A. W. Young, 2 J. A. Boling and N. W. Bradley 9 University o f K e n t u c k y , 3 L e x i n g t o n 4 0 5 0 6
Summary Abomasal cannulated steers were used to quantitate various nitrogenous components reaching the abomasum during a 68-day adjustment period to either soybean meal (SBM) or urea supplemented ear corn rations (11% crude protein, CP). Mean estimates showed that total nitrogen (N) reaching the abomasum per day amounted to 64.3 g and 71.7 g for steers fed the urea and SBM rations, respectively ( P < .05). Signigicant ( P < .05) increases in the peptide N fractions, boundamino N and free-amino N, accounted for the 11.5% increase in total N on the SBM ration. Following introduction of urea into the basal corn ration, abomasal N decreased for 14 days and then increased linearly (P < .06). For steers on the SBM ration, abomasal N decreased for 8 days and then increased linearly (P < .05) at a daily rate of 250 mg N greater (P < .05) than was observed on the urea ration. No significant differences in plasma amino acids were detected; however, a significant (P < .05) linear response to time was noted for most of the essential amino acids on the SBM ration. Plasma urea-N levels (mg/100 ml) were significantly ( P < .01) elevated for steers fed the urea ration (7.80 vs 6.47). Increases in plasma urea levels were associated with
adjusting steers to both rations ( P < .05). Throughout the trial daily increases in plasma urea amounted to .08 mg/100 ml and .03 mg/100 ml for the urea and SBM rations, respectively (P < .05). Results of this study showed that quantitatively more N reached the abomasum of steers fed the SBM ration over the 68-day trial. Increased N post-ruminally should be consistent with improved performance of cattle started on natural protein supplements as any "compensatory adaptation" to the urea ration was not apparent.
Introduction Utilization of nonprotein nitrogen (NPN) involves its conversion to ruminal ammonia and synthesis into microbial protein (McDonald, t948). Such utilization may improve with length of time fed (Reid, 1953; McLaren et al., 1965; Virtanen, 1966; Ludwick et al., 1971) and amount to .2% increase in the daily absorbed N retained (Smith e t al., 1960). Several recent reviews of N metabolism (McLaren, 1964; Chalupa, 1968; Oltjen, 1969; Helmer and Barfley, 1971) devote sections to discussion of possible adaptation responses to NPN. However, the bulk of the literature points to rather inconclusive elucidations of the adaptation phenomenon, particularly as to the area or site o f such adaptations. The study reported herein was instigated to measure the importance of adjusting steers to different N sources as reflected bY (1)quantitative and qualitative changes in nitrogenous constituents reaching the abomasum, (2) changesin plasma
1This paper (74-5-94) is published with the approval of the Director of the Kentucky Agricultural Experiment Station. 2Present Address: Department of Meat and Animal Science, University of Wisconsin, Madison. a Department of Animal Sciences. 775
JOURNAL
OF ANIMAL
SCIENCE, vol. 40, no. 4, 1975
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N I T R O G E N M E T A B O L I S M IN T H E BOVINE: A D J U S T M E N T TO N I T R O G E N SOURCE AS R E F L E C T E D BY C H A N G E S IN A B O M A S A L N I T R O G E N AND P L A S M A COMPONENTS 1
776
YOUNG ET AL.
urea levels, and (3) changes in plasma free amino acids. Experimental Procedure
TABLE 1. COMPOSITION OF RATIONS FED TO STEERS DURING RATION ADJUSTMENT EXPERIMENT
Ingredient
International reference numbera
Ration (%) Soybean meal Urea
G r o u n d ear
corn Soybean meal (44%) Urea 281 Ground limestone Dicalcium phosphate Salt Vitamin A, IU/kg Analyses: b Crude protein, % Gross energy, kcal/g Chromic oxide, g/kg feed
4-02-850
89.00
97.08
5-04-604 5-05~070
9.40 --
-1.21
6-02-632
.60
.50
6-01-080 6-04-152
-1.00
.20 1.00
7-05-143
2,200
2,200
10.84
11.02
3.96
4.00
2.15
2.20
aAtlas of Nutritional D a t a o n U n i t e d S t a t e s and Canadian Feeds. 1971. National A c a d e m y o f S c i e n c e s . W a s h i n g t o n . D. C. b A n a l y s e s are o n air-dry basis.
