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.
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.