Milk or blood urea nitrogen is indicator of diet adequacy and nitrogen utilization efficiency in lactating dairy cattle.
As a management tool for dairy farmers, MUN offers a simple and noninvasive approach to examine protein status of rations fed to dairy cattle.
Through routine monitoring of MUN, dairy farmers can adjust dietary protein levels to better match protein requirements of their cows and potentially increase profitability by reducing feed costs.
Milk urea nitrogen also is an effective means to estimate nitrogen excretion from lactating dairy cattle. MUN can be used to assess the impacts of excess nitrogen feeding to dairy cows in a watershed.
Condensed by Milkproduction.com staff from a paper originally presented at the Florida Ruminant Nutrition Days, 2007 by Rick Kohn, Department of Animal and Avian Sciences, University of Maryland
Reducing N excretion is the best way to reduce N loss (runoff, volatilization and leaching) from dairy farms. The use of milk or blood urea nitrogen to identify inefficiencies in protein nutrition and estimate nitrogen excretion is reviewed here.
Cows absorb N into their blood stream by the diffusion of ammonia across the rumen wall and transport of amino acids and peptides from the small intestine. Ammonia is toxic to the cow and is rapidly converted to urea in the liver. Absorbed amino acids and peptides not utilized for milk synthesis are deaminated in the liver for energy, and then converted to urea. This urea becomes part of the blood urea N pool. The blood urea N pool has three ultimate fates: recycling, secretion in milk, or excretion in urine. Recycling of urea via saliva, and across the rumen wall, can provide N for rumen microbial protein synthesis. Urea is also filtered from the blood by the kidney and excreted in urine. Blood flow through the kidney is constant within an animal, which ensures a constant blood filtration rate regardless of urine volume (Swensen and Reece, 1993).
As milk is secreted, urea diffuses in and out of the mammary gland, equilibrating with blood urea. Because of this process, milk urea nitrogen (MUN) is proportional to blood urea N (Roseler et al., 1993; Broderick and Clayton, 1997)), and total urinary N excretion is linearly related to MUN (Ciszuk and Gebregziabher, 1994; Jonker et al., 1998). MUN may be used to monitor nutritional status of lactating dairy cows and improve dairy herd nutrition. Several researchers have explored the relationship of MUN to dietary protein and energy. Variation in MUN has been suggested to be related to the protein to energy ratio of the diet consumed (Roseler et al., 1993). The concentration of MUN changed only slightly with N intake when the protein to energy ratio was held constant, but increased with an increase in this ratio.
A reanalysis of data from 35 conventional lactation trials found MUN was affected by protein to energy ratio. However, MUN was not affected by total energy (Mcal/d), non-protein N intake (g/d), dietary concentration of energy (Mcal/kg), or neutral detergent fiber (%) (Broderick and Clayton, 1997).
With adequate dietary energy, MUN indicates protein status. MUN concentration increased when different forms of protein were fed in excess of National Research Council recommendations with no difference in milk production (Roseler et al., 1997). Conversely feeding protein below recommendation reduced MUN concentration and milk production because N was limiting in the diet. High levels of readily degraded protein are reported to increase MUN concentrations (Baker et al., 1995).
Predicting urinary and fecal N, intake and utilization efficiency
Milk production per cow, milk protein percentage, and MUN can be entered in a mathematical model to estimate urinary and fecal N excretion, N intake, and N utilization efficiency of lactating dairy cows (Jonker et al., 1998). Urinary N is predicted from MUN. Originally, urinary N (g/d) was predicted as 12.54 times MUN (mg/dl) for typical Holstein cows. However, urinary N for smaller breeds was being under predicted by the model.
In 1998, Dairy Herd Improvement Association laboratories changed the way standards were derived in the US. As a result, reported MUN values decreased by an average of 4 mg/dl (Kohn et al., 2002). By accounting for body weight and the new MUN analysis, urinary N (g/d) can be predicted as .026 times body weight (kg) times MUN (mg/dl) for any breed of dairy cow (Kauffman and St-Pierre, 2001; and Kohn et al., 2002).
The proportion of N absorbed in the body, although not that excreted in feces, is consistent across various types of feedstuffs (Jonker et al.,1998). Therefore, assuming that most N is either secreted in milk or urine by mature dairy cows, N intake (g/d) can be predicted as: (urinary N (g/d) + milk N + 97) / 0.83. The endogenous losses are represented as 97 g/d and the fraction of feed N digested is assumed to be 0.83. Since all N consumed by mature (not growing) cows must eventually leave the animal, fecal N can be predicted as intake N – urinary N – milk N. Finally, N utilization efficiency for mature cows is equal to milk N times 100 and divided by N intake. This model was evaluated using data from several published research studies.
