Use of BUN and MUN as Guides for Protein and Energy Supplementation in Cattle

Monitoring of Bun of Mun is a technique that can be used for measuring protein and energy status in cattle from biological samples obtained at strategic times relative to production cycles, feeding changes, and seasonal availability offorage. These indicators should be used as an adiunct to other measures such as body weight and body condition score that reflect the integrated effects of nutrition over time. On the other hand, metabolic indicators such as BUN or MUN, are used to assess shortterm or real-time changes in nutritional status. The focus of this review will be on protein and energy, and will attempt to provide information on assessing which of these two nutrients are limiting particularly when cattle are consuming tropical forages of varying quality. 

How BUN and MUN Work

Digestible protein in the diet of ruminants is either degraded in the rumen or escapes to the abomasum and small intestine where it is degraded to amino acids and small peptides then absorbed into the portal blood system. Nitrogen from protein that is degraded in the rumen is used for microbial protein synthesis either by incorporation of free amino acids or small peptides liberated by the process of proteolysis or by incorporation of ammonia nitrogen that arises from deamination of amino acids. Nonprotein nitrogen (NPN) such as urea also can be made into ruminal microbial protein following enzymatic conversion or breakdown of the NPN to ammonia in the rumen. Yield of microbial protein produced in the rumen is maximized when the ratio of available energy (fermentable organic matter) to protein (nitrogen) is optimized. 

When there is an excess of nitrogen relative to energy in the rumen, ruminal ammonia concentration increases. Unused ruminal ammonia enters the portalblood through the rumen wall and is transported to the liver where it is detoxified by conversion to urea. The liver also produces urea from ammonia derived from deamination of amino acids arising from postruminal digestion and systemic protein turnover. Urea then circulates in the blood to the kidneys and is excreted with the urine or it can diffuse from the blood back into the rumen, into saliva and back into the rumen, or diffuse from the blood into milk in the case of lactating females. When there is a deficiency of dietary protein, ruminal ammonia concentrations are relatively low and the proportion of nitrogen recycled back to the rumen as urea is increased. As a result of these metabolic transactions, BUN is highly correlated with ruminal ammonia (Thornton, 1970; Hammond, 1983a; Hennessy and Nolan, 1988), and MUN is highly correlated with BUN (Roseler et al., 1993; Baker et al., 1995; Butler et al., 1996). Therefore, in healthy ruminants BUN and MUN concentrations are indicative of the protein to energy ratio in the diet (i.e., DOM:CP ratio), and factors that are reported to affect BUN concentrations can be taken to have a similar effect on MUN concentration 

When energy intake is held constant, increasing dietary protein increases BIJN concentrations. Isocaloric semipurified diets with three levels of CP were fed to steers at equal intakes of 5 kg/day (Hammond, 1983ab). Mean BUN concentrations increased from 2.6 mg/dL to 11.1 mg/dL as dietary CP increased from 6 to 18% of the diet. For growing steers, BUN levels between 11 and 15 mg/dL were associated with maximum rates of gain (Byers and Moxon, 1980). With finishing steers, maximum performance was associated with BUN concentrations of 7 to 8 mg/dL (Preston et al., 1978). Balanced diets for lactating dairy cows were associated with average BUN concentrations of 15 mg/dL (Roseler et al., 1993) and average MUN concentrations of 15 to 16 mg/dL (Baker et aL., 1999). 

Increased solubility or degradability of dietary protein can lead to increased ruminal ammonia concentrations resulting in increased BUN concentrations. In steers fed isocaloric diets that differed widely in nitrogen solubility, there was an average difference in BUN of over 6 mg/dL (Hammond, 1983ab). Also, steers fed corn-cottonseed hull diets had higher BUN concentrations when supplemented with urea than when supplemented with soybean meal (Burris et al., 1975). In lactating dairy cows, an imbalance of degradable and undegradable intake protein increased BUN and MUN (Roseler et al., 1993). Similarly, an imbalance of ruminally degradable and ruminally undegradable protein increased BUN and MUN in lactating dairy cows, but this increase was not as great as an increase in BUN and MUN caused by excess CP (Baker et al., 1995). However, varying dietary nitrogen solubility by varying source of dietary nitrogen has not always resulted in altered concentrations of BUN. In a study with yearling steers fed corn silage diets and supplemented with soybean meal or urea to meet 85 or 100% of requirements for CP, BUN concentrations did not differ between sources of protein (Cross et a1., 1974). Likewise, feeding lactating dairy cows 16% CP diets containing soybean meal or formaldehyde-treated soybean meal did not significantly affect BUN concentrations (Folman et al., 1981).

