Milk Urea Nitrogen Testing

Development of the test

Optimising reproductive and productive performance involves balancing the protein to energy ratio in the dairy cow diet. Protein type and energy availability are important. Nitrogen is expensive and is wasted if fed in excess of requirement. In addition, high protein in the diet can negatively affect reproductive performance. Milk urea nitrogen (MUN) tests have been developed as a means of determining how much protein a cow consumes and how well it is utilized. The amount of urea in the milk is directly related to blood urea levels, which led to the development of MUN tests. Blood urea nitrogen levels fluctuate throughout the day according to feeding times, but milk is more stable being collected in the udder usually over a long period of about 12 hours.

High levels of MUN indicate nutritional problems caused by any of the following:

  • High protein levels
  • Protein imbalance with excessive degradable intake protein and high or low intake of bypass protein
  • Amino acid imbalance (Protein synthesis is dependent on there being the right amino acids in the correct quantities being present at the same time.)
  • Excess soluble intake protein,
  • A shortage of readily fermentable carbohydrates to capture the ammonia
  • Poor rumen environment for microbial growth
  • Partial dehydration (High water intakes increase urine production which will reduce MUN values)
  • Heat stress (an increase in environmental temperature from 18 to 28°C resulted in MUN levels rising from 14 to 19.3 mg/dl).
  • A combination of the above.

Low levels of MUN indicate insufficient ammonia in the rumen from too little protein in the diet, which reduces milk and protein yields. Excessive N excretion also has a negative impact on the environment.

Why high MUN is undesirable

1. Effect on production

Dietary protein is converted to urea by micro organisms in the rumen of the dairy cow. This ammonia, along with readily available carbohydrate energy source, is used in the synthesis of microbial protein. When excess protein is consumed, there is insufficient energy and the microbes cannot use the ammonia produced quickly enough. The ammonia is readily absorbed into the blood stream and converted to less toxic urea in the liver.

This conversion requires a lot of energy that could otherwise be used for milk production. It costs the cow 7.3 kcal (0.03 MJ) for every gram of ammonia that is converted to urea. It is estimated in New Zealand that the energetic costs of converting ammonia to urea and the low carbohydrate availability associated with high protein pasture could decrease milk yields by 11 l per day when ryegrass protein content increased from 20 to 35% of herbage dry matter. This urea is mostly excreted in the urine, but some enters the saliva, uterine fluid and milk.

2. Effect on fertility and health

Reports on the effects of high MUN on cow fertility are controversial. Conception rates have been seen to decrease by 10 to 15% with MUN levels are greater than 19 mg/dl. High MUN may not affect oestrus, but pregnancy rates are higher in cows with normal MUN test results. Travaskis & Fulkerson (1999) found no relation between high MUN and poor reproductive performance. However, fertility and embryo development are generally poor in high producing cows, as they are in a negative energy balance in early lactation.

The extra protein fed to early lactation cows, that battle to consume enough dry matter to meet their requirements for amino acids, may also affect reproduction and reduce fertility by altering the uterine environment and impairing the survival of sperm, ova and embryos. High protein concentrates are fed to cows in early lactation because cows mobilize body fat as an energy source and therefore need that extra protein. Cows on pasture have been found to have MUN values greater than 25 mg/dl. In theory, this should have a negative effect on reproduction, but cows on pasture are no harder to breed than cows that are not on pasture.

Diets with high protein concentrations can lead to health problems. Excess protein may reduce the cow’s resistance to infection or ability to recover from reproductive problems.

Effective and economical MUN testing involves:

1. Establishing a baseline MUN for your herd

In order to properly interpret MUN values for a herd, baseline MUN values collected over several months should be established. Changes in rations and hence MUN values can be compared to the baseline values. The values should also not be interpreted on their own but in conjunction with the whole feeding program on the farm including forages, concentrate and water intake. Cows that are tested should be healthy. All cows in small herds and 10% of a large herd should be tested. Individual samples from cows are more useful than samples taken from the bulk tank in tracking down problem causes, although values form individual cows are highly variable.

2. When to collect samples

Samples should always be taken at the same time. High producing cows often have higher MUN levels than lower or late lactation cows. Once the baseline has been established, testing should be done every three months or if there has been a major ration change, (when cows go onto pasture, when new forage is used). Changes in the processing of raw materials also have an effect. Reducing particle size and increasing the moisture content of grains can alter rumen protein and carbohydrate use. If conception rates drop and dung consistency changes, MUN can be a useful test for diagnosing the problem. Loose dung and a strong smell of ammonia in the parlour indicate excess protein or insufficient available carbohydrates in the diet.

3. Interpretation of results

According to Cornell University, herd averages should be between 12 and 18mg/dl and individual cow values between 8 and 25mg/dl. A decision to change the feed should be based on herd average results. Individual cow results are useful for identifying cows with digestive problems. Cows with high SSC or altered milk solid components can give unreliable MUN test results. MUN is affected by stage of lactation. In a trial done at the Oklahoma State University, it was found that MUN concentrations increased during early lactation, levelling of after three weeks. There is a tendency for butterfat and protein to be lower when MUN values are higher than 18 mg/dl. MUN levels are higher in spring than other times of the year, particularly for cows grazing ryegrass, which can have crude protein values in excess of 30%. Cows grazing kikuyu pasture can have a significantly higher level of MUN (43mg/day) than when grazing ryegrass (35mg/day).

Benefits from testing MUN

  • An ability to better meet dairy cow nutritional requirements
  • Lower feed costs through minimising wastage
  • Increased reproductive performance
  • Increased milk protein yield
  • Nitrogen excretion into the environment is minimised.

(Hutjens & Barmore 1995); (Travaskis & Fulkerson 1999); (Dugmore 2002); (Grant 2003); (Brinton & Hopkins 2003); (Stallings, 2003); (Ferguson 2003); (Muroya et al. 1997).

Date published: 2004-06-18

Jane Holiday