TECHNICAL NOTE DAIRY17

Milk Urea Nitrogen Testing –Practical Implications

Technical Note prepared by Dr. Aidan Cushnahan, Dairying Technologist, Greenmount College

ISBN 1 85527 590 2

April 2003

What is milk urea?Urea is a nitrogenous compound found in the bloodof dairy cows, which diffuses freely into the milkmaking up around 2.5% of the nitrogen found inmilk. Urea is produced by one of two processes:

• Breakdown of protein and non protein nitrogen(NPN) in the rumen into ammonia which isconverted to urea.

• Breakdown of excess amino acids in the liver.

Excessive production of urea is associated withinefficient use of protein in dairy cow diets and hasbeen implicated with reductions in dairy herd fertilityin some situations. Most of the urea produced isremoved via urine but some passes into the milk. Atest now exists that allows the urea content of milk tobe measured, which gives an indication of levels ofurea within the bloodstream. Samples may becollected on an individual cow basis or as a singlesample taken from the bulk tank.

Typical concentrations of milkureaMilk urea concentrations are usually expressed aseither a percentage or as mg/dl (dl is anabbreviation for decilitre, where 10dl is equivalent toone litre). Values recorded under local conditionsand their interpretation are given in Table 1.

Table 1: Range in milk urea concentrations and their interpretation

Percentage mg/dl Interpretation

0.025 25 Low0.035 35 Average0.045 45 High0.055 55 Very high

Low values (25mg/dl) indicate a low overall proteinlevel in the diet. High values (greater than 35mg/dl)would indicate surplus protein in the diet andpossible health / feed issues for the farmer toconsider.

Factors affecting milk ureacontentChanges in milk urea concentration are generallyassociated with changes in the diet of the dairycow. For example, low urea contents areassociated with low amounts of dietary protein and/or degradable protein, for example, low D-valuemature grass or silage or diets containing largequantities of maize silage without appropriateprotein supplementation. High urea concentrationsare associated with diets containing large amountsof protein and/or degradable protein combined withlow supplies of fermentable energy, spring grassbeing an example.

Other factors however will also affectconcentrations of milk urea. These include:

• Breed

• Body weight of cow

• Change in body condition

• Parity

Using milk urea testing as amanagement toolThe use of bulk milk urea samples will be of limitedvalue to give an insight into herd nutrition.Evidence on the effectiveness of milk ureaconcentration as an aid to identify fertility problemsis also conflicting and there is some debate aboutidentifying threshold levels for milk urea, that is,concentrations above which problems may occur.

How to use milk urea test resultsMilk urea testing may prove more effective insituations where cows are being individuallysampled or where bulk tank samples are takenfrom herds operating a tight calving pattern and feda uniform diet. In these situations milk urea testingmay be used in conjunction with other records toidentify possible imbalances in herd nutrition andfine tune ration formulation.

Low milk urea and low milk yields

In this situation it is likely that a positive milk yieldresponse will be obtained from feeding extraprotein. In situations where milk urea levels arehigh and cows are losing condition in early

lactation, it would be advisable to check the overallcrude protein content of the ration. If crude proteinlevels are considered to be adequate, then correctthe energy deficit by feeding additional energysources such as cereals or sugar beet pulp.

High milk urea - cows holdingconditionWhere milk urea concentration is high and cowsare not losing condition, the general interpretationis that while high urea levels may not be causingany problems, the protein in the diet is being usedinefficiently by the cow. Reductions in the amountof degradable protein and/or crude protein andincreases in the amount of fermentable energybeing fed may be recommended as a result.

Discuss the implications of milk urea testing withyour local Greenmount Dairying DevelopmentAdviser before making any changes to dairy cowdiets. Farmers interested in using milk ureanitrogen as a nutritional management tool shouldconsider plotting urea levels on a chart through theyear to establish a benchmark for their herd. This inturn can be used in future years to show possibledeviations from the norm.

• Stage of lactation

• Season

• Health status

• Water intake