Milk Urea Nitrogen

What is urea? Urea is an end-product of protein metabolism that is synthesized from ammonia primarily in the liver. Animals convert excess ammonia to urea because ammonia is toxic while urea is not. The concentration of urea in milk is referred to as milk urea nitrogen (MUN) while the concentration of urea in blood is referred to as blood urea nitrogen (BUN). Urea concentration is typically analyzed on-farm as MUN because milk is easier, less invasive and less expensive to sample. In addition, because urea will readily diffuse from the blood in the mammary gland into milk, changes in BUN will be reflected in MUN.

Sources of MUN:

- Protein or non-protein nitrogen in feed that is broken down in the rumen by microbes is converted to ammonia. If microbes have energy available from fermented carbohydrates, the ammonia can be utilized by the microbes to synthesize microbial crude protein. Unused ammonia in the rumen is absorbed into the blood, transported to the liver and converted to urea.

- Protein that is not degraded in the rumen will reach the small intestine un-degraded. In the small intestine, some of this protein will be degraded to amino acids. Amino acids that are not utilized by the animal cannot be stored in the body and are instead utilized in the liver as an energy source or used to synthesize glucose. In the process, ammonia is produced, which is then converted to urea. Protein will also be utilized by the liver when the diet is deficient in energy.

What is the fate of urea? Urea is either excreted in the urine via the kidneys or recycled back into the rumen through saliva or through the rumen wall.

Why analyze for MUN? Milk urea nitrogen is an inexpensive tool to indicate the balance between dietary protein and energy, and how efficiently the cow is utilizing the protein or nitrogen to produce milk. As concerns increase about nitrogen in the environment, MUN is a good low cost way of monitoring excretion of excess nitrogen into the environment.

How do I interpret MUN values? Optimal MUN values ranging from 8.5 to 12.0 mg/dL have been suggested.

The MUN values on farms can vary significantly due to a variety of non-dietary and dietary factors. Non-dietary factors such as breed, parity, days in milk, milk production and feeding to milking interval can all affect MUN values. Diet or ration has the greatest effect on MUN values, however, with high MUN values indicating excess protein in the diet or a deficiency in energy. An imbalance between rumen degraded protein and energy from fermented carbohydrates should also be considered. This can occur due to a lack of rapidly degraded carbohydrates in the rumen or an excess of rumen degradable protein (RDP). Excess rumen undegradable protein (RUP) can also be a factor. Feed ingredients that contribute a significant amount of RDP to the diet include urea, soybean meal and alfalfa haylage. Protein sources that are less degradable in the rumen include distillers grains, brewers grains, fish meal and blood meal. Low MUN values may indicate the diet is deficient in protein or that energy from fermented carbohydrates in the rumen is in excess of RDP.

How useful are MUN values? Although research is limited, a recent Canadian study suggests monitoring MUN values is a useful tool on-farm. Milk urea nitrogen was determined on individual cows for 11 months. Values were reported by parity (first, second and ≥ three lactations) and within stage of lactation (early, mid and late). Using 38 herds as an “intervention group”, producers were provided reports indicating whether any group had high (>14 mg/dL) or low (10 < mg/dL) MUN values. In addition, a list of potential dietary reasons for the MUN level was provided. Producers then decided whether or not to make any dietary changes in response to MUN concentration. Within the intervention group, they reported herds that made a feed change based on MUN concentrations increased milk yield 2.4 lb/d in the following month as compared to herds that did not make a feed change. However, the actual reason for the milk change is not known as the diet change could have corrected a nutrient imbalance or it could have been a change in feeds resulting in a change in nutrient content and/or intake of the ration. Whatever producers did, MUN was a good monitor to alert nutritionists or producers that efficiency of dietary nitrogen utilization could be improved.

In summary: Milk urea nitrogen should be considered as an on-farm benchmarking tool to help make decisions regarding dietary changes. To get the most out of MUN values, collect milk samples from the same milking from individual cows, utilize one laboratory for analysis and do not include cows with a high SCC count. Establish a MUN baseline for your herd from several tests to determine the average and normal variation in your herd. A baseline and variation is important for assessing diet changes and the effect they have on MUN values.

By Jim Linn and Mary Raeth-Knight