Milk Urea Nitrogen

Milk Urea Nitrogen (MUN) often is routinely included in the lab report received from you milk plant. Sometimes high MUN levels have been implicated with lower  fertility. Research trials on this have been inconclusive (Elrod and Butler, 1993; Elrod et al., 1993; Ferguson et al., 1993; Butler et al., 1996 Carroll et al., 1988). Grazing herds have high dietary Rumen Degradable Protein/ moderate energy diets and typically have high MUN levels but not lower fertility.

TMR diets have been trending lower in total protein as nutritionists have begun to utilize amino acid balancing and protein fraction balancing instead of crude protein as the standard for formulation. As diets have dropped in protein fewer high MUN tests have come back from dairy plants as well. Monitoring MUN is not a measure that its time has passed however.

MUN is measured in milligrams per deciliter (mg/dl) ranging from below 6 to as high as 20 on individual

cows with a slightly lower range typically for bulk tank samples. Season, time of day of the sample, breed, age of the animal and stage of lactation are all factors in this number but the largest factor is related to the ration fed the cows. The amount and type of protein, the amount and type of carbohydrate and the manner in which they are fed are the largest factors affecting MUN.

High MUN is an indicator of protein that has been fed but not utilized by the cow. Using the milk sample to identify this ration inefficiency can help reduce excess purchased protein. Often high MUN is associated with milk production that has not been realized. Reaction to a high MUN may be one or a combination of several feeding changes. It may not be the total amount of protein that is the problem, rather the rate at which the protein is broken down in the rumen and if there is a complimentary carbohydrate source available to be utilized by rumen bacteria. The amount of protein and carbohydrate fractions such as starch and sugar must be in the right ratio and have complimentary availability so that rumen microbes can build new proteins and utilize dietary protein.

Monitoring and reacting to changes in MUN also have value as dairy producers implement increasingly close nutrient management plans.

A response to a high MUN if ration protein level is moderate may be an increase in grain feeding, it might be a review of the particle size of the grain or the starch availability in the silage, maybe introduction or increase of a sugar source is indicated. If high protein forage such as alfalfa haylage is fed a change of the types of supplemental proteins or inclusion of other forages in the diet may be a solution. The answer to the problem is not always an easy one but without MUN typically the problem would not even be noticed. Very low MUN may suggest increasing amounts of protein sources.

In response to heat stress or crowding cows may change their meal size and eating frequency. These changes may affect MUN levels without any change in the diet fed to the herd.

As you can see MUN can be used as an indicator of the consistency of the diet and the cow’s environment. If MUN has been steady but is starting to be more random it has served like a parakeet in the coal mine to give us early notice of potential problems and providing opportunity to improve cow environment and diet formulation.

For more information regarding dairy nutrition, please visit UW-Extension Dairy Cattle Nutrition.

– Matt Lippert
Agricultural Agent
UW-Extension Wood County

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