass. The grass mixes with water in the rumen and is broken
down with stomach juices and microbes.

The grass then enters the reticulum, where it’s softened and made into small wads called cuds.
Each cud then returns to the cow’s mouth and is chewed 40 to 60 times – for about one minute.

The chewed cud is swallowed into the omasum, where it’s pressed to remove water and broken
down further.

The cud then enters the fourth stomach – the abomasum – and finally digested. The digested
grass passes through the small intestine, where all the essential nutrients the cow needs to stay
healthy and strong are absorbed.  

It takes 50 to 70 hours for a cow to turn grass into milk. Depending on the breed, a cow can make
between 25 and 40 litres of milk a day.

Nutrients from the grass are turned into milk by four mammary glands in the udder. The milk is
released from the udder through the teat, but this won't occur if the cow is stressed or
uncomfortable. Suction from a calf or milking machine helps draw out the milk. The teat has a
muscle called a sphincter which stops the milk dribbling out when the cow isn’t being milked.
For every litre of milk the cow makes, more than 400 litres of blood must travel around her udder
to deliver the nutrients and water for making milk. A cow has about 45 litres of blood in her body,
so her blood is continually travelling around her udder to help with milk production.


*Photo Courtesy of Billings Farm & Museum 

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