FACTORS AFFECTING BLOOD UREA NITROGEN AND ITS USE AS AN INDEX OF THE NUTRITIONAL STATUS OF SHEEP. D. T. Torell I, I. D. Hume 2 and W. C.

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1 FACTORS AFFECTING BLOOD UREA NITROGEN AND ITS USE AS AN INDEX OF THE NUTRITIONAL STATUS OF SHEEP Summary D. T. Torell I, I. D. Hume 2 and W. C. Weir 3 University of California, Davis Three experiments were conducted to study factors affecting blood urea nitrogen (BUN) and the use of BUN as an index of the nutritional status of ewes. In experiment 1, 315 ewes of varying ages grazing on mature annual grass were flushed by supplementing daily with 0, 90, 180, 270, 360 and 450 g pelleted alfalfa hay just prior to and during the breeding season. Lambing percentage (lambs born x 100/ewe present at lambing) increased from 98% (no supplement)-to 130% when 360 g pelleted alfalfa were consumed (P <.10). Both BUN (P <.05) and weight gain (P <.05) increased during the flushing period when alfalfa was fed. Simple correlation coefficients (r) were: supplemental N intake vs. BUN, 0.99; BUN vs. weight gain, 0.95; BUN vs. lambing percent, 0.95; supplemental N intake vs. gain, 0.95; and weight gain vs. lambing percent, In experiment 2, blood samples from 10 each of lambs, yearlings and ewes were taken five times during a day to determine the effect of animal age, time of day, and variation in BUN levels among animals. Neither age nor sampling time had any significant effect on BUN level but variation between animals within age class was significant (P<.01). The minimum number of samples necessary for 95% of the values to be within + 10% of the sample mean was calculated to be 10. In experiment 3, 19 to 25 ewes from each of 26 commercial flocks throughout northern California were sampled for BUN prior to breeding. Since the standard deviations were rather constant irrespective of BUN level, the minimum sample size necessary for 95% of the 1 Department of Animal Science, HoplandField Station, Hopland, Ca Present address: University of New England, Armidale, New South Wales, Australia. s Department of Nutrition. values to be within -+ 10% of the sample mean varied between flocks with regression of Y = X (r = -.761) where Y is the sample size and X is BUN value. Introduction The use of blood urea nitrogen (BUN) levels in ruminants for assessing the value of feed protein was suggested by Lewis (1957), on the basis that BUN levels appeared to reflect ammonia production in the rumen. Addis et al. (1947) showed that in humans, increasing protein intake increased serum urea concentration, and Puchal et al. (1962) found that BUN levels were inversely proportional to protein quality as reflected by growth and feed efficiency in baby pigs. Plasma urea concentration has been suggested to be a realistic predictor of both nitrogen utilization (Egan and KeUaway, 1971) and nitrogen intake (Nolan, Cocimano and Leng, 1970) of sheep. Preston, Schnakenberg and Pfander (1965) found a close relationship between protein intake and BUN levels in growing-finishing lambs. However, as Preston et al. (1965) pointed out, this relationship can be modified by several other factors, not all of which are yet defined. Certainly starvation and low nitrogen diets can increase BUN levels as body protein is catabolized (Leibholz, 1970). Torell, Hume and Weir (1972a) found a close relationship between BUN levels in range ewes during flushing and their subsequent lambing performance. Flushing of range ewes may be the most efficient way to increase fat lamb production in the Mediterranean environment of California (Torell et al., 1972a,b,c). The experiments presented here examine some of the factors which may effect BUN levels in range ewes, and hence the usefulness of BUN as an index of the lambing response which may be expected from flushing under different conditions. 435 JOURNAL OF ANIMAL SCIENCE, vol. 39, no. 2, 1974

