Milk Urea Nitrogen Testing To Improve Protein Utilization In Dairy Cattle

Agdex#:	414/54
Publication Date:	December 2003
Order#:	03-117
Last Reviewed:	December 2003
History:	New
Written by:	Jodi Calberry - Program Assistant/OMAFRA

Table of Contents

1. Background
2. How Often to Test
3. MUN Test Costs
4. Summary
5. References

Background

In addition to being an essential nutrient of all plants and animals, protein is the most expensive nutrient fed to dairy cattle. When protein is consumed it gets broken down into smaller compounds such as peptides, amino acids and ammonia in the rumen. While the peptides and amino acids can be absorbed in the small intestine and used directly by the cow for growth and lactation, rumen microbes can use ammonia for microbial growth and protein synthesis (Van Soest, 1994).

The amount of nitrogen (N) required by the microbes is determined by the amount of available carbohydrate. If dietary protein is fed above the level needed by the microbes, the ammonia will be converted to urea in the liver and excreted in the urine. Feeding more energy will increase the microbes’ need for N and promote the use of excess ammonia (Van Soest, 1994).

A proper balance of protein and energy, or more specifically rumen degradable protein and rapidly fermentable carbohydrate, allows the cow to make the best possible use of protein in the diet. This could mean higher production, lower feed costs, and less environmental impact from N in manure.

As a guideline, 30%–40% of the diet should be represented by non-structural carbohydrate to maintain optimum rumen health and function (Nocek, 1997). It is difficult however to make recommendations for crude protein (CP). The amount of CP to include in the diet will depend on milk yield, milk protein percentage, growth rate, body size, energy content and type, as well as amino acid composition and degradability of dietary protein. Generally, the amount of CP in the diet will need to be increased if the requirement for rumen degradable protein (RDP) is not met (NRC, 2001).

Monitoring urea nitrogen (UN) concentration is an effective way to determine whether or not dietary protein and energy are in proper balance. There are two ways to test for urea concentrations: from samples of either blood or milk . A blood sample is an invasive and time consuming procedure, while milk urea nitrogen (MUN) can be tested using samples submitted for fat and protein analysis.

The type of dietary protein fed will determine how much ammonia is produced and consequently, the concentration of MUN. Feedstuffs, such as alfalfa hay, canola and oats, which are rapidly degradable in the rumen, result in higher ammonia production. Milk urea nitrogen concentration tends to increase when this type of protein is fed. Feedstuffs that are more slowly degraded in the rumen, such as brewers and distillers grain and corn gluten meal, produce less ammonia, which result in a lower urea nitrogen concentration. See Table 1, Crude Protein Content of Common Feed Ingredients and their Rate of Protein Degradability, for a more detailed analysis. The balance of rumen degradable and undegradable protein is something you should consider when formulating your ration for crude protein content.

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According to Ontario Dairy Herd Improvement (DHI), the normal range for MUN concentration is between 10–16 mg/dl. When MUN concentration is below this range, more degradable protein may be needed to meet the microbial N requirement for protein synthesis. Cows will respond with higher milk yield to this dietary change. When MUN concentration is above the normal range perhaps too much total or degradable protein or too little energy is being fed. Routine feed and ration evaluation is important at this point — knowing the nutrient content of your feed will help determine whether protein or carbohydrate is causing the imbalance. Balancing the ration to meet dietary requirements will improve productivity as well as profitability.

Besides the effect of protein and energy, MUN concentration may also vary by season, month, parity group, stage of lactation and time of sampling. Researchers (Godden et al., 2001) found that concentrations were highest from July to September and in cows between 60–150 days in milk. Milk urea nitrogen was lowest in first lactation cows as well as cows in their first 60 days in milk or after 150 days in milk. The rate of decline between mid and late lactation was larger in animals in their second lactation or greater (Godden et al., 2001).

(Adapted from NRC, 2001)

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Depending on the frequency, time of feeding and when you sample the milk, MUN concentration may change. Concentrations are lower in the morning. Therefore, if you sample at both morning and evening milkings, be consistent in sampling times to ensure an accurate picture of MUN levels, or test regularly to establish a baseline for the herd. As MUN concentrations are at their highest 2–6 hours post-feeding, continuous feeding or feeding with high frequency may eliminate the morning/evening effect (Godden et al., 2001; Gustafsson and Palmquist, 1993).

Remember that milk urea nitrogen concentrations may also increase in the spring as pastures become available. Alfalfa and grass pastures are especially high in readily fermentable nitrogen.

Most farmers test MUN to assist with dietary management. There are some who believe MUN also has benefits for breeding. However, contrary to popular belief MUN may have limited value as a monitoring device for reproductive performance. Research (Godden et al., 2001) has shown that good fertility can be accomplished across a wide scope of MUN concentrations.

How Often to Test

Test at least once a month to establish an effective baseline for the herd. However, retest MUN when:

• there is a change in the ration
• new feeds are used
• feeding from a new silo
• pasture feeding cows.

Currently, 73% of Ontario farmers use DHI services and 18% of these farmers test for MUN.

Generally, sample every cow in the herd. For larger sized herds it is acceptable to test a representative sample of cows based on diet. In this case, sample at least 10% of the cows. Some producers may consider bulk tank sampling rather than testing individual cows. But due to diet, higher producing cows have elevated MUN concentrations. So bulk tank sampling has its limitations.

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MUN Test Costs

The cost of a MUN test varies with location. In Ontario, the 2003 price is $0.25 per test per cow. For example, testing 50 cows every month would cost $150 per year. This price seems quite economical.

Protein is an expensive dietary nutrient required by the cow, as well as the rumen microbes, for growth and production. With the right amount of energy, microbes can make efficient use of the ammonia produced in the breakdown of dietary protein and synthesize microbial protein to help meet the cow’s requirements. But if dietary protein is too high, and ammonia production is greater than microbial need, ammonia will spill over into the blood stream. This process is wasteful of both protein and carbohydrate, as energy is needed in order to detoxify ammonia to urea.

Summary

Ultimately, MUN is a useful tool in monitoring protein utilization in dairy cattle as it acts as a signal to indicate when protein and energy are out of balance. Knowledge of your herd’s MUN levels will help detect nutritional problems, help in meeting protein requirements more precisely while reducing feed costs, and lessen the harmful effect that manure N can have when it is over-applied to the land.

References

1. Godden, S. M., Lissemore, K. D., Kelton, D. F., Leslie, K. E., Walton, J. S., and J. H. Lumsden. 2001. Factors associated with milk urea concentrations in Ontario dairy cows. J. Dairy Sci. 84:107-114.
2. Godden, S. M., Kelton, D. F, Lissemore, K. D., Walton, J. S., Leslie, K. E., and J. H. Lumsden. 2001. Milk urea testing as a tool to monitor reproductive performance in Ontario dairy herds. J. Dairy SCI 84:1397-1406.
3. Gustafsson, A. H. and D. L. Palmquist. 1993. Diurnal variation of rumen ammonia, serum urea, and milk urea in dairy cows at high and low yields. J. Dairy SCI 76:475-484.
4. Nocek, J. E. 1997. Bovine acidosis: implications on lameness. J. Dairy SCI 80:1005.
5. National Research Council (NRC). 2001. Nutrient requirements of dairy cattle. Seventh revised edition. National Academy Press, Washington, DC.
6. Van Soest, P. J. Nutritional ecology of the ruminant. Second ed. 1994. Cornell University Press. United States. 291–303.

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