D A I R Y I n i t i a t i v e s N E W S L E T T E R
V o l u m e 8 I s s u e 2 F a l l 1 9 9 9

Don't Waste Your Protein

SANDRA GODDEN, College of Veterinary Medicine, University of Minnesota
JIM LINN, Department of Animal Science, University of Minnesota

Feeding excess protein to dairy cows is expensive and wasteful. Protein or nitrogen fed in excess of requirements for body maintenance, milk production, growth, and reproduction must be excreted. Excretion of excess nitrogen from the body means loss of income and is of environmental concern. Milk urea nitrogen (MUN) is an inexpensive and easily conducted test to help assess protein feeding to dairy cows.

Where MUN Comes From

The crude protein (CP) you feed your cows has three fates:

It can be broken down by the microorganisms in the rumen to ammonia.
It can bypass the rumen and be broken down into amino acids and absorbed from the small intestine.
It can bypass the cow entirely and be excreted in the manure.

Some of the ammonia released in the rumen from CP breakdown can be recaptured by the microorganisms to form bacterial CP. Bacterial CP then leaves the rumen and is digested in the small intestine, the same as the true proteins in feeds that bypass the rumen.

Ammonia not recaptured by the microorganisms is absorbed into the blood, where it is transported to the liver for conversion into urea. Amino acids and other small protein fractions absorbed from the small intestine in excess of requirements are also converted to urea in the liver. Urea is a nontoxic compound for handling excess nitrogen in the body until it can be filtered through the kidney and excreted in the urine.

Urea in the blood, referred to as blood urea nitrogen (BUN), also has three potential fates:

It can be recycled back to the rumen through saliva and directly through the rumen wall. Because urea is a relatively small molecule and has a high affinity for water, it rapidly flows back into the rumen, where it can be converted into microbial protein.
It can be removed by the kidney and excreted in the urine. The amount of nitrogen or urea in the urine is directly proportional to the concentration of BUN.
Because urea has a high affinity for water, as BUN passes through the mammary gland, it readily diffuses into milk to achieve an equal concentration of urea between blood and milk. Milk urea nitrogen (MUN) levels are similar to BUN levels, but usually lag behind peak BUN values by about two hours.

Measuring MUN

Urea can be measured in either blood or milk. However, it's cheaper and easier to collect milk samples than blood samples. Milk samples may be submitted as bulk tank or individual cow samples. The difference is that a bulk tank sample doesn't allow for evaluation of groups fed different rations. Given the relatively low cost of analysis, we recommend that MUN be analyzed for individual cows. MUN may be measured in milk samples routinely collected on DHI test day. Test costs in Minnesota range from 10 to 15 cents per cow, depending on which lab you use and the test method used. Non-DHI samples sent to DHI labs for analysis may cost approximately 45 cents per cow sample (includes butterfat, total protein, and MUN) or $5.00 for a bulk tank sample (includes butterfat, total protein, and MUN).

Interpreting MUN Results

Because MUN levels are affected by a large number of cow-related factors, including age, stage of lactation, health status, water consumption, and dry matter intake, cows fed the same ration often have very different MUN values. For this reason, we recommend you use MUN results from a minimum of 10 cows for diagnosing potential feeding problems. Don't try to draw conclusions from one or just a few cows' MUN values.

There is diurnal (day-to-night) variation in MUN, with p.m. tests often averaging 1 to 2 mg/100 ml higher than a.m. tests. This factor may create considerable test-to-test variation, making it difficult to know when important changes in feeding management have occurred. Test routinely to establish a baseline so you know what is normal variation for the herd.

Herd or group average MUN levels for Holstein, Ayrshire, and Guernsey herds are commonly in the range of 10 to 18 mg/100 ml. Jersey and Brown Swiss breeds tend to run about 2 mg/100 ml higher. New York and Pennsylvania data suggest a herd average target range of 10 to 16 mg/100 ml. While these averages may be used as a reference for interpreting MUN values, it should be understood that the MUN value for optimum milk production and health of an animal has not been defined. Dairy producers and their nutrition advisors should establish a baseline for their herd and monitor changes in MUN to determine if feeding changes have occurred or problems exist.

Ways to Use MUN

Use MUN to assess the amount of protein in the diet. A high MUN value may mean you're feeding more protein than the cows require for their production level.

If the amount of protein fed is correct, a high MUN value could mean you're feeding excess rumen degradable protein--urea, soybean meal, alfalfa haylage, or other readily degradable sources of protein. Replace some of the highly degradable rumen protein feeds with a slower or less degradable protein source.

A third option is to evaluate the amount of rumen degradable carbohydrate sources in relation to degradable protein sources. If the rumen bacteria do not have a readily available source of carbohydrate when ammonia is being produced from the breakdown of feed proteins, the bacteria will not be efficient at capturing ammonia and converting it into bacterial protein.

Another use of MUN is to evaluate the amount of urinary nitrogen (UN) being excreted. Maryland researchers found the amount of UN can be predicted from MUN by the following formula:

UN (grams/day) = 12.54 x MUN

For example, if MUN is 16 mg/100 ml, cows are excreting 12.54 x 16 = 200 grams of nitrogen/day or 1,254 grams of crude protein (200 x 6.25). A cow producing 80 pounds of milk requires about 3,650 grams of crude protein per day, so at this level about 34 percent of the protein consumed is being excreted in the urine. If MUN were decreased to 13, only 28 percent of the diet protein would be excreted each day.

Summary

Single MUN tests are relatively useless in solving feeding problems. Establish a MUN baseline over four to six months and evaluate changes in relation to the baseline values. If conducted routinely, and if a good baseline level for MUN has been established in the herd, MUN tests may be a management tool to improve protein utilization efficiency in dairy cows, decreasing feed costs and nitrogen excretion.

Top Tips for Controlling Costs

Who knows best what a dairy farmer can do to keep costs down? A dairy farmer who's done it, that's who!

If you're on the Internet, you can share your tips and learn what works for others through the new "Producer Tips" page that's part of Minnesota's online dairy network, www.minnlink.org. To submit a tip, e-mail it to support@minnlink.org. To review tips, simply go to the Web site and click on "Producer Tips."

If you're not on the Internet, send your ideas to Dairy Initiatives Newsletter (see address on back page) and we will add them to the page for you. We will also share a few of the top tips in the next issue of Dairy Initiatives Newsletter.

MinnLink is a resource for Minnesota dairy farmers and other dairy industry personnel created with support from the Dairy Leaders Roundtable, the Dairy Council of the Upper Midwest, and the Central Minnesota Dairy Advisory Team. It offers dozens of links to Web sites for dairy-related news, education, government information, weather, databases, and much more. Check it out!

www.minnlink.org

D A I R Y I n i t i a t i v e s N E W S L E T T E R
Volume 8 Issue 2 Fall 1999