MILK UREA NITROGEN TEST (MUN)
Allen Young
USU Extension Dairy Specialist


What is the MUN test?

The MUN test, as the name implies, is a measure of urea in milk samples. Urea collects in milk as a consequence of excess ammonia leaving the rumen and getting converted into urea in the liver. The urea produced in the liver moves throughout the body in the bloodstream and eventually is found in the blood, milk, and urine. It is eventually excreted from the body as a waste product. In addition, conversion of ammonia to urea in the liver uses energy that could have been put to better use by making milk. Again, a waste.

What does it tell you?

Excessive ammonia leaving the rumen is wasteful from a nutritional standpoint. It signifies that there is an imbalance between the rumen soluble carbohydrates and protein needed for microbial synthesis. This means that a low MUN value results from either too little soluble protein in the ration or an excess of soluble carbohydrates. Conversely, a high value means there is too much soluble protein or too little soluble carbohydrates. Excessively high MUN values have been associated with poor conception and early embryonic deaths.

If you have MUN values too high or too low, have your nutritionist look at your ration to determine where it can be fine-tuned.

What are normal values?

Currently, we are trying to evaluate what normal values are for Utah and surrounding states. Northeast and Pennsylvania DHIA's have been doing MUN analysis for several years and recommend either 12 - 16 mg/dl (Northeast) or 10 - 14 mg/dl (Pennsylvania) as normal values. In Utah we are recommending 12 - 16 mg/dl.

Cautions:

1. Variations between animals can be quite large, so looking at individual animals does not give any useful information. Always sample a minimum of at least 7 - 8 cows per group or time period. Usually it is easier to sample the whole herd because the cost is relatively cheap ($0.10 per cow) and for ease of testing. Variations between animals are due to time since last feeding, time of day of sampling, the ration fed, and what ration was actually eaten by the cow (selective eating of forage compared with grain), etc. So far, variations between cows within a herd are greater here in Utah and surrounding states than what is found in Pennsylvania. It is very important to never make any management decision for the herd or even an individual cow based upon the results of a single sample from one animal.

2. Bulk tank samples, while interesting, give no useful information regarding management of the herd because high or low values can be masked by a bulk tank sample average and won't tell you where to look for problems in the ration or group.

3. Finally, while reading most values from a DHIA record is fairly straight-forward and the interpretation relatively unambiguous, MUN values require caution in interpretation and it is strongly advised that a dairy producer consult with a competent nutritionist before making any major changes in a ration based upon MUN levels. Ration changes should be made slowly and in small increments.

Do you have to do something special to have the test run on your herd?

No. The test is run on a separate wet chemistry machine at the RMDHIA lab using the same milk sample that is used to determine fat, protein, and somatic cell counts. All you have to do is have your technician make a note that you want the test run.

The MUN test can be a valuable supplement when used with other tools to determine how well the cows in your herd are responding to a given ration. The use of this test can help determine if you are feeding excess protein (a costly problem), or if adding a little more soluble concentrate to the ration can give an increase in milk production. Small changes in the cost of a ration can make a big difference in your cash flow. For more information, contact Rocky Mt. DHIA. ©