Download Interpreting MUN results

Interpreting MUN (Milk Urea Nitrogen)
Results
Bill Woodley
Shur-Gain Ruminant Technical Services Manager
Ammonia is an interesting compound for the
dairy cow. The rumen microbes rely on the
release of ammonia from dietary protein to
increase microbial protein production. The
microbial protein can then be used postruminally to support metabolic functions such
as milk protein production. But ammonia in the
blood is a toxic compound for the dairy cow. To
mitigate this effect, the dairy cow detoxifies
ammonia by converting it to urea in the kidney
and liver. The majority of this conversion occurs
in the liver while the kidney is used to remove
waste material, such as urea, as well as excess
fluid. The dairy cow can handle high levels of
urea with relative safety because in this form it
is non-toxic.
Because urea readily diffuses into body tissue
spaces with water, it will be a normal
constituent of milk (82.5% water) where it is
measured as the NPN (non-protein nitrogen)
content of milk protein. This value represents
the difference between total milk protein
(Canadian payment system) and true protein
(US payment system). The difference between
total protein and true protein is usually in the
range of 0.18-0.19 points.
Measuring MUN improves our understanding of
protein efficiency in the dairy herd. The level of
MUN can be influenced by:



Feeding excessive levels of protein;
primarily rumen degradable protein but
excessive escape protein can also
influence MUN levels.
Breakdown of tissue protein
Water intake
Rumen Degradation of Protein
The rumen is a sophisticated fermentation “vat”
that requires the proper combination of
nutrients to optimize function. When the rumen
microbes degrade protein and amino acids,
ammonia is released. The ammonia can be used
by other microbes or be absorbed by the cow.
The absorbed ammonia is then converted to
urea and much of it can be excreted in the urine
or the milk.
Urea can also be re-cycled through the saliva
and introduced into the rumen through cud
chewing, where it provides microbes with a
nitrogen source to optimize fibre digestion.
This nitrogen recycling is an evolutionary
survival trait which was needed when cows
were grazing on poor quality forages during the
dry season. The system is more efficient when
lower protein diets are fed.
Rumen efficiency is influenced by:

Amount
of
degradable
protein
consumed. Excess levels of degradable
protein will lead to high levels of
ammonia being produced in the rumen.
Because the excess can’t
be
incorporated into microbial protein,
then it will be absorbed through the


rumen wall and converted into urea at
the liver.
Amount
of
rumen
available
carbohydrates. If there is an imbalance
in rumen available carbohydrates and
protein in the diet, then microbial
production is compromised and excess
ammonia is produced.
Lack of cud chewing. The production of
saliva, which is the re-cycling
mechanism for urea, is highly
influenced by cud-chewing and
rumination. Low fibre (cud chewing)
diets can further exaggerate the excess
ammonia.
Tissue Metabolism of Protein (small effect)
Tissue protein breakdown will affect MUN levels
but only to a small degree in a properly
balanced diet. The rate and extent of tissue
catabolism is influenced by total energy being
absorbed through the digestive tract (ruminally
and post-ruminally absorbed).
Ensuring
adequate energy in the diet through proper
rumen function and post-ruminally absorbed
energy (fatty acids, escape carbohydrates) will
reduce MUN.
Water Intake (small effect)
Because urea is passed out of the body in urine,
water intake can influence MUN. Higher water
intake will help to reduce MUN due to the
clearing of urea in the urine. Herds that have
restricted water intake (availability, quality,
tingle voltage) may experience higher MUN
than herds with higher water intake.
How to Interpret MUN Levels
MUN measurements are recorded as mg/dl
(milligrams/decilitre). In the past a range of
between 10 mg/dl and 16 mg/dl was considered
“normal” for a Holstein dairy herd based on
historical data. But with a shift to amino acid
balancing and an increased concern for the
environment, a range of 8 mg/dl to 12 mg/dl is
considered acceptable or normal. Concerns
arise when the herd is experiencing high (>16
mg/dl) or low (<8 mg/dl) levels.
High MUN (>16 mg/dl)
Research indicates that herds with a high MUN
(>16 mg/dl) are associated with decreased
fertility. This was originally linked to early
embryonic mortality due to excess ammonia
levels circulating through the blood, even
though blood ammonia levels are very tightly
controlled by the cow. The other scenario is
related to lower energy consumption and
inefficiencies of converting protein to energy
and ammonia to urea. This can be an energyexpensive process and shifts energy partitioning
away from reproduction.
Herds with high MUN (>16 mg/dl) may be
reflective of:





Excessive rumen degradable protein in
the diet
Excessive total protein in the diet
Imbalance of rumen available protein
and rumen available carbohydrates
Lack of total diet energy
Protein used as an energy source
(energy-expensive)
Low MUN (<8 mg/dl)
Lower MUN can be a sign of:


Excellent protein and energy efficiency
Lack of protein in the diet
There has been a shift to lower MUN as dairy
nutrition models move to balancing for amino
acids rather than crude protein. Crude protein is
a “crude” measurement of nitrogen, which may
be present as NPN (e.g. urea and nucleic acids)
as well as true protein (amino acids). The rumen
microbes can utilize nitrogen in many forms,
but the cow requires metabolizable protein
(absorbed amino acids) for metabolic functions.
Balancing for amino acids can result in a lower
protein diet. Amino acids may also play a pivotal
role in the conversion of ammonia to the nontoxic urea.
Low MUN has been associated with lack of
protein – both degradable and metabolizable
which is then linked to lower milk production.
Adding additional protein to the diet may
increase MUN levels. But if there is no
subsequent increase in milk production, then
the low MUN levels are indicative of excellent
protein efficiency.
Conclusion
MUN measurement is a management tool to
evaluate protein efficiency. The trends to lower
MUN can be healthy for both the cow and the
environment, as long as they are considered in
conjunction with other management tools. High
producing dairy herds with well-balanced diets
will have optimum production and reproduction
with MUN levels between 6-8 mg/dl.
Protein Metabolism Pathway