Young, Allen

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MILK UREA NITROGENTEST (MUN)Allen Young Dairy Extension Specialist Utah State University, Logan, UtahDecember 2001 AG/Dairy-01WHAT IS MILK UREA?Protein eaten by the cow is classified into two components, rumen degradable protein(RDP) or rumen undegradable protein (RUP). Rumen undegradable protein is sometimesreferred to as rumen by-pass protein because it resists breakdown in the rumen and is brokendown in the small intestine. Protein degraded in the rumen is used by the rumen microbes tomake microbial protein, which in turn is used by the cow to make milk. In order for the rumenmicrobes to make protein they need a source of soluble carbohydrate and soluble (RDP) protein,primarily in the form of ammonia. Excess ammonia in the rumen moves through the rumen wallinto the blood stream. Ammonia is very toxic to most mammals and needs to be removed fromthe body. Blood ammonia is converted to non-toxic urea in the liver. Urea is a small, water-soluble compound that equilibrates throughout all parts of the body and is found in the blood,milk, and urine. Urine is the primary route for excretion from the body as a waste product.Conversion of ammonia to urea in the liver requires a substantial amount of energy that could beused to make milk.WHY USE MILK UREA FOR A TEST METHOD?There is a high correlation between blood levels of urea and milk, but milk is preferredbecause it is non-invasive, easy to collect, less variable during the day (average of day), and is apractical method of sampling compared with blood. The test requires no special on-farmequipment and can be analyzed using the same milk sample taken during a regular DHIA test.All you have to do is have your DHIA technician make a note on the barn sheet that you wantthe test run.WHAT ARE RECOMMENDED CONCENTRATIONS?Concentrations for MUN in the United States are expressed as mg/dl (100 ml = 1 dl ordeciliter). We recommend 12 - 16 mg/dl. Northeast and Pennsylvania DHIA recommend either12 - 16 mg/dl (Northeast) or 10 - 14 mg/dl (Pennsylvania) as recommended concentrations.WHAT DOES MUN TELL YOU?If MUN concentrations are outside of the recommended levels, it signifies that there is animbalance between the rumen soluble carbohydrates and protein needed for microbial synthesis.If the MUN concentration is low there is probably either too little soluble protein in the ration oran excess of soluble carbohydrate. Conversely, a high MUN concentration means there is toomuch soluble protein or too little soluble carbohydrates. Excessive RUP or protein in general canalso result in high MUN concentrations. Dairy farmers and nutritional consultants have usedMUN concentrations to determine if:! their ration is properly balanced,! if the ration on paper or mixed is the same as what the cows are eating,! if the values used by the computer are the same as the ingredients that went intothe ration (i.e., if you change bags of haylage, is the protein % the same asbefore),! if there are undetected equipment problems that changed the ration.While most DHIA production variables such as fat %, protein %, and somatic cell countare fairly straight-forward and the interpretation relatively unambiguous, MUN values requirecaution in interpretation and it is strongly advised that a dairy producer consult with a competentnutritionist before making any major changes in a ration based on MUN concentrations. Rationchanges should be made slowly and in small increments.HOW GOOD IS THE TEST?The answer to that question depends on the type of machine being used to analyze forMUN. There are two types of machines, infrared (IR) and wet chemistry. The IR machine is thesame one that is used by DHIA to determine the levels of protein %, fat %, and somatic cellcount. MUN is determined by using a different wave length of light than the other milkcomponents. This test is quick and easy to run, but the wavelength of light used picks up othernitrogen compounds in milk. This will result in greater variability between cows depending onthe level of these other nitrogenous compounds. Research has shown that IR can underestimatehigh and overestimate low MUN.The wet chemistry method is superior to the IR (primarily the Foss 4000) in the accuracyof the analysis because all it measures is urea - nothing else. The main problem is this analysis ismuch slower and requires a separate piece of equipment. The wet chemistry method is themethod of choice based on accuracy. Rocky Mountain DHIA uses the wet chemistry method.IS THERE