Research and Reviews: Dairy

Special Circular 169-99

Effect of Breed and Concentrations of Dietary Crude Protein and Fiber on Milk Urea Nitrogen

A. J. Kauffman and N. R. St-Pierre^{1
The Ohio State University Department of Animal Sciences}

Abstract

This study was conducted to clarify the relationship between urinary nitrogen (UN) excretion and concentration of milk urea N (MUN) to confirm whether this relationship is linear or suggests nonlinearity, and to determine if there is a difference in the relationship between breeds. Eight multiparous cows (four Holstein, four Jersey) were fed four different diets in a 2 x 2 factorial arrangement of crude protein (CP) and neutral detergent fiber (NDF). Levels of crude protein were formulated for a low of 14% and a high of 18%, while NDF levels were formulated for a low of 30% and a high of 40%. No differences were found due to the level of NDF in the diet. Dietary crude protein had a significant effect on yields of milk, milk fat, and milk protein, as well as concentration of MUN. Crude protein also had a significant effect on the nitrogen (N) balance of the animals with effects seen in nitrogen intake, fecal and urinary N excretion, milk N yield, and apparent N digestibility. The relationship between UN and MUN was found to be linear over the range of MUN values observed and was found to be different for the two breeds.

Introduction

The environmental impact of nutrient waste from agriculture is becoming an area of concern as ways to produce more food with less land are sought. The encroachment of residential areas contributes to the declining number of acres for agricultural production and creates an area that is highly sensitive to pollution. Animal agriculture is being targeted as a source of non-point pollution that affects water sources, including streams, ground water, and other bodies of water. One of the three major nutrients included in this issue is nitrogen (N) (Kohn et al., 1997). Nitrogen is released into the environment through the urine and feces of livestock as undigested N fractions and as the end products of N metabolism. High-producing dairy cows are no exception and will contribute greatly to the release of N into the environment.

The importance of proper protein balance in dairy cow rations has long been investigated (Crampton and Harris, 1969). From a production standpoint, the impact is direct through feed costs and indirect through milk response of the cows to protein supply (Roffler et al., 1986). The required levels of rumen degradable protein (RDP) and rumen undegradable protein (RUP) have also been studied, and much progress has been made in identifying optimal levels for maximum daily production of milk. In a mature, non-growing lactating animal, N retention should be near zero, because N intake and output (milk N + fecal N + urine N) amounts are very close to equal. Therefore, the total amount of N ingested must be recovered from the sum of milk N, plus fecal N, plus urinary N, since no other significant means of N utilization or elimination exist.

The end products of protein digestion and catabolism are N-containing compounds, which are the predominant source of N in the waste products of high-producing ruminant animals (Church, 1993). One of these end products is urea, which is primarily taken from the blood by the kidneys and eliminated in urine, although a small amount diffuses from the blood to the milk. Milk urea nitrogen has been proposed as a method to evaluate the protein nutrition of the dairy cow (Roseler et al., 1993; Butler et al., 1995). Butler et al. (1996) suggested that high MUN concentrations are negatively associated with conception rate. Recently, Jonker et al. (1998) proposed a model in which MUN was used as a method to quantify N excretion into the environment. A proper understanding of the relationship between MUN and other forms of N release from the body is critical for MUN to be a reliable indicator of N flow to the environment.

The concentration of MUN and the amount of UN are determined predominately by the concentration of urea in the blood, or blood urea nitrogen (BUN). The concentration of BUN is a function of, among other things, the amount of nitrogenous compounds in the diet. Nitrogen intake includes N from true protein as well as N from nonprotein sources. The concentration of MUN has been shown to be highly correlated with BUN (Roseler et al., 1993). This is because urea appears to diffuse readily from the blood to the milk across the epithelial cells of the mammary gland, though this diffusion occurs with a lag time (Gustafsson and Palmquist, 1993). Therefore, MUN can be used as an estimate of BUN without the need for taking a blood sample (Roseler et al., 1993). A high MUN concentration may indicate an excess of protein in the diet, which has economic implications of money spent on unutilized protein (Harris, 1995). A low MUN concentration may indicate a protein deficiency which, if corrected, could result in an increase in milk production.