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Six yearling Hereford steers weighing from 250 to 300 kg were fitted with permanent abomasal cannulae as described by Dougherty (1955). Prior to and following surgery steers were fed 1.8 kg of ground ear corn and 2.7 kg of alfalfa hay daily. Three weeks after surgery the steers were paired on the basis of size and randomly assigned to either the urea or SBM ration shown in table 1. During the 7 days preceding the initiation of the experiment only ground ear corn (containing .22% chromic oxide) was fed to reduce possible carry-over effects of exogenous proteins other than corn. Steers were fed either 4 or 5 kg of their respective rations in equal morning and evening feedings. Rations fed during the experimental period were calculated to contain 11.0% CP, utilizing either SBM or urea as supplemental N sources. Rations were calculated to be isocaloric and isonitrogenous and analyzed essentially so. Each ration contained .22% chromic oxide as an indicator for quantitating N reaching the abomasum, and both rations were pelleted in an attempt to minimize sorting and feed refusals. Salt, steamed bonemeal and ground limestone were offered free-choice. Additionally, both rations surpassed established N.R.C. (1970) requirements for total protein and TDN for growing steers of their mean trial body weight (300 kg) for no gain and approximated the requirements for .25 kg daily gain. Samples of abomasal contents were collected so that each 2-hr interval following the morning and evening feeding would be represented in the composited sample. To reduce stress to the animal these collections were made over a 3-day period so that 6 hr elapsed between each sample collection. These samples were immediately frozen, and subsequently all 12 samples from each steer were thawed and composited to one sample for that particular steer. This sampling procedure was repeated at predetermined intervals to give values representing mean days 2, 8, 14, 20, 26, 35, 44, 53 and 68. Nitrogenous components of abomasal fluid were fractionated by the technique presented by Potter et al. (1969). Total N analyses were
conducted on whole abomasal contents by macrokjeldahl procedures (A.O.A.C., 1960). Chromic oxide content of feed and abomsal contents was determined using the procedure of Hill and Anderson (1958). Results of these analyses were used to compute the total quantity of N reaching the abomasum by conventional N:Cr2 03 ratio techniques. Jugular blood samples were collected in heparinized tubes 0, 2, 4 and 6 hr after the morning feeding, and were taken on days 0, 7, 14, 21,28, 35, 42, 49 and 63. Plasma free amino acids were determined for individual steers on samples taken at the initiation of experimental period, and at 3, 6 and 9 weeks. To obtain a more representative sample of the plasma amino acid pool, a 2 ml sample of plasma from each of the 2-hr bleedings was composited for amino acid
777
NITROGEN METABOLISM IN THE BOVINE
N i t r o g e n intake,
Results
In agreement with earlier reports (Potter et al., 1969; Tucker and Fontenot, 1970; Potter et al., 1971), more N (P < .05) reached the abomasum of animals fed SBM-supplemented rations than from those fed urea rations (table 2). Only on mean day 2 was the reverse true (figure 1). It may be speculated that substantial
80.9
82.2
N i t r o g e n in abomasum, g/day a
g/day
71.7 • 3.7 b
6 4 . 3 • 3.1
N i t r o g e n in abomasum, % of intake
88.6
78.2
aRation differences axe significant (P < .05). bSEM.
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ruminal washout of NPN to the abomasum occurred during the first few days on the urea ration. After this initial period, ureolytic activity of the rumen microorganisms was probably sufficiently high to permit rapid hydrolysis of the urea. Quantitative fractionations of the abomasal nitrogenous compounds revealed that the 11.5% more N on the SBM ration was attributable to a significantly (P < . 10) increased NPN fraction (table 3). Subfractionation of the NPN showed that the peptide N fractions, bound-amino N and free-amino N, were significantly (P < .05) higher in abomasal contents of steers fed the SBM ration. A somewhat different pattern of abomasal constituents was Yij = p + Ri + Lj + (RxL)ij +/~ (Xij -- X) previously reported (Potter et al., 1969) in that + eij increased protein N and similar NPN values were found for SBM - vs urea-supplemented Yij = dependent variable rations. Differences in the degree of gastric # = mean common to all observa- proteolysis could account for this difference tions since Masson (1950) has presented evidence that bacteria undergo digestion to varying Ri = effect of the ith ration degrees in the abomasum. Lj = effect of the jth level of feed Abomasal N reached its lowest value 8 days intake after the steers were exposed to the SBM ration (RxL)ij = interaction of ration (Ri) and and then increased rapidly (figure 1). However, level (Lj). following introduction of urea into the basal /3 = linear regression of Yij on time corn ration, abomasal N continued on a within Ri and Lj downward trend for 14 days and then increased at a slower rate. I f one considers the entire eij random errors, assumed to be feeding trial, total N reaching the abomasum independent and normally dis- increased for both rations (figure 1) and a significantly linear response was noted for the tributed. SBM ration ( P < .05) and urea ration (P< .06), as seen in table 4. As can be seen from the Where rations accounted for a significant portion of the partitioned sum of squares in the analysis of variance or for clarity of the results, TABLE 2. TOTAL NITROGEN IN THE ABOMASUM the ration was deleted from the above- OF STEERS FED SBM OR UREA (MEAN VALUES mentioned model and separate regressions ADJUSTED TO A COMMON TIME) established for each ration. Tests of homoRation geneity of these regressions were performed Item SBM Urea according to Steel and Torrie (1960). determinations. Amino acid analyses were similar to the ion-exchange chromatographic procedure outlined by Hamilton (1963). Plasma proteins were precipitated from the samples prior to analysis with 5-sulfosalicylic acid (5% W/V). Plasma urea nitrogen analyses were conducted on all plasma samples according to the procedure described by Skeggs (1957). Since the primary objective of this experiment was to study ration adjustment over the feeding period, a regression analysis was used in the statistical treatment of the data (Snedecor, 1956). The following model was applied to the variables studied.
