About MUN 

Urea is a small, water soluble molecule that is produced primarily by the liver. The liver produces urea
in order to excrete either the excess nitrogen that comes from the cow’s protein metabolism, or the ammonia produced by the rumen. This last source is generally the most significant. The urea produced by the liver is transported in the bloodstream to the kidneys, where it is filtered and becomes the main form of nitrogen excreted in the urine. Because urea is water soluble, the concentration in the blood and that of the other body fluids (notably the saliva and the milk) become balanced while it is in the circulatory system. The urea in the saliva can be recycled to the rumen and used as a source of nitrogen for the microorganisms. As for the urea in the milk, the equilibrium allows us to look at the concentration of urea in the milk as a reflection of the concentration of urea in the blood. This blood level fluctuates over the course of the day, notably with feed intake. Since the animals are milked twice (or three times) a day, the concentration of urea in a sample taken at one milking is a reflection of the average blood level since the last milking.

New Research Confirms the Ideal Range for MUN

Integrating the MUN results with the feed and production data provides us with a data bank which is unique worldwide and allows us to shed some light on the factors which affect the concentration of urea in the milk and the optimal levels.
MUN is a good indicator of the efficiency of protein use from a ration. The higher the concentration of urea, the larger the proportion of wasted feed protein, and the more excretion of nitrogen there is in the urine.

Variation According to Parity and Stage of Lactation

We have also evaluated the effect of parity and stage of lactation on MUN, as illustrated in Graph 1. The graph shows that primiparus cows have an average urea concentration of around 1 mg N/dL lower than multiparus cows. A lower intake of dry matter, along with a different protein metabolism due to continued growth, is probably the reason for this difference. As for the stage of lactation, the effect is the same, regardless of the lactation number: the concentration is lower at the beginning of lactation and reaches a plateau at around the 4th month of lactation. The evolution of dry matter intake and rations more rich in grain are what brings about this effect.


Optimal Concentration

Graph 2 shows us milk and protein production are at their maximum when the MUN concentration falls in the range between 8 and 14 mg N/dL. Consequently, this has become the new target range (the ideal range was previously estimated at between 10 and 16). Values lower than eight do not allow for maximum production, probably due to a lack of protein availability in the rumen. Values higher than 14, however, will not support high production.
High values reveal a less than optimal use of feed protein, and consequently, significant nitrogen waste. Not surprisingly, more detailed analyses also reveal that cows at the beginning of lactation (100 days and less) respond a little better with a higher level of MUN than cows that are more advanced in their lactation. The data also shows that the optimal range is probably slightly higher for the colored breeds.
Article written by STEVE ADAM, Agr., Dairy Production Expert – Animal Welfare, Comfort, and Behaviour, R&D Valacta