Practical Application of MUN Analyses

Abstract

Urea is the detoxified form of protein waste in the mammalian body. In the ruminant, excess rumen ammonia is absorbed from the rumen through the rumen wall to the blood stream. It is carried to the liver and is converted into urea by the liver. Urea can be recycled through the blood stream back to the rumen.

The protein waste, ammonia, originates either from the diet or from normal tissue breakdown throughout the cow's body. The high producing milking cow has most of this urea originate from un-used dietary protein. Ropstad, et.al.²¹ nicely showed the relationship between dietary protein, rumen ammonia, and milk urea in 21 adult and 7 first lactation Norwegian Red cows (Figures 1 and 2). The excess protein can be from any of the protein fractions. We tend to think only of soluble protein, but it can also originate from insoluble degradable, or undegradable protein.⁶ Urea is extremely water soluble and is carried by the blood into all tissues, including the lungs, kidney, rumen, small intestine, uterus and the mammary gland.

Blood and plasma and serum urea nitrogen (BUN, PUN, SUN) are synonyms for urea levels taken from blood samples. Milk urea nitrogen (MUN) measures the level of urea in milk. Until recently, it was believed that milk urea nitrogen levels were about 85-90% of blood urea nitrogen. Very recent work done at the University of Pennsylvania¹ (Figure 3) and Cornell University has demonstrated that milk urea nitrogen is nearly equal to blood urea nitrogen; MUN/BUN= .96-.98. The apparent difference in prior studies was due to improper sample preparation, with milk fat and/or milk protein interfering with the correct determination of milk urea level.

It is believed today that urea diffuses into and out of the mammary gland, and that urea in milk will equilibrate with blood in a short time span. Gustafsson and Palmquist¹¹ showed that serum urea peaked 1.5-2.0 hrs after rumen ammonia peaked in cows fed once daily. Milk urea changes lagged behind serum urea changes by 1.0-2.0 hrs (Figure 4). When urea in serum was increasing, milk urea lagged and was lower; when serum urea was decreasing, milk urea lagged and was higher.