The objectives of this study were to look at the relationship between MUN and UN excretion. Jonker et al. (1998) proposed a model for predicting UN excretion based on MUN values (Figure 1). This model was developed using three separate digestibility and N balance studies. A strong linear relationship between UN and MUN was derived, although a nonsignificant curvilinear trend at higher MUN was observed. One possible reason for the lack of significance of a curvilinear function is the wide range of UN excretion values at a given MUN value (Figure 1). Also, the applicability of the UN-MUN relationship may be reduced substantially from a lack of standardization of the MUN assay. Faust and Kilmer (1996) showed that MUN values can differ on the same sample when done in different laboratories, revealing the importance of using a consistent laboratory when monitoring MUN on the farm.

Materials and Methods

Experimental Design

Four Holstein and four Jersey cows at approximately160 days in milk were used in two 4 x 4 Latin squares, with breed as the main plot. Experimental periods were three weeks, with the first two weeks used for adjustment of the cows to the experimental diets and the last week for collection of blood and a five-day total collection of urine and fecal samples. Treatments were in a 2 x 2 x 2 factorial arrangement with breed as the first factor, percentage of crude protein in the diet as the second factor (14%, low; 18%, high), and NDF as the third factor (30%, low; 40%, high). The forage to concentrate ratio was maintained constant across diets at 50:50. The forage consisted of corn silage and alfalfa hay in a 2:1 ratio, DM basis. The concentrate consisted of ground corn, soybean meal (SBM), soybean hulls, distillers grains, and other minor ingredients to balance for vitamins and minerals. Crude protein and NDF were adjusted by changing the relative proportions of corn, SBM, and soybean hulls (Table 1). Daily samples of feed offered and feed refused were taken for each cow and composited for the five-day collection period. All feed samples were kept frozen until analyzed in the laboratory.

Sample Collection/Analysis

Cows were housed in tie stalls at the Ohio State University Waterman Dairy Farm in Columbus. Urine was collected using an external harness, without the use of indwelling catheters, to minimize the possibility of urinary tract infection.

Feces were collected by placing metal plates over the gutter behind the cows. The feces were shoveled into separate containers for each cow numerous times each day. Urine and feces were collected daily and composited for each cow during each five-day collection period. All weekly samples were kept frozen until analyzed. Milk protein, fat, somatic cell count, and MUN were analyzed by Ohio DHI (DHI Cooperative, Inc., Powell, Ohio). Milk urea nitrogen was analyzed using a Skalar segmented flow analyzer. Some of the milk samples were subsequently measured for MUN using Sigma Kit #535 (Sigma Chemical Co., St. Louis, Mo.) to verify the DHI values. Both methods utilized a reaction of urea with diacetylmonoxime to form a reddish color that is measured with a spectrophotometer. Nitrogen was analyzed on dry feed samples and wet urine and fecal samples using the Kjeldahl method.

Results and Discussion

Dietary NDF and its interaction with CP and breed did not have significant effect on any of the measured variables. Likewise, there were few significant interaction effects of breed with CP. Thus, presentation of results will focus on the main effects of breed and CP.

Breed Effects

The effects of breed on DM intake, production, digestibility, and N partitioning are presented in Table 2. The production parameters were as expected for both Holstein and Jersey, with the Holstein cows eating more and producing more milk protein and fat (lb/day). The Jersey cows had higher milk protein and fat percentages as expected. The Holstein cows digested more DM because they consumed more, but there was no difference in the apparent digestibility of the DM.

CP Effects

The effects of dietary CP level are reported in Table 3. The higher CP diet significantly affected milk yield, milk protein and fat yields, and the MUN concentration. There were small, nonsignificant, numerical differences in the percentages of milk protein and fat. Dry matter intake, pounds of DM digested, and the apparent DM digestibility were not affected. Significant differences were observed for N partitioning. The high CP diet resulted in a 122 g/day increase in N intake over the low CP diet. Of this extra 122 g/day that were ingested, 17 g were excreted in the feces, indicating that this portion was not digested. The remaining 105 g were digested, but of those, 90 g were excreted in the urine. Only 10 g/day of the potential 15 g remaining (out of 122 g) were incorporated into milk N, with the other 5 g/day being retained. Since the animals on this trial were mature animals in mid-lactation, there should theoretically be no retained N. It is typical to see a small apparent retention of N with this type of study, because feces and urine cannot physically be completely collected and the small losses are algebraically calculated as N retained. The numbers for N retention in this trial are within expected ranges. The apparent N digestibility increased with the higher N diet.