2 436 TORELL, HUME AND WEIR Experimental Procedures BUN was measured on sera by the Auto Analyzer method of Skedds (1957) and Marsh, Fingerhut and Kirsch (1957). Blood was collected from the jugular vein, refrigerated overnight, the sera decanted and frozen. All analyses were made during the same run on all samples within the experiment. Repeatability tests showed the procedure variation as measured by the coefficient of variation was less than 1% while the animal variation was 19%. Experiment 1 Relationship Between BUN Levels at Breeding and Subsequent Lambing Performance of Range Ewes. Three-hundred and fifteen predominantly Corriedale ewes 189 to 789 years old, were allotted at random (after stratification by age) to six groups. Stratification by age was deemed necessary because reproductive performance improves with age (Torell et al., 1972a). The ewes grazed one pasture on the University of California's Hopland Field Station in the northern Coast Range of California consisting of annual grasses (mainly 8romus spp.), annual forbs and some subterranean dover (Trifolium subterraneum L.) (Van Dyne and Heady, 1965), and was completely dry at time of breeding (August to September). During the treatment period (18 days prior to and the first 18 days of breeding) the ewes were flushed by supplementing daily with 0, 90, 180, 270, 360 and 450 g pelleted alfalfa hay 4 (19.3% crude protein), so supplemental nitrogen intake varied between 0 and 13.8 g/day (table 1). The supplement was oven dried at 52 C before weighing. Ewes were gathered at 10 am each day, supplemented individually and returned to the range at 2 pm. Interference with normal grazing behavior was minimized, since at this time of year sheep graze only during the cooler parts of the day (early morning and evening) and at night. Blood samples for BUN analysis were taken on day 19 (beginning of breeding) and day 37. The ewes were bled 24 hr. after supplementation. All ewes were weighed on days 1 and 37, each time after an overnight shrink to minimize differences in gut fill. After flushing the ewes were mated to six Corriedale rams for the 18 days of supplementation. These rams remained with the ewes on range for an additional 21 days. While 4 Alfalfa hay, s - c -, 17-20% protein, no ewes were being supplemented, the rams were fed alfalfa hay. The ewes remained together on the annual range pasture and lambed in a barn where individual lambing performance could be recorded. Lambing performance was analyzed by chi-square (Steel and Torrie, 1960). Experiment 2 Effect of Animal Age, Sampling Time, and Animal Variability on BUN Levels. Blood samples were taken five times during the same day (7 am, 10 am, 1 pm, 4 pm and 7 pm) from each of 10 lambs, 10 yearlings and 10 mature ewes, all of which had grazed together on a dry annual range pasture at the Hopland Field Station for at least 3 weeks before sampling. The effects of animal age and time of day on BUN levels were analyzed by analysis of variance (Steel and Torrie, 1960). The minimum number of blood samples necessary for 95% of values to be within + 10% of the sample mean was determined by the method described by Harker, Torell and Van Dyne (1964), in which minimum sample size (n) is determined by the formula n = t2s2/d 2 (Snedecor, 1956) where t is from the value intake, s z is the variance and d is the deviation from the mean. Experiment 3 Minimum Sample Numbers Required in Commercial Flocks. Blood samples were taken for BUN analyses from 19 to 25 ewes from each of 26 commercial flocks throughout northern California approximately 2 weeks prior to breeding. Lambing performance records were obtained, wherever possible, from the entire flock [up to 982 ewes (table 4)]. The ewes sampled were purebred and grade Columbias, Corriedales, Targhees, Rambouillets, Romneys, Romeldales, Romnelets and Suffolks. The pastures varied from dry annual range to irrigated pasture. The minimum number of blood samples necessary for 95% of the values to be within + 10% of the sample mean under a wide variety of commercial conditions was determined as described in experiment 2. Results Experiment 1. Lambing percentage (lambs born 100/ewe present at lambing) increased from 98% (8.7% twin births) without supplement to 130% (29.0% twin births) with 360 g pelleted alfalfa hay/day (P< 20)(table 1).

