The milk urea nitrogen (MUN) test is a still an effective management tool for improving dietary efficiency in dairy cows. Routine bulk-tank MUNs should not be used by themselves. Rather, milk samples should be taken and their results reviewed with other milk and component data, forage/diet analysis and reproduction records to get an overall picture. Therefore if necessary, quick and effective changes can be made to the diet and/or other management.
Testing milk urea nitrogen (MUN) gives the producers an actual tool to evaluate whether the proteins in rations are efficiently being converted into body function, tissues and milk. Although often MUN bulk tank testing is preferred over individual cow testing, the underlying idea of MUN is the ruminal microbes are digesting feed proteins into ammonia. Ammonia that does not get used in microbial protein synthesis is quickly absorbed through the ruminal wall, transported to the liver, where it is converted into urea. This urea is spilled back into the bloodstream. As the blood flows into the udder, some of this urea quickly diffuses into milk.
A general range of 11 to 16 mg/100 ml milk is considered to be acceptable MUN levels in bulk-tank milk tests, although there are no actual official MUN standards. Its general acceptance among producers and nutritionists is based upon information derived from university and extension field studies.
For example, some European studies have shown a broad linear relationship between MUN and the amount of protein formulated in the diet. These and other researchers have shown as ration protein for lactating cows increase, MUN values increase, urinary nitrogen increases and the efficiency of protein utilization by the rumen microbe population decreases.
Excessive MUN values are usually explained as a reflection of too much soluble protein (lush alfalfa protein) or not enough readily available starch-energy (grain) in the diet. Dietary imbalances are thought to prevent ruminal microorganisms from turning ammonia into microbial protein that helps cows meet their protein requirements. Other smaller contributors to high MUN values are pH imbalances or a poor rumen environment for good fermentation of the diet.
MUN values taken from individual animals on a farm can be affected by a number of non-protein factors such as size of the cows, stage of lactation, herd health status, dry matter intake and water consumption. Variation in individual animal MUN tests is one of the main reasons that bulk-tank MUN testing is often preferred.
TEST INDICATES WASTED PROTEIN
Despite bulk-tank or individual MUN tests, there is much agreement that high MUN tests could be a sign of wasting dietary protein in the barn. It is also coupled with the belief that excessive MUN values are a link to a possible cause of poor reproduction in cows.
Cornell University (1996) showed that a concentration of MUN greater than 19 mg/100 ml in the milk of early lactation cows was associated with a 20 per cent decrease in pregnancy rates. They explained high MUN values are associated with an increased energy demand in the cow’s body to detoxify ammonia in the bloodstream and convert it into MUN. Associated high ammonia levels in the uterus (originating from the bloodstream) may cause unfavorable conditions, which often leads to early embryonic deaths.
Although, there is a lot of focus upon the MUN values that exceed the normal 11 to 16 mg/100 ml milk, producers are advised to develop their own MUN baseline that is normal for their herd. When this personal baseline changes by more than two to three points; look for dietary or animal changes in one’s own herd. It is finally advisable to record and review weekly averages rather than large day to day variations.
Producers might notice large/subtle trends or out-of-range changes over significant time in their herds’ MUN bulk-tank tests. To adjust and return to more modest MUN levels, they might look into the following areas:
1. Diet formulation. Review the total protein content of the ration as well as the rumen degradable protein (RDP), dietary soluble protein levels (SP) and available non-fibre carbohydrate (NSC) levels. It is also a good idea to know the rumen undegradable protein (RUP) levels; too much RUP versus RDP might lead to too-low MUN levels.
2. Adequate effective forage fibre. Cows not receiving enough effective forage fibre can lead to subclinical acidosis (SARA). When rumen acidosis occurs, resident microbe function is often inhibited, which could lead to not enough ammonia being turned into microbial protein, thus contribute to high MUN values.
3. Ration changes. Using a new bag of lush alfalfa haylage at the same significant level as a more grass-based alfalfa silage may cause a dramatic rise in MUN values. It is a good idea to test all forages, know their nutrient analysis, before implementation.
4. Poor feed mixing and sorting. Inadequate mixing could lead to technical erratic MUN levels in the milk of individual cows (which might not be shown in bulk-tank MUN tests). Sorting by milk cows could lead to a similar situation. In contrast, excessive mixing of the ration could cause effective fibre breakdown, SARA conditions, and excessive MUN tests.
5. Sampling and analytical errors. An automatic infrared instrument is used to measure MUN. Organic molecules such as protein, fat and urea molecules in normal cow’s milk have a unique spectrum when heated and analyzed for accurate results. Very high fat and protein component levels in milk as well as high somatic cell counts have been shown to produce erroneous MUN results.
While there may be occasional blips in MUN tests, there is no need to panic. Even long-term MUN values can be adjusted. It is a matter of noting undesirable MUN in the lactation herd and taking corrective action as needed. †