NEWS for North Dakotans
Agriculture Communication, North Dakota State University
7 Morrill Hall, Fargo, ND 58105-5665

March 26, 1998

New Tests Look at Milk to Evaluate Dairy Cow Protein Status

Tests for urea nitrogen in milk give dairy producers a new tool to troubleshoot protein-related nutrition problems, notes a North Dakota State University dairy specialist.

The tests measure the amount of nitrogen present in the milk. The nitrogen comes from protein which is derived in the rumen from ammonia and carried to the udder via the blood, explains J.W. Schroeder of the NDSU Extension Service.

If ammonia is not used by rumen microbes by converting it to microbial protein, it is absorbed into the blood. Blood ammonia can shift blood pH and can be toxic to cows. To prevent this toxic effect, the liver converts ammonia to urea and excretes most of it in the urine. But some of the urea is also released into the blood as blood urea nitrogen (BUN), the milk (as MUN) and uterine fluid, or recycled to the rumen by saliva.

MUN tests, available in North Dakota through Heart of America Dairy Herd Improvement Association, measure the portion of nitrogen in the milk in the form of urea. As the level of blood urea nitrogen increases, so does the level of urea in the milk.

"By measuring MUN levels, we have an indication of protein status and urea levels throughout the cow," Schroeder says. He notes that high levels of MUN can reflect several nutritional problems:

• Protein imbalance with high levels of total protein, excessive degradable intake protein, high or low undegradable intake protein, amino acid imbalances or excess soluble intake protein.
• A shortage of rumen fermentable carbohydrate including starch, sugar or fiber to capture available rumen ammonia.
• Poor rumen microbial environment, reducing microbial growth.

Schroeder says that low levels of MUN are also reason for concern because they can reflect inadequate ammonia in the rumen for optimal microbial growth leading to a protein shortage in the cow.

Herd or group levels less than 12 milligrams per 100 milliliters and above 18 milligrams per 100 milliliters can reflect losses of nutrients, higher feed costs, health effects and reduced milk production, Schroeder says. He notes that MUN levels from individual cows can vary greatly, so summarizing 10 or more individual cow MUN values is recommended to give a more accurate picture of a herd's protein status.

If group MUN values are over 18 milligrams per 100 milliliters, several losses can be occurring.

• Significant energy losses occur when a cow must convert ammonia to urea and excrete it in urine.
• If purchased protein (nitrogen) is excreted as urea, the financial loss could be 20 cents per cow per day.
• High levels of BUN can reduce conception rate due to energy loss in early lactation (negative energy balance), change of pH in the uterus and mineral shifts in the uterus.
• High levels of protein can affect cow health and the immune system, especially in cows with metabolic problems.
• High BUN levels have an environmental impact as excessive nitrogen is excreted in manure effecting odor and water quality.

Schroeder recommends conducting MUN tests to establish a baseline value. "Producers can use that value to compare to new test results after rations have been changed or when protein-related problems are suspected," he says. "A review of test results by milk production and feeding groups can reveal patterns that indicate potential problems." He notes that MUN values in cows fresh less than 35 days are variable and difficult to interpret.

DHIA field technicians can provide more information on using this technology. For the name of a DHIA field technician, contact your county extension agent or state dairy specialist.

####

Source: J.W. Schroeder (701) 231-7663

Editor: Tom Jirik (701) 231-9629