3 FACTORS AFFECTING BLOOD UREA NITROGEN 437 z z r~ z.< 9 z < a~ o z M M < [.-, i,m t-..- e4 ~ 9. ~dn e~ ~ ~,~ ~ ~.~ L-.- r162 ~~4+I+~ COO v o ef~ ~ocuo ~'~ ~ ~ ~ ooo "~='~ -~,--- ~ ~,xx o "o "O +1 oq "el When the 64 yearlings were excluded, however, the response to treatments was significant (P <.01) (100.0% lamb drop of ewes present on the basal treatment vs. 132% on 360 g pelleted alfalfa hay/day). The lambing response of the yearling ewes (189 years old at breeding) was less (P<.01) than that of the 2~A - to 789 ewes. The improvement in reproductive performance with supplementation was due both to fewer dry ewes and an increase in the number of twins. These effects were significant (P <.05 and P<.10 for dry ewes and twins, respectively) only when the yearling ewes were excluded from the chi-square analysis. Both BUN level (P <.05) and weight gain (P <.05) increased during flushing. Based on group means, there were close relationships (table 2) between supplemental nitrogen intake during flushing and the mean BUN levels at breeding, weight gain during flushing and subsequent lambing performance. Use of individual values resulted in smaller correlation coefficients between these parameters due to the variation between animals on the same treatment. Experiment 2. BUN levels from lambs, yearlings, and mature ewes sampled at five times during a day did not vary with age or sampling time. The only significant source of variation was between-animal, within-age (P<.01) (table 3). Mean standard deviations were 0.70 for lambs, 0.72 for yearlings, and 0.68 for adults, indicating that age of animal does not affect the variation. Ten samples were necessary for 95% of values to be within _+ 10% of the sample mean. Experiment 3. The quality of the pasture tended to be reflected in the mean BUN levels; BUN levels fell when dryland pasture growth ceased. The minimum sample size necessary for 95% of the values to be within + 10% of the sample mean varied between flocks, and was inversely related to the mean BUN level (r = -.761, P<.01) (figure 1). There was no over-all correlation between the mean BUN flock level just prior to breeding and the percent of twin births during the subsequent lambing. Discussion In experiment 1, mean BUN levels at breeding were useful predictors of response in lambing performance from flushing. A similar close correlation was also observed in a concurrent investigation (Torell et al., 1972a). In experiment 3, where great variation was

4 438 TORELL, HUME AND WEIR TABLE 2. SIMPLE CORRELATION COEFFICIENTS (r) FROM EXPERIMENT 1 Wt gain during Item flushing BUN a Individuals Supplemental N intake BUN a 0.22 Means Supplemental N intake 0.95* 0.99* Wt gain BUN a * _ Lambingb % 0.94* 0.97* 0.95* *P <.05. aserum blood urea nitrogen from ewes at breeding (day 19). blambs born X 100/ewe present at lambing. BLOOD UREA NITROGEN (m! "~) r o -761 Figure 1. Sample size (number of sheep to be bled) required for 95% of the blood serum urea nitrogen levels to fall within 10% of sample mean as influenced by the BUN level. involved, there was no overall correlation between the mean BUN in a flock and twinning percent. Breed of ewe, twinning ability of ewes within an individual flock, season of breeding and the nutritional status of the flock are all factors that will affect the number of lambs born. In experiment 3, BUN levels reflected what the ewe had eaten in the preceding 2 or 3 days, whereas breeding extended for 2 months or more, during which time the ewes were often shifted between pastures of varying nutritive values. Thus the relationship between BUN level and lambing performance would be closer within a flock (experiment 1) than between flocks (experiment 3). The lambs, yearlings and mature ewes sampled in experiment 2 were all Targhee-type sheep grazing a pasture that supplied a maintenance ration for the adult sheep. Combs, Mukhoty and Groves (1968) showed a marked difference in BUN levels between Lincoln and Southdown and male and female lambs during a TABLE 3. MEAN BUN LEVELS a FOR RANGE EWES IN EXPERIMENT 2 Time of sampling Lambs Age of ewes Yearlings Mature 7am am lpm pm pm amg/lo0 ml serum + standard deviatiom

