Over feeding or under feeding protein, or feeding unbalanced protein to carbohydrate ratio rations can decrease feed efficiency, increase feed costs and lower milk production.
MUN analysis can be used to identify potential problems in your feeding program.

Milk Urea Nitrogen (MUN) is correlated to dietary nitrogen and nitrogen balance of a cow.
If protein in the diet is deficient relative to the cow’s requirements, MUN concentrations will be low.

MUN, the concentration of urea in milk, is tool to monitor the efficiency of crude protein utilization in dairy cows and it can help improve the efficiency of milk production, reduce feed costs, and reduce environmental problems associated with dairy production.

Low MUN: It indicates a possible dietary protein deficiency, which can result when the rumen bacteria yield is reduced, thereby limiting milk production and milk protein yield. - Below: 5~20 mg/dl

High: It can be associated with excess dietary protein or an imbalance of ruminal protein, protein fraction and energy.
These factors can also be related to reduced milk yield, true protein, and feed efficiency. - Above: 15~20 mg/dl

High MUN values indicate wasted feed protein and more energy being used by the cow to excrete that extra protein.
It also means that excess nitrogen is being excreted into the environment.

* Low and High value can be interpreted differently by regulation, nutrition policy of farm, dairy company.