778
YOUNG ET AL.
8O
~ 7s
/
/
Dependent Rationb variable (y)a SBM Urea Total N .55 + .17 c .30 • .15 d es . l / -''J \ ; ssM Protein N .33 -+ .06 c .21 • .16 d Nonprotein N .22 • .11 .09 + .08 eo~ ",1 Bound amino N .12 + .05 .08 + .04 Free amino N .02 • .01 .01 • .01 D A Y OF T R I A L Purine-pyrimiFigure 1. Total nitrogen reaching the abomasum of dine N - - . 0 1 • .01 .02 • .01 steers at intervals t h r o u g h o u t t h e experiment. Undetermined N .06 • .02 - - . 0 2 -+ .02 ..I
regression coefficients, the rate of increase in total N to the abomasum was 250 mg/day aExpresscd as g/day reaching the a b o m a s u m . greater for the SBM ration over the 68-day trial. bRegzession coefficient (~) + SE~ expressed as Protein N increased in a linear manner on both change per day (g) of the feeding period. and dbinear response is significant P < .05 and rations. The increasing quantities of post- P < c.06. ruminal nitrogen observed in this study might explain the increased nitrogen retention asOne hypothesis concerning the reduced sociated with adjusting lambs (Welch et al., performance associated with feeding urea 1957; Smith et al., 1960; McLaren et al., 1965; rations is that microbial protein synthesis may Schaadt et al., 1966; Ludwick et al., 1971) or be insufficient to meet the amino acid needs of dairy cows (Virtanen, 1966) to urea diets. the tissues. If an adaptation to urea feeding Although NPN increased on both rations, large occurred, it might be postulated that plasma variability in the data refused establishment of amino acid patterns would be influenced trends. accordingly. At the near maintenance levels of intake in this study, it is doubtful that TABLE 3. N I T R O G E N F R A C T I O N S IN ABOMASAL increased quantities of amino adds presented to CONTENTS OF STEERS FED SBM OR the intestines for absorption would become UREA (G/DAY) accumulated in greatly increased concentrations in the plasma pool. However, the linear increase Rationa in most essential amino acids as a function of Fraction SBM Urea time on the SBM ration (table 6) is suggestive Protein N 2 0 . 2 -+ 1 . 7 20.8 + 1.7 that had the animals remained on their diets Nonprotein N b 51.5 + 3.4 4 3 . 4 + 2.5 longer or at greater levels of intake treatment Bound amino responses might have been apparent. This Nc 26.0 • 1.7 21.2 • 1.2 observation is confirmed in feedlot situations Free amino with these rations, i.e., that steers receiving Nc 7.0• .4 5.8• .3 SBM" supplements to ear corn finishing rations Purine-pyrimihave elevated plasma levels of certain essential dineN 9.6• .6 9.4• .5 amino acids (Little et al., 1966; Freitag et al., Undetermined N 9.0 • .9 7.1 • .7 1968; Young et aL, 1973). Plasma urea levels for the 6-hr period following feeding were significantly higher for aMean values -+ SEM adjusted to a c o m m o n time. b and CRation differences are significant P < .10 steers on the urea ration (table 5). Over the and P < .05, ~espectively. 68-day trial plasma urea levels increased linearly
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T A B L E 4. REGRESSION COEFFICIENTS F O R OBSERVING THE I N F L U E N C E OF LENGTH OF THE FEEDING PERIOD ON NITROGEN F R A C T I O N S IN ABOMASAL CONTENTS OF STEERS FED SBM OR U R E A RATIONS
NITROGEN METABOLISM IN THE BOVINE TABLE 5. PLASMA FREE AMINO ACID AND UREA NITROGEN CONCENTRATIONS OF STEERS FED SOYBEAN MEAL OR UREA RATIONS
SBM
Urea
A m i n o acid, u r n / 1 0 0 ml Total amino acids 178.4 • 13.6 T o t a l essential 84.6 • 5.9 Total nonessential 93.8 +- 7.8 Threonine 10.6-+ 1.3 Valine 16.4 • 1.1 Methionine 2.2 • .2 Isoleucine 8.0 -+ .7 Lettcine 12.5 • 1.3 Phenylalaline 4.6 • .4 Lysine 10.9 • .9 Histidine 7.4 • .7 Arginine 12.1 • 1.0 A s p a r t i c acid Serine G l u t a m i c acid Proline Glycine Alanine Tyrosine Urea n i t r o g e n , rag/100 mlb
