Editor's note: The following answer is provided by Mike Hutjens, professor emeritus at the University of Illinois.
Q: Our milk urea nitrogen (MUN) levels are low, ranging from 8 to 9 mg/dl. I am planning to add 0.2 pounds (90 grams) of urea to the ration. My nutritionist suggested this may not be right. What do you think?
A: Two questions need to be considered before adding urea as your nitrogen source to raise MUN values.
A MUN value between 8 and 9 mg/dl could be optimal if your cows capture recycled urea while meeting their protein requirements. Check milk protein test, levels of rumen degradable protein, rumen undegradable protein, soluble protein, and total ration protein content (metabolizable and/or crude protein), and fecal scores. If these values are optimal, your MUN values are acceptable for your herd.
If the answers to the previous point are not correct, adding protein/nitrogen is recommended. But, check your soluble and rumen degraded protein levels. If these values are high (for example soluble protein over 34 percent of the crude protein), adding feed-grade urea will raise MUN values as the rumen microbes convert urea to ammonia in the rumen; excess rumen ammonia is absorbed in the blood as ammonia; blood ammonia is converted to blood urea nitrogen (BUN) and excreted in the milk and urine and recycled. However, this source of nitrogen does not provide amino acids to the cow to improve protein status of the dairy cows while raising MUN values.
MUN results are useful data, but carefully interpret the results.