Milk urea

Milk urea is an approximate indicator of dietary crude protein relative to cow requirements.

The digestion of protein in the rumen by microbes releases ammonia. If there is high ammonia production, a greater amount is absorbed through the rumen wall into the blood stream and converted to urea in the liver. Most of this urea is excreted in the cow’s urine, although some passes into the milk or is recycled back to the rumen.

Increased urea production can occur when dietary crude protein is above animal requirements, or when the rumen microbes’ ability to capture the ammonia into microbial protein is exceeded. This typically occurs when the diet contains a large amount of rumen degradable protein or there is a deficit in readily available carbohydrates that provide the microbes with energy.

If the diet is deficient in protein, then less urea is excreted, and more is recycled back into the rumen and converted to ammonia for the microbes.

Interpreting milk urea values

In New Zealand pasture-based systems, milk urea levels are higher than in systems where cows are fed a total mixed ration (i.e. USA, Western Europe). This is due to the high amount of crude protein (often above 20%) in good quality pasture, relative to cow requirements.

When pasture makes up more than 60% of the lactating cow’s diet, milk urea levels can be greater than 30 mg/dL. Research suggests that these high milk urea values are not detrimental to the health or reproductive performance of the cow.

When pasture is less than 60% of the lactating cow’s diet, milk urea may be used as a tool to help guide ration changes (e.g. help to decide when to put in or to pull out protein or energy supplements).

Milk urea should be used alongside other information, such as feed testing, complete diet assessment, and pasture residuals, and tools such as FeedChecker and the Supplement Price Calculator to determine marginal cost benefits.

Factors affecting milk urea

The type and amount of dietary protein and energy relative to animal requirements are important factors affecting milk urea in pasture-based systems.

Other factors include supplementary feeding, water intake, cow condition, stage of lactation, time of year, genetics, milking frequency, rumen health, and liver function among others.

Can milk urea values help to improve farm efficiency or the farm’s environmental footprint?

Positive associations between the dietary supply of N (from crude protein), urinary N excretion and milk urea support the use of milk urea as an on-farm indicator tool in pasture-based systems. However, these relationships are less precise than in housed systems where cows are fed more consistent diets.

The DairyNZ-led Low N Systems programme is investigating how bulk milk urea could be used as an indicator of herd dietary N surplus and increased urinary N loss risk. Used alongside other metrics and on-farm information, bulk milk urea could help farmers identify opportunities to improve the efficiency of N use from pasture, N fertiliser and supplementary feeds, and to manage the N loss risk of their farm system. For more detail, see Low N Systems.