Target MUN concentrations
NRC (1989) dietary N recommendations and typical lactation curves for daily milk production, milk fat percentage, milk protein percentage, and body weight change were used to determine target MUN concentrations (Jonker et al., 1999). Using the modification recommended by Kauffman and St-Pierre (2001) and Kohn et al. (2002) suggests a peak MUN concentration of 11.6 mg/dl occurred at 78 days in milk (DIM) for a 10,000 kg per year lactation. Higher average milk production increases target MUN levels. Mean MUN (weighted by milk production) for a 12,000-kg lactation was 12.7 mg/dl with a peak MUN concentration of 14.5 mg/dl occurring on day 76. Milk production drives the N requirement of lactating dairy cows. As milk production increases, when cows are fed according to NRC recommendations, predicted MUN concentrations increase linearly because of higher N intake and N excretion. Subsequently, target MUN concentrations are extremely sensitive to changes in milk production.
Target MUN concentrations were much less sensitive to changes in milk fat and protein percentages, body weight, and parity (Jonker et al., 1998). MUN was lower in milk from Jersey cows compared with milk from Holstein cows (Rodriguez et al., 1997). These differences were likely due to five factors: body weight, milk production, milk fat and protein percentage, and N intake. Renal clearance rates and blood volume may increase as animal size increases (Swenson and Reece, 1993) and could affect differences observed between breeds as well.
The target values developed by Jonker apply only to lactating cows weighing 600 kg. Combining the effect of bodyweight on protein requirements with effect of body weight on the relationship between MUN and urinary N excretion enables calculation of target MUN for smaller or larger cattle (Kohn et al., 2002). A Jersey cow with a body weight of 400 kg would be expected to have a mean MUN that is 3 mg/dl higher than a Holstein with a bodyweight of 600 kg for the same production level.
Protein feeding level relative to requirement affects target MUN concentrations the most; MUN is very sensitive to overfeeding protein. Feeding 10% above NRC recommended N intake increases MUN concentration 26% (Jonker et al., 1999). The excess N intake elevated feed costs and urinary N excreted to the environment.
While the model described above provides a precise number for target MUN concentrations, an acceptable range around the target exists. Under typical production conditions, most dairy farms should have MUN concentrations between 8 to 12 mg/dl.
MUN Pilot project
A field study with 1156 dairy farms in Maryland and Virginia (USA) analyzed MUN for six months. The mean and standard deviation in N feeding parameters were calculated from the survey data and the first month’s milk analysis. Nitrogen intake, urinary and fecal N, and N utilization efficiency were determined for each herd using the model of Jonker et al. (1998), as modified by Kauffman and St- Pierre (2001) and Kohn et al. (2002). Crude protein requirements were determined from NRC (1989) recommendations assuming a one-group TMR was fed (Stallings and McGilliard, 1984). The protein required was assumed to be that needed by the 83th percentile cow with respect to protein requirements for the entire milking herd. This approach prevents under feeding of most cows. Excess N feeding was determined as the difference between observed N intake and that predicted to be required.
Observed MUN was 12.7 mg/dl but feeding according to NRC (1989) for the 83nd percentile cow would have resulted in a MUN of 11.0 mg/dl. Survey farms fed 6.6% more N than recommended by NRC and this overfeeding resulted in a 16% increase in urinary N and a 2.7% increase in fecal N compared to feeding to requirements. Most (71.5%) farms appeared to feed more than recommended amounts of protein with an average excess of 61 g/d or 11% of required N. Urinary N excretion ranged from 143 g/d for the 17th percentile herd to 247 g/d for the 83rd percentile herd. Similarly, herd efficiency ranged between the same percentiles from 24.5% to 32.3%. The tendency to overfeed N and herd N efficiency was not associated with herd size (P > 0.1).
MUN increased in the spring when lush pastures high in protein were available.
As was hoped, feedback on herd MUN levels encouraged farms that appeared to be underfeeding protein to increase protein feeding during the course of the program. Conversely, farms that appeared to be overfeeding protein appeared to decrease dietary protein over the 6 month survey period.