MUN as Predictor of UN Excretion

The relationship between UN excretion and MUN is shown in Figure 2. The observations from each individual cow are connected by a straight-line, and the two bold lines represent the best-fit line of all data points for each breed. The slopeds of the best-fit lines for the different breeds are significantly different, indicating that the relationship between UN and MUN is different for the two breeds. The equation for Holstein cows based on the best-fit line is UN (g/day) = 17.6 (± 0.56) * MUN (mg/dl), and the equation for Jersey cows based on the best-fit line is UN (g/day) = 11.8 (± 0.62) * MUN (mg/dl). It is possible that what is observed here is merely a difference in the body weight of the two breeds. Further analysis is being done to determine if this is a true breed effect (genes) or simply an effect of the difference in body size.

Conclusions

Holstein and Jersey cows responded differently to the level of CP in the diet. This difference may be a true breed difference or a difference in the way cows of different body weight respond to the level of CP in the diet. In either case, the model should not be applied indiscriminately to every animal, but rather used according to the specifics of the animal in question. Additionally, it is important in a study to use the same laboratory for the analysis of MUN in order to obtain consistent results.

References

Butler, W. R., D. J. R. Cherney, and C. C. Elrod. 1995. Milk urea nitrogen (MUN) analysis: field trial results on conception rates and dietary inputs. In: Proc. Cornell Nutr. Conf. Feed Manuf., Rochester, N. Y. Cornell University, Ithaca, N. Y. Page 89.

Butler, W. R., J. J. Calaman, and S. W. Beam. 1996. Plasma and milk urea nitrogen in relation to pregnancy rate in lactating dairy cattle. J. Anim. Sci. 74:858.

Church, D. C. 1993. The Ruminant Animal Digestive Physiology and Nutrition. Waveland Press, Inc., Prospect Heights, Ill.

Crampton, E. W. and L. E. Harris. 1969. Applied Animal Nutrition. 2nd Ed. W. H. Freeman and Co., San Francisco, Calif.

Faust, M. A. and L. H. Kilmer. 1996. Determining variability of milk urea nitrogen reported by commercial testing laboratories. 1996 Dairy Report. Iowa State University, Ames.

Gustafsson, A. H. and D. L. Palmquist. 1993. Diurnal variation of rumen ammonia, serum urea, and milk urea in dairy cows at high and low yields. J. Dairy Sci. 76:475.

Harris, B. Jr. 1995. MUN and BUN values can be valuable management tools. Feedstuffs. Oct. 9. pg. 14.

Jonker, J. S., R. A. Kohn, and R. A. Erdman. 1998. Using milk urea nitrogen to predict nitrogen excretion and utilization efficiency in lactating dairy cows. J. Dairy Sci. 81:2681.

Kohn, R. A., Z. Dou, J. D. Ferguson, and R. C. Boston. 1997. A sensitivity analysis of nitrogen losses from dairy farms. J. Environ. Mgt. 50:417.

Roffler, R. E., J. E. Wray, and L. D. Satter. 1986. Production responses in early lactation to additions of soybean meal to diets containing predominantly corn silage. J. Dairy Sci. 69:1055.

Roseler, D. K., J. D. Ferguson, C. J. Sniffen, and J. Herrema. 1993. Dietary protein degradability effects on plasma and milk urea nitrogen and milk nonprotein nitrogen in Holstein cows. J. Dairy Sci. 76:525.

¹ For more information, contact at: The Ohio State University, 221A Animal Science Building, 2029 Fyffe Road, Columbus, OH 43210; (614) 292-6507, Fax (614) 292-1515; email:st-pierre.8@osu.edu