1.7 12.3 11.5 7.4 37.2 19.0 4.5
• + • +• • +
6.47 -+
.2 1.3 1.0 .8 3.7 1.4 .4 .30
1 8 0 . 6 • 14.8 84.9 -+ 7.5 95.7 10.9 15.8 2.2 8.3 13.2 4.5 11.0 7.6 11.5
-+ • • • • • • • • •
8.3 1.7 1.5 .2 .9 1.4 .5 1.2 .7 1.1
1.9 13.3 12.8 7.6 35.7 20.0 4.4
• • • • • • •
.1 1.3 1.2 .7 4.0 2.0 .4
7.80 +
.36
aMean values adjusted to a common time • SEM. bRations differ significantly (P < .01).
(P < .05) on both rations (table 6 and figure 2). However, the rate of increase was much greater (.08 mg/lO0 ml vs .03 mg/100 ml) on the urea ration. Discussion
Nitrogen arriving at the abomasum represents the net between dietary intake, preabomasal losses and extra-dietary (endogenous) influxes of N into the stomach compartments. At constant dietary intakes, increased quantities of abomasal N associated with length of exposure to the rations could have resulted from one or more of several factors. Improved ration utilization, as manifested in increased rates of microbial protein synthesis, w o u l d result in increased quantities of N available post-ruminally. It is generally agreed that ureolytic activity of fumen microorganisms far exceeds their protein synthetic capabilities. Further, any adaptation at the microflora level
is generally agreed to be quite rapid and, when measured as the ability of the microorganisms to assimilate ammonia, is largely accomplished in 7 (Lewis, 1960) to 19 days (Caffrey et al., 1967). Such a rapid adaptation would be consistent with increases in abomasal N observed in early phases of the trial. However, microbial adjustments fail to explain the prolonged and linear increases in post-ruminal N observed. Increasing the N recycled to the rumen should provide 'additional ammonia for microbial growth during periods of low rumen ammonia levels provided adequate carbohydrates are available. Most studies reveal an increase in urea returned to the rumen when ruminal ammonia levels, and ultimately plasma urea levels, are elevated (Lewis et aL, 1957; Houpt, 1959; Hirose et al., 1960). In studies with cattle, Vercoe (1969) and Thornton (1970) reported that maximal transfer of urea from blood to the rumen occurs at plasma urea levels of 12 mg/100 ml and 8 to 10 mg/100 ml, respectively. From the data of Vercoe (1969), it was estimated that 17 to 20 g N could be transferred to the rumen daily. Certain workers (Houpt and Houpt, 1968) suggest that transfer of urea across the rumen wall is a function of rumen ammonia concentration rather than blood urea concentration. However, most of the studies reviewed by Waldo (1967) show that total endogenous urea influx (salivary plus direct transfer) is largely governed by blood urea levels. Linear increases in plasma urea with time in our study would be consistent with increased urea return to the rumen. Data presented by Wisconsin workers (Satter and Roeffler, 1973) indicate average rumen ammonia levels for rations with this TDN and CP content would be fairly low (~3 mg/100 ml). Therefore, recycled urea would be utilized quite efficiently, resulting in increased microbial N reaching the abomasum. However, in the absence of urinary N excretion data, the relative magnitude of increased recycled N to the total N status of the animal cannot be ascertained. On the average (table 2), less N reached the abomasum than was consumed indicating urinary losses occurred. Additionally, such losses were probably greater on the urea ration with its correspondingly higher plasma urea levels. This observation agrees with the reciprocal relationship of plasma urea levels and
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Rationa Item
779
780
YOUNG ET AL.
