Editor’s note: This article was written by Noelia Silva-del-Rio, dairy advisor, University of California Cooperative Extension, Tulare County and originally appeared in the California Dairy Newsletter.
What is Milk Urea Nitrogen?
Milk urea nitrogen (MUN), the concentration of urea in milk, is a tool to monitor the efficiency of crude protein utilization in dairy cows.
In the rumen, microbes degrade dietary protein to ammonia. When ammonia is coupled with fermentable carbohydrates, rumen microbes are able to capture the nitrogen and synthesize amino acids and microbial protein. However, excess ammonia in the rumen is absorbed across the rumen wall and taken to the liver to be converted to urea. Blood urea is freely diffusible to milk, and therefore, MUN reflects the urea concentration in blood.
What factors influence Milk Urea Nitrogen?
The factors that influence MUN concentrations the most are the concentration and balance of nutrients in the diet. Feeding the following rations will result in wasted feed protein and high MUN values:
Rations high in crude protein.
Rations high in rumen degradable protein and soluble protein.
Rations low in fermentable carbohydrates.
Other factors that affect the MUN concentrations are:
Water intake: increasing water intake and urine production decreases MUN.
Dry matter intake: MUN is at its highest six hours after feeding and at its lowest prior to feeding.
Time of feeding related to milking: MUN are usually lower in a.m. samples than p.m. samples.
Level of production: MUN is higher in high producing herds than in low producing herds.
Method of feeding: separate ingredient feeding increases MUN more than TMR feeding.
Parity: MUN is higher in multiparous cows.
Season: heat stress increases MUN values.
Milking frequency: herds milked three times a day usually have higher MUN than those milked two times a day.
How should I use MUN values?
Research studies suggest that the most desirable MUN for Holstein cows range from 10 to 14 milligrams per deciliter (mg/dl). High concentrations of MUN (> 14 mg/dl) indicate an excess in protein feeding and/or deficiency in rapid fermentable carbohydrates. Low concentrations of MUN (< 10 mg/dl) indicate protein-limited diets. If MUN values are outside the normal range, the ration, the milk components, the feeding program and the nutrient balance should be evaluated.
As previously described, many factors affect the MUN values across herds and within herds. The variation is so wide that MUN in individual Holstein cows ranges from 5 to 20 mg/dl. So every herd can have a different optimal MUN concentration. Individual cow samples should be summarized into groups to establish the different baselines. The cow group baselines may range from 8 to 16 mg/dl.
Changes in MUN baseline greater than two to three points should be investigated to identify the factors causing the shift. High cow-to-cow variation within a group, even if MUN falls within the normal range, suggests feed bunk problems such as feed mixing, delivery or sorting.
What should I do if MUN falls outside the normal range?
If MUN indicates inefficiency in protein feeding, you should evaluate your feeding program:
Are the cow rations formulated to target the nutrient requirements especially for crude protein, rumen degradable protein, rumen undegradable protein, starch and sugars?
Is the ration balanced based on current feedstuff lab analysis?
Do you routinely reformulate the rations based on dry matter?
Are your employees following the proper loading instructions? Are they consistent in their mixing and delivery time practices?
What is the feed efficiency and conversion of nitrogen from feed to milk?
For instance, MUN may increase when cows are switched to corn silage that is less processed or has lower fermentable carbohydrates. Similarly, an increase in MUN is expected when cows are offered a new alfalfa hay higher in crude protein, or a new protein source with a larger fraction of rumen degradable protein.
Take home message
MUN is another tool to monitor the nutritional protein status of dairy cows. MUN values should only be interpreted after examining the entire feeding program.
MUN values are highly variable across herds, so the greatest benefit is to evaluate a group of cows within a herd, and the cow-to-cow variation within a group.
Source: University of California Cooperative Extension