Diagnostic value of milk protein

It is very important to observe the trends of protein variation in milk and be aware of the relationship with other components. The ratio of milk fats to proteins can be a sufficiently reliable indicator in the diagnosis of acidosis or ketosis.

Determination of subclinical ketosis

As the ratio of milk fats to proteins approaches the threshold of 1.5, we can suspect subclinical ketosis in cattle. In such a case, the changes are associated with an energy deficit in the body due to the physiological state of cattle (e.g. after calving) or an improperly balanced diet.

The amount of fat is not constant during milking either. At the beginning of milking, the amount of fat in milk is about 1 percent, rising to 8 or even 12 percent in the last spurts of milk. Therefore, constant monitoring of milk fat can be important not only in the assessment of the health status of the animal, but also as an indicator of the proper performance of the milking process.

Fat-protein ratio in suspected ketosis
>1,4 There is a high probability that a cow has subclinical ketosis, the risk of clinical ketosis increases
1,3-1,4 Increased risk of subclinical ketosis
1,2 Optimal ratio of fats and proteins

Determination of subacute acidosis (SARA)

Fat-protein ratio in suspicion of acidosis
1,2 Optimal ratio of fats and proteins
1,0-1,1 Increased risk of subacute acidosis (SARA)
<1,0 The cow is more likely to have subacute ruminal acidosis (SARA), the risk of clinical acidosis is increased

If the ratio of fat to protein is <1.0, it can be stated that the cow has subacute acidosis (SARA). In this case, the cause of the disease should be identified and eliminated as soon as possible. This can occur as a result of an excessive amount of grain in the diet or an excessive amount of concentrated feed given during a single feeding. Acidosis can be caused by insufficient digestible fibre or if the feed is too finely chopped or the raw materials are contaminated with mycotoxins can also be one of the causes of acidosis.

Urea is the final product of nitrogen metabolism in the body. The protein and urea content in milk is of important diagnostic importance, since from the data obtained we can judge the nitrogen-energy balance in the rumen, i.e. provision of cattle with protein and energy feed.

Diagnostic significance of protein and urea concentrations in milk

Protein concentration in milk (%) Urea concentration in milk mg%
<15 15-25 >25
>3,6 Protein deficiency and energy excess
in feed
Excess energy in feed Excess protein and energy in feed
3,2-3,6 Deficiency of protein and a slight
excess of energy in the feed
Appropriate balance of protein and
energy in the feed
Excess of protein and a slight deficiency
of energy in the feed
<3,2 Deficiency of energy and protein in feed Energy deficiency in feed Excess protein and energy deficiency in feed

*R. Antanaitis, V. Žilaitis. Assessment of the health of a herd of dairy cattle: methodological publication for veterinary faculty students, veterinarians. LSMU, VA. Kaunas, Lithuania. 2016.

Protein determination methods

Kjeldahl method

The reference test for determining the protein content of milk has been used since ancient times. It is designed to determine the total and actual protein content. The method is reliable and accurate, but can only be performed in laboratories, as special equipment, reagents, and qualified personnel are required. Also, this test takes a relatively long time, so it is not very practical to use it in milk processing plants or farms.

Laboratory spectroscopic tests

True proteins are more accurately determined by infrared analysis of their absorption and emission spectra. This methodology is very popular all over the world and is used in many laboratories to analyse the constituents of milk. Most often, such equipment is adapted for laboratory use, is quite expensive and requires qualified specialists. However, with the development of technology, there are opportunities to make this type of methods more practical and adapted for use not only in laboratories, but also on farms.

BROLIS HerdLine milk analyzer

The in-line milk analyzer BROLIS HerdLine is like a small laboratory on your dairy farm. The analyzer examines the composition of each cow’s milk during each milking. This “mini-spectroscope” is installed in the milking stalls or milking robot in the milk line and does not require additional reagents and special maintenance.

The analysis of fat, protein and lactose provides a proper evaluation of the health, productivity and economic efficiency of dairy cattle. The data collected during milking is processed in real time and can be viewed using the BROLIS HerdLine application.

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What affects milk protein?

Milk proteins are synthesised in the body of cattle from nitrogen compounds that the animal digests in its feed. True proteins make up 95-97 percent of total protein. There are no protein reserves in the body, so the nitrogenous substances that enter the digestive tract with feed are absorbed and carried by blood to the glandular tissue of the udder, where milk proteins are synthesised. The protein composition of milk has a significant influence on its nutritional value and technological properties. Casein makes up 75 to 85 percent of total milk protein, with whey protein accounting for the rest. Their composition depends on the animal’s genetics, feeding, time of year, lactation and state of health. Therefore, by applying purposeful breeding of cows through proper feeding and care, there should be an increase in both the milk yield and also the protein content of the milk.


The level of protein in milk is highly dependent on the proper balancing of the cow’s diet. There are often situations when the protein content of cow’s milk is low due to the insufficient levels of energy and protein in the feed. If the amount of nitrogen is unbalanced, it is removed from the body in the form of urea through milk and urine. Therefore, based on the amount of urea in milk, we can judge the level of ration balancing. Protein in milk increases when raw materials containing biologically valuable proteins (e.g. soybean meal, rapeseed meal, etc.) are included in the cow’s rations. Protein in milk is reduced if the concentrated feed in the cow’s diet consists only of bell plants (barley, wheat, triticale, rye). These raw materials contain insufficiently digestible nitrogen compounds.

Lactation period

The milk protein content is also affected by the lactation period. As milk yields increase in early and peak lactation, protein levels tend to be lowest. During this period, there are not enough microbial proteins for the synthesis of milk proteins, therefore, in order to increase the body’s supply of amino acids, the diet should be supplemented with proteins that are not broken down in the rumen, obtained from soy, rapeseed, and sunflower feed materials.


The breed of cattle has a great influence on the protein content of milk. Jersey cattle are one of the most protein-rich breeds of cattle, and Holstein Friesian cattle are the least protein-rich in milk. Therefore, when improving cattle herds, cows should be inseminated with the sperm of bulls that increase protein content.