This fact sheet has been developed to support the implementation of the Natural Resources Conservation Service Feed Management 592 Practice Standard. The Feed Management 592 Practice Standard was adopted by NRCS in 2003 as another tool to assist with addressing resource concerns on livestock and poultry operations. Feed management can assist with reducing the import of nutrients to the farm and reduce the excretion of nutrients in manure.
The Natural Resources Conservation Service has adopted a practice standard called Feed Management (592) and is defined as “managing the quantity of available nutrients fed to livestock and poultry for their intended purpose”. The national version of the practice standard can be found in a companion fact sheet entitled “An Introduction to Natural Resources Feed Management Practice Standard 592”. Please check in your own state for a state-specific version of the standard.
Milk processing plants and DHI can provide dairy managers with milk urea nitrogen (MUN) values on bulk milk and individual cow milk samples. Milk Urea Nitrogen is a useful tool that can allow dairy managers to monitor changes in the feeding and management of their herds. The following points can allow you to interpret MUN test results from your herd.
Milk urea nitrogen is the fraction of milk protein that is derived from blood urea nitrogen (BUN). In Holstein’s, MUN normally represents about 0.19 percentage points of the normal 3.2% total milk protein.
Casein and/or whey proteins that contribute amino acids for human use or cheese production are not included in MUN values. Average MUN values will range from 10 to 14 milligrams per deciliter (usually reported as a whole number such as 12). When cows consume feed containing protein, If bacteria cannot capture the ammonia and convert it to microbial protein, the excess ammonia is absorbed part of the protein is degraded to ammonia by rumen microbes (rumen degraded protein or RDP). across the rumen wall. Because ammonia can shiftblood pH, the liver converts ammonia to urea to be excreted or recycled. Urea diffuses freely across cell membranes, therefore MUN concentrations represent blood urea concentrations. Thus, if BUN values are elevated, MUN will be elevated. If MUN values are high, your herd is possibly wasting feed protein along with excreting excess nitrogen into the environment. If MUN values are too low, the rumen bacteria yield can be reduced thereby limiting milk production and milk protein yield.
The key factor is providing adequate rumen available carbohydrates to provide the energy for the rumen microbes to convert ammonia into microbial protein. The following feeding situations could lead to higher MUN values in your herd.
Every herd can have a different optimal MUN depending on the time of feeding relative to milking time, total mixed rations (TMR) compared to component-fed herds, cow eating patterns, and other factors that affect BUN values. The power of a MUN tests is to monitor changes in feeding and management programs within a herd.
If the rumen does not maintain a minimum level of ammonia, milk yield and milk protein yield may drop because of reduced microbial protein synthesis. If your herd MUN is low, consider adding supplemental protein, different protein sources and/or other ration change and then monitor your herd for changes in MUN concentrations.
Herd MUN values are similar to herd somatic cell counts when interpreting results. DHI processing centers may provide MUN group averages summarized by lactation number, days in milk, and milk production. Pennsylvania workers recommend a minimum of 8 to 10 cows per group in order to calculate an unbiased group MUN value. There are a number of factors that can influence your MUN values. These include:
AM-PM samples – The AM MUN value is usually lower than PM samples taken from the same herd. When comparing MUN values in your herd between months, be sure to account for differences in sampling times.
MUN is one tool to evaluate ration protein and energy status. Remember that MUN’s can be impacted by heat stress (MUN values are higher in the summer). Evaluate the following management factors along with herd or group MUN values.
Wisconsin workers have developed an equation to predict the loss of nitrogen based on body weight and MUN values. Other equations are also available and could be used.
Urinary excretion of nitrogen = Body weight x 0.0129 x MUN (mg/dl)
Two examples are calculated below using a low (10 mg/dl) and average (14 mg/dl) MUN’s.
1500 lb Holstein cow x 14 MUN x 0.0129 = 271 grams of urinary nitrogen 1500 lb Holstein cow x 10 MUN x 0.0129 = 194 grams of urinary nitrogen
The difference of 77 grams represents a loss of one pound of dietary protein or 2.2 lb of soybean meal plus the added environmental risks of disposing of the urinary nitrogen. This is equal to about 52 lbs. of N excreted per cow during a 305-day lactation.
Jonker, J.S., R.A. Kohn and J. High. 2002. Use of milk urea nitrogen to improve cow diets. J. Dairy Sci. 85:939-946.
Kauffman, A.J. and N.R. St-Pierre. 2001. The relationship of milk urea nitrogen to urine nitrogen excretion in Holstein and Jersey cows. J. Dairy Sci. 84:2284-2294.
Nousiainen, J., K.J. Shingfield and P. Huhtanen. 2004. Evaluation of milk urea nitrogen as a diagnostic of protein feeding. J. Dairy Sci. 87:386-398.
Wattiaux, M.A., E.V. Nordheim and P. Crump. 2005. Statistical evaluation of factors and interactions affecting Dairy Herd Improvement milk urea nitrogen values in commercial Midwest dairy herds. J. Dairy Sci. 88:3020-3035.
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This fact sheet reflects the best available information on the topic as of the publication date. Date 6-20-2007
This Feed Management Education Project was funded by the USDA NRCS CIG program. Additional information can be found at Feed Management Publications.
This project is affiliated with the LPELC.
Extension Dairy Specialist
University of Illinois, Urbana
Larry E. Chase
Extension Dairy Nutritionist
Cornell University, Ithaca, NY
Dave Casper – Agri-King, Inc.
Jim Drackley – University of Illinois