Milk: an incredible life perpetuating commodity which can also be used as a health monitoring tool.

What is milk?

Milk originates as the special fluid produced by mammals to feed their young – young that range from Wildebeest young where the precocious new born calf is ready to run with the herd within hours of birth to mammals where the young has to be supported by its mother or parents for years (as is the case with humans!).

What are the constituents of milk?

A heifer starts to produce milk as soon as she has calved and the first milk is colostrum – in the cow critically important to the calf as it contains antibodies to provide passive immunity to protect the new-born until it begins to develop its own immune response. As lactation continues, the constituents of the milk change considerably from predominantly protein to a more balanced combination of fat, protein and carbohydrate (see table below):

Table 1: Typical composition of colostrum and milk in cows

Milk TypeProteinFatCarbohydrateUnits
Full milk33.038.050.0g/kg

How can milk be used to monitor health?

Milk is produced by cells in the alveoli of the mammary gland. Tight junctions between cells form a barrier – the blood –milk barrier – between blood and milk, ensuring that the two systems are separate.  In the healthy mammary gland blood constituents may enter the milk passively or by active transport. This system ensures that milk has a consistent character, albeit within a range, for its constituents.

These systems ensure that levels of identical constituents of blood and milk can be present at different levels in the blood vs milk. For example the level of progesterone in blood is typically lower than that of milk.

However, progesterone is metabolised by the cow’s liver and passes out in milk, thus maintenance of levels in both blood and milk requires an ongoing supply of appropriate nutrients or raw materials for progesterone production. Problems can be seen in some high yielding dairy cows, possibly because of a dilution effect with the amount of progesterone in milk or higher metabolism (or possibly a combined effect of the two). Because levels in milk and blood are different but related to one another, low levels in milk will reflect low levels in blood, hence milk is a useful diagnostic medium for monitoring progesterone levels. If this occurs in early pregnancy as the cow is producing large volumes of milk, this may result in early embryonic death as there is insufficient progesterone to support the pregnancy.

Milk testing options at RRL

Here are RRL we provide a milk testing service, run by our Ridgeway Science arm. This service allows you to submit milk and plasma samples for progesterone testing. Ridgeway Science also provide a plethora of other progesterone testing alternatives:

Milk ELISA – an enzyme based assay in a microtitre plate designed for use in laboratories.

P4Rapid – a pen side, on farm test designed to be used and interpreted by farmers, vets and anyone working with animals. This test gives you near instant results to allow to make decisions on when to inseminate your cow.

HeatAid – a version of the Milk ELISA mentioned above, but converted to be on farm freindly. This was also designed with vets and farmers alike in mind.

For queries on any of these products, advice or if you would like to purchase anything please contact us on, give us a call or visit our website here.


Langer, P. Differences in the composition of colostrum and milk in eutherians reflect differences in immunoglobulin transfer. Journal of Mammology. 2009. 90(2):332-339.

Kobayashi, K et al. Lipopolysaccharide disrupts the milk-blood barrier by modulating claudins in mammary alveolar tight junctions. PLoS ONE. 2013. 8(4): e62187.

Wall, S K et al. Blood-derived proteins in milk at start of lactation: indicators of active or passive transfer. J Dairy Sci. 2015. 98:7748-7756.

Bruckmaier R M et al. Triennial lactation symposium/bolfa: pathogen-specific immune response and changes in the blood-milk barrier the bovine mammary gland. J Anim Sci. 2017. 95:5720-5728.