Increasing dietary energy intake while holding protein intake constant would be expected to decrease BUN. This was demonstrated in an experiment with bulls where diets were formulated and rationed to provide 75 or 150% of maintenance energy requirement but equal CP intake (Chase et al., 1993). At the high level of energy intake BUN averaged 5.6 mg/dL and at the low level of energy intake BUN averaged 9.7 mg/dL. The effect of increased level of intake on BUN concentration appears to be similar to the effect associated with increased energy intake. Steers on both high quality (legume hay, 24% CP) and low quality (grass hay, 45% CP) forage diets had decreased BUN with increased intake (Vercoe, 1967). Increased frequency of feeding has been associated with lower BUN (Thomas and Kelly, 1976), probablydue to more efficient use of nitrogen in the rumen.

Dietary and Nutritional Factors Affecting BUN and MUN in Cattle under Grazing Conditions of Fed Ad Libitum Amounts of Forage

Less work has been reported with regard to BUN and MUN in grazing cattle than in cattle under controlled feeding situations. Predicting response to protein or energy supplementation in grazing cattle using BUN as a guide is complicated by the fact that forage intake is not known and varies among animals. Although neither energy nor protein intake is known in free-grazrng cattle, the protein to energy ratio of the diet should be reflected in BUN concentrations. 

A similar trial was conducted inAustralia (Hennessy andWilliamson, 1990) using steers and heifers fed hay from a pasture that was predominantly carpetgrass (Axonopus ffinus). The basal hay diet (5.3% CP) resulted in an average BUN concentration of 2.1 mg/dlL that was increased to 10.5 mg/dL with urea supplementation or 8.5 mg/dL with protected casein supplementation. These increases in BUN were associated with significant increases in ADG (from .1 kg/day on hay alone to .3 kg/ day on hay supplemented with urea, and to .6 kg/day on hay supplemented with protected casein). These data suggest that the nitrogen supplemented in the less degradable form of protected casein was more efficiently used, resulting in a grater increase in ADG and a lesser increase in BUN compared to supplementation with urea.

Other dietary factors that affect the efficiency of protein utilization also can affect BUN concentrations. Addition of sulfur to diets of sheep and cattle deficient in sulfur resulted in decreased BUN concentrations associated with increased animal performance (Kennedy and Siebert, 1972). Factors not already discussed that may affect BUN concentrations other than diet include health of the animal, physiological state, use of growth promotants, and breed. The magnitude of differences caused by these factors, except for certain disease conditions, is generally less than the dietary factors presented above, but the differences can be significant. Severe nutritional depletion as a result of prolonged under nutrition (Ward et al.1992; Hayden et al., 1993) or disease can cause catabolism of tissue protein and result in high concentrations of BUN. Renal disease can interfere with excretion of urea and also result in high BUN concentrations. In dairy cows, BUN increased as cows progressed from the dry stage through early lactation and the lactating pregnant period, and BUN increased with increasing age (Peterson and Waldern, 1981). In beef cattle, the use of growth promotants generally decreases concentrations of BUN (Preston et a1.,1978; Galbraith, 1980; Eisemann et al., 1989). Use of feed additives such as monensin to increase feed efficiency has resulted in no change (Steen et al., 1978) or small increases in BUN (Raun et a1.,1976; Thompson and Riley, 1980). We have observed lower concentrations of BUN in Hereford cows compared to Senepol cows (Hammond et al., 1992) and lower concentrations of BUN in Angus bulls compared to Senepol bulls (Chase et al., 1993) suggesting differences in protein utilization between breeds. Higher BUN in Shorthorn x Hereford cattle compared to Africander cattle was associated with a trend toward lower nitrogen balance in the Shorthorn x Hereford (Vercoe and Frisch, 1970). Others have observed lower concentrations of BUN in Hereford compared to Brahman cattle (Hunter and Siebert, 1985), lower BUN in Angus compared to Brahman (Olbrich, 1996), and lower BUN in Angus x Hereford cattle compared to Brahman crosses (Coleman and Frahm, 1987).