Economic and environmental impact of over feeding protein
The environmental and economic impact of overfeeding dairy herds in the Chesapeake Bay (Virginia and Maryland, USA) drainage basin were estimated from the field study described above. Seventy one percent of farms fed N above NRC (1989) recommendations for the 83rd percentile cow. This excess N would be excreted in urine. Less than 25% of excreted N is typically available to be recycled to crops, leaving 75% of manure N to be lost to the environment. Thus, 7.6 million kg of N would have been lost to water resources due to overfeeding of N by farmers. This figure represents 7.9% of the total non-point source N loaded to the Chesapeake Bay each year. In addition, crops would be grown to produce this excess feed N, and N losses would result from the fields where these crops were produced. The cost of feeding excess soybean meal in place of corn grain was $32.94 per cow per year, or $17.86 million per year. The potential exists to both increase dairy farm profitability and decrease non-point N loading to the environment by keeping herd MUN levels within targets. However, many dairy farms maintain high production with MUN concentrations lower than the target, indicating a potential for feeding below NRC recommendations and further reducing N loading.
Using MUN for diet evaluation
High MUN levels are often attributed to specific causes, including too much RDP, too little energy, imbalance of carbohydrate and protein ratios, and too much RUP. None of these causes alone determine MUN; high (or low) MUN concentrations depend on a combination of factors. In simplest terms, high MUN concentrations indicate a general excess of intake N relative to the animal’s level of milk production. Excess N might be the result of excess protein. Alternatively, an imbalance of available protein to fermentable carbohydrate may result in energy limiting milk production by the cow. Because of reduced production, the protein cannot be used, and high MUN results.
When average MUN concentration is outside target ranges, the cause needs to be determined. A minimum of 10 cows should be sampled from a management group to determine an average MUN value for that group. Bulk tank samples may save money, but will not show differences among different management groups of cows. The first area to consider when MUN concentrations are outside the target range is milk production. Are the cows producing what they are expected to produce and what the ration is balanced for? If the cows are producing less than expected, excess protein consumption results in elevated MUN levels. The reason for lower milk production needs to be examined. Lower than expected milk production can be caused by management (e.g. too high expectation) or ration formulation (e.g. not enough energy). A next logical step, if milk production is as expected, is to examine the ration formulation. Is the ration formulated to meet all the nutrient requirements of the cow, not just total N?
When ration formulation appears to be correct, nutrient composition of actual feed ingredients may differ from the values used in ration balancing. Poor feed sampling technique and/or frequency may result in incorrect nutrient profiles for feedstuffs.
If feed nutrient content errors do not explain high MUN concentrations, the actual feeding process should be examined. Is the TMR mixed thoroughly? An improperly mixed TMR can result in inadequate distribution of nutrients with some cows getting more than their share. Is the ration being fed according to how it was balanced? Careful attention must be made in order not to over- or under-feed any particular diet ingredient.
If the cause of high MUN level is still not isolated, feed intake levels must be examined. Are the cows eating the expected amount of feed? And, are they sorting out preferred ingredients? Feed leftover at the end of the day should look like the ration which was originally fed. If the cows are able to sort through the ration, concentrate may be consumed preferentially over forage and high MUN levels may occur.
Conditions can exist where MUN levels may actually be low, indicating a protein deficiency in the diet and potentially lost milk production. Low MUN levels suggest the cows’ diet does not contain adequate available protein. Do any of the feed ingredients have heat damage reducing its digestibility? If a dried brewers grain (or other dried byproduct feed) being fed is dark brown, it may have a significant portion of bound protein which the animal is unable to use. If forages were heat damaged during the ensiling or hay preservation process, the protein digestibility may be reduced. This may cause the diet to be low in absorbed protein and may result in a low MUN level. When MUN levels are extremely low, production may be limited because of a protein deficient diet. Suspicious feeds should be analyzed for acid detergent insoluble nitrogen or bound protein.
Using Blood Urea Nitrogen
Blood or plasma urea nitrogen (BUN) can be used in much the same way as MUN. Within a study, BUN concentration was an excellent predictor of urine N excretion per day (Kohn et al., 2005). However, there was considerable variation from study to study. On average for herbivores, urine N could be predicted as 0.013 x BW x BUN. This lower coefficient compared to the one used for MUN results from the higher concentration of BUN as compared to MUN even though both are highly correlated (Roseler et al., 1993; Kauffmann and St-Pierre, 2001).
Milk or blood urea nitrogen is indicator of diet adequacy and nitrogen utilization efficiency in lactating dairy cattle. As a management tool for dairy farmers, MUN offers a simple and noninvasive approach to examine protein status of rations fed to dairy cattle. Through routine monitoring of MUN, dairy farmers can adjust dietary protein levels to better match protein requirements of their cows and potentially increase profitability by reducing feed costs. Milk urea nitrogen also is an effective means to estimate nitrogen excretion from lactating dairy cattle. MUN can be used to assess the impacts of excess nitrogen feeding to dairy cows in a watershed.
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