5 FACTORS AFFECTING BLOOD UREA NITROGEN 439 TABLE 4. MEAN BUN LEVELS AND TWIN BIRTHS OF EWES FROM COMMERCIAL FLOCKS IN EXPERIMENT 3 No. ewes BUN Minimum Twin births Flock sampled a mg/100 ml number of samples a % Green annual pasture (June): NAC ~0 Partly green native pasture + dry subterranean clover (June-July): Dry native pasture (July-August): NA NA NA NA NA NA Irrigated pasture (August): afor BUN analyses. bfor 95% of values to be within + 10% of sample mean. CNot available. growing-fattening period. They relate this difference to the fattening process and the "expectation that Southdowns fatten earlier than the faster growing Lincolns and that females fatten earlier than uncastrated males." This indicates that there might be a nutritional level age (growth stage) interaction which was not measured in this experiment. The number of samples necessary for a required degree of accuracy varies with the BUN level. The variance is about the same for all BUN levels, thus more samples must be taken when the BUN level is low. Using the individual data in experiment 2 (five collections/sheep/day), it would require four samples per sheep to bring the accuracy for the individual within + 10% of the mean 95% of the time, thus papers which report differences in BUN when only a few animals are used should be given critical scrutiny. Literature Cited Addis, T., E. Barrett, L. J. Poo and D. W. Yuen The relationship between the serum urea concentration and the protein consumption of normal individuals. J. Clin. Invest. 26:869. Combs, W., H. Mukhoty and T. D. D. Groves Relationship between blood urea nitrogen and five weight changes in Lincoln and Southdown lambs. Univ. of Alberta Feeders Day, p. 40. Egan, A. R. and R. i~. Kellaway Evaluation of nitrogen metabolites as indices of nitrogen utilization in sheep given frozen and dry mature herbages. Brit. J. Nutr. 26:335. Harker, K. W., D. T. ToreU and G. M. Van Dyne Botanical examination for forage from esophageal fistulas in cattle. J. Anim. Sci. 23:465.

6 440 TORELL, HUME AND WEIR Leibholz, Jane The effect of starvation and low nitrogen intakes on the concentration of free amino acids in the blood plasma and on the nitrogen metabolism in sheep. Australian J. Agr. Res. 23:723. Lewis, D Blood-urea concentration in relation to protein utilization in the ruminant. J. Agr. Sci. 48:438. Marsh, W. H., B. Fingerhut and E. Kirsch Determination of urea nitrogen with the diacetyl method and an automatic dialyzing apparatus. Amer. J. Clin. Path. 28:681. Nolan, J. V., M. R. Cocimano and R. A. Leng Prediction of parameters of urea metabolism in sheep from the concentration of urea in plasma. Proc. Australian. Soc. Anim. Prod. 8:22. Preston, R. L., D. D. Schnakenberg and W. H. Pfander Protein utilization in ruminants. I. Blood urea nitrogen as affected by protein intake. J. Nutr. 86:281. Puchal, F., V. W. Hays, V. C. Speer, J. D. Jones and O. V. Canton The free blood plasma amino acids of swine as related to the source of dietary proteins. Skedds, L. T., Jr An automatic method for colorimetric analysis. Amer. J. Clin. Path. 28:311. Snedecor, G. W Statistical Methods (5th Ed.) Iowa State College Press, Ames. Steel, R. G. and J. H. Torrie Principles and Procedures of Statistics. McGraw-Hill Book Co., New York. Torell, D. T., I. D. Hume and W. C. Weir. 1972a. Flushing of range ewes by supplementation, drylot feeding, or grazing of improved pasture. J. Range Manage. 25:356. Torell, D. T., I. D. Hume and W. C. Weir. 1972b. Effect of level of protein and energy during flushing on lambing performance of range ewes. J. Anim. Sci. 34:479. Torell, D. T., I. D. Hume and W. C. Weir. 1972c. Biuret as a nitrogen supplement for flushing range ewes. J. Anim. Sci. 35:606. Van Dyne, G. M. and H. F. Heady Dietary chemical composition of cattle and sheep grazing in common on a dry annual range. J. Range Manage. 18:78.