Milk urea nitrogen (MUN) levels can vary by 20 percent, depending on how long it has been since the animal has eaten.
This was borne out recently by research at Kansas State University. (See chart below.)
Milk samples used in MUN analysis are usually taken in conjunction with monthly DHIA testing, and it may not be practical to vary from that schedule. Nevertheless, you need to account for differences in results, depending upon the feeding schedule.
Hours post-feeding – MUN concentration (mg/dL)
2 – 13.65
4 – 14.30
6 – 14.46
8 – 13.94
10 – 13.16
12 – 12.04
Source: Dairy Day 2000 proceedings, Kansas State University