Approaches and Applications for Using BUN and MUN

The most common application of the use of BUN and MUN is as a retrospective diagnostic tool to analyze biological responses to protein or enerSy supplementation, change in pasture or forage on offer, or change in pasture management. In a two year study (Hammond et al., 1992), we monitored BUN in cows and heifers wintered on bahiagrass hay and supplemented with molasses and rhizoma peanut-grass hay or molasses and a 20% crude protein (CP) range cube. Rhizoma peanut-grass hay fed at the rate of 2.3 kg/day or feeding .9 Kg/day of the range cubes resulted in similar BUN concentrations and performance. More importantly BUN concentrations indicated that we did not initiate protein supplementation early enough in the winter (average BUN of 3.7 mg/dL in early January) and that we supplemented with protein longer in the spring than necessary (average BUN of 16.3 mg/dL in late April). Appropriate strategic changes in our winter supplementation program have subsequently increased the efficiency with which we use winter supplements.

Increasing concern over the environment may create yet another application particularly for MUN. As producers are encouraged to increase efficiency of nitrogen utilization from the perspective of minimizing release of excess nitrogen into the environment, measuring average herd MUN by sampling from the bulk tank may be a way of monitoring efficiency of nitrogen utilization on a whole farm basis.

Technical Notes

Often, BUN is used as a generic term for plasma, serum or whole blood urea nitrogen, although small differences in values can occur depending on the sample matrix (Hammond, 1983b; Hammond, 1992). Urea in milk can be measured with typical analytical procedures used for BUN. In our laboratory, we use an automated colorimetric procedure based on the diacetyl monoxime method (Marsh et al., 1965) that includes a dialysis step to remove proteins for the analytical stream. If a manual method of this colorimetric procedure is used, a deproteinization step is required. Alternatively, urea can be measured by analyzíng ammonia concentration before and after incubation with urease. Ammonia can be quantified by titration or colorimetrically. With milk samples, it is also necessary to remove fat by centrifugation. 

Currently, preservatives are used in milk samples but this may not be necessary if samples are kept cool or frozen (Miettinen and Juvonen, 1990; Butler et al,. 1996). A commercial dipstick (Azotest", Compagnie Chimique d'Aquitaine, Lalande de Pomerol, France) has been evaluated and shown to give only semiquantitative results (Butler et al., 1996). Milk testing laboratories in the U.S. and some other countries are beginning to offer MUN analysis along with other routine milk tests. For laboratories such as these with large numbers of samples analyzed daily, automated infrared instrumentation is available. 

Sampling time is an important consideration for BUN and MUN. Peak BUN concentrations occur several hours after feeding (Thomas and Kelly 1976; Hammond 1983b; Elrod and Butle, 1993; Gustafsson and Palmquist, 1993), and changes in MUN lag behind changes in BUN by about to 2 hours (Gustafsson and Palmquist, 1993). In beef cows that were supplemented with cottonseed meal tlvice a week (Monday and Thursday ), BUN ranged from a high of 14 mg/dL in the afternoon after supplementation to a low of 7 mg/dl 2 days after supplementation (Hammond and Chase, 1996). With frequent feeding, however, there is little diurnal (Folman et al., 19811) or prandial (Thomas and Kelly 1976) variation in BUN. To control diurnal and prandial variation, sample just prior to feeding in fed or supplemented cattle or early in the morning in cattle on pasture. When this is not possible, it is important to be consistent with regard to sampling time if results are to be compared. When sampling milk, strip samples from a single quarter and composite samples have given similar results (Gustafsson and Palmquist, 1993; Butler et a1.,1996), and strip samples before and after milking are also similar. Milk samples from a bulk tank can give a generalized indication of the entire herd weighted by the production contributed by each cow. 

High BUN or MUN and Decreased Reproductive Efficency

High dietary protein (nitrogen) intake resulting in BUN or MUN of greater than 19 to 20 mg/dL has been associated with an altered uterine environment and decreased fertility (reduced conception rate, decreased pregnancy rate) in Iactating dairy cows and heifers (Elrod and Butler, 1993; Elrod et al., 1993; Ferguson et al., 1993; Butler et al., 1996). However, high protein intake and high BUN have not always been associated with reduced reproductive efficiency (Caroll et al., 1988) and is therefore not pathognomonic. Also, there is an energy cost associated with the conversion of excess ammonia to urea by the liver, and this is at the expense of energy use for other productive purposes. 

Portions of this review were presented at the Tropileche Workshop, CIAI, Cali, Colombia (Hammond and Chase, 1996) and the 8th Annual Florida Ruminant Nutrition Symposium (Hammond, 1997); the research conducted at USDA, ARS, Subtropical Agricultural Research Station was in cooperation with the University of Florida, Institute of Food and Agricultural Sciences.

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