A DIFFERENCE BETWEEN HOLSTEINS AND JERSEYS?Research from Pennsylvania suggests that the average MUN for Jerseys is about 2-3mg/dl higher than Holsteins. Work at USU, using DHIA records, showed that Holsteins averaged15.2 mg/dl while Jerseys averaged 13.7 mg/dl. Research has shown that MUN concentration is afunction of body weight with larger breeds having a higher MUN.HOW IS MUN RELATED TO MILK YIELD, PROTEIN % OR FAT%?Work at USU suggests that MUN levels change similar to milk yield - lower during thefirst 30 days in milk (DIM), then increases and peaks at approximately 60 - 70 DIM, thendecreases. We suggest that changes in ration composition was the driving factor in MUNchanges. MUN was related to protein % in Holsteins with lower (within normal ranges) MUNconcentrations associated with higher milk protein %. This relationship was not seen in Jerseys.Milk fat% was not related to MUN within normal fat % levels.RELATIONSHIP BETWEEN MUN AND REPRODUCTIONThere is a great deal of controversy regarding the relationship between high urea levelsand reproduction. There are many controlled studies that show a clear relationship between highlevels of MUN and impaired reproduction. Other studies show no relationship, particularly inthose studies using averaged DHIA records from several commercial dairies. While there arephysiological reasons for impaired reproduction, such as early embryonic deaths and changes inUtah State University is an affirmative action/equal opportunity institution. Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperationwith the U.S. Department of Agriculture, Jack M. Payne, Vice President and Director, Cooperative ExtensionService, Utah State University, Logan, Utah.(EP/DF/12-2001)the uterine environment, the lack of response to groups of commercial dairies may be due tovariations in management that are hidden due to averaging. Current research suggest thatexcessive body weight loss, enhanced by high MUN, may be a critical factor. As noted earlier,high blood ammonia requires energy to be converted into urea in the liver. This energydiversion, especially during the negative energy balance found in early lactation, can lead toexcess weight loss. Excessive body weight loss is most often associated with reproductiveproblems. If a manager is able to minimize weight loss, even with high MUN, then reproductiveproblems may be minimized or not develop. Body condition scoring and MUN concentrationscan be useful tools for troubleshooting and eliminating possible reproductive problems on adairy.OTHER COMMENTS:1. Variations between cows within a herd can be large, so results from individualanimals are not representative of herd MUN concentrations and should not beused. Always sample a minimum of at least 7 - 8 cows per group or time period. Itis easier to sample the whole herd and this is recommended because the cost percow is cheap ($0.10 per cow, RMDHIA) and ease of handling in the lab.Variations between animals are due to time since last feeding, time of day ofsampling, the ration fed, what ration was actually eaten by the cow (selectiveeating of concentrates compared with forage), water intake, etc. It is veryimportant to never make a management decision for the herd or even anindividual cow based on the results of a single sample from one animal. Othershave recommended that a baseline period of 6 - 8 months be used to determineyour herd concentrations.2. Bulk tank samples are interesting and useful for getting a general picture of howthe herd, as a whole, is doing; however, it does not supply enough managementinformation such as which string(s) are high or low, or if a certain stage oflactation is out-of-line. These can all be masked by a bulk tank sample and won’ttell you where to look for a ration or group problem. Averaging of individualcows or groups of cows is recommended.The MUN test can be a valuable tool, when used with other management tools, todetermine how well the cows in your herd are responding to a given ration. The use of this testcan help determine if you are feeding excess protein (a costly problem), or if adding a little moresoluble concentrate to the ration can give an increase in milk production. Small changes in thecost of a ration can make a big difference in your cash flow. For more information, contact yourcounty extension agent, Rocky Mountain DHIA or your DHIA Technician.

Milk Urea Nitrogen Test (MUN)

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Milk Urea Nitrogen Test (MUN)

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