TABLE 6. INFLUENCE OF LENGTH OF THE FEEDING PERIOD ON PLASMA AMINO ACIDS AND UREA FOR STEERS FED SOYBEAN MEAL OR UREA RATIONS Ration a
Dependent variable
Urea
30.09 16.46 13.63 2.24 3.42 .52 1.47 1.84 1.10 2.31 1.62 1.93
• + • +• ++ + + + ++
10.74 b 4.74 b 6.20 1.05 .86 b .16 b .53 b 1.03 .31 b ,79 b .53.b .76 b
20.22 7.84 12.37 1.26 1.08 .36 .45 .93 .57 .59 1.31 1.27
.64 .99 1.88 .87 4.43 3.55 1.26 .03
+ + + + + ++ +
.15 b 1.03 .83 b .60 2.89 1.08.b .35 I) .01 b
.61 1.83 1.66 1.27 4.34 2.06 .61 .08
Aspartic acid Serine Glutamic acid Proline Glycine Alanine Tyrosine Urea nitrogen, mg/100 ml cd
+ 11.69 • 5.94 • 6.55 + 1.36 + 1.21. + .16 b + .73 • 1.13 +.36 + .96 +.54b • .85 + ++ • + • • •
.10b 1.05 .93 .56 b 3.19 1.64 .29 .01 b
aRegression c o e f f i c i e n t s (~) + SEn expressed as change per 3-week period in units of the d e p e n d e n t variable. b Linear reaponse is significant (po< .05). CChange per day of the feeding period i n units of the d e p e n d e n t variable. d R a t i o n regressions differ significantly (P < .O5)
urinary N losses (Vercoe, 1969; Thornton, 1970). Recoveries of abomasal N in excess of dietary intake on the SBM ration during the later phases confirms the influx of endogenous N. Since our measurements were made on abomasal contents, it is apparent that endogenous N secretions into this organ accounted for a portion of the total N influx. Nitrogen influx via the gastric secretions and tissue slough probably represents a net increase in N since absorption of ammonia from this organ has been shown to be minimal (McDonald, 1948). Absorption of products of protein is generally assumed to be minimal in the monogastric stomach, and a similar situation probably exists in the abomasum (Gray et al., 1958). Relatively few studies are available quantitating gastric secretions in cattle, and the single reference to German studie s cited by Hill (1965) states that 30 to 35 liters are produced daily in the abomasum of the cow. Based on N contents reported for
sheep abomasal juices of 15 to 33 mg N/100 ml (Phillipson, 1964), from 4.5 to 11.6 g of endogenous N could be secreted daily in the abomasum of cattle. Similar estimates of influxes of 5.5 to 11.0 g N are obtained by extrapolating data on endogenous N secretions
.~ 10.0
s /
9.0 8.0
/ ~ 7.0 ~ 6,0 5.0 ~
~
UREA
~, 4.0 2
8
14
20
26
35
53
68
DAY OF TRIAL
Figure 2. Plasma urea concentrations measured throughout the experiment. Each value represents an average of samples taken at 0, 2, 4 and 6 hr after the morning feeding.
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Amino acid, pm/100 ml Total amino acids Total essential Total nonessential Threonine Valine Methionine Isoleucine Leucine Phenylalanine Lysine Histidine Arginine
SBM
NITROGEN METABOLISM IN THE BOVINE
infusions of several proteins have been reported (Hatfield, 1970). In the case of sheep fed rations low in N and infused per duodenum with casein, c o n c o m i t a n t improvements in appetite were observed (Egan, 1965). Importance o f increased appetite in adjusting feedlot cattle to rations cannot be over-einphasized. Literature Cited A.O.A.C. 1960. Official Methods of Analysis. (9th Ed.). Association of Official Agricultural Chemists. Washington, D. C. Burroughs, W., A. H. Trenkle and R. L. Vetter. 1971. Some concepts of protein nutrition of feedlot cattle (metabolizable protein and metabolizable amino acids). Vet. Med. Small Anim. Clin. 66:238. Caffrey, P. J., E. E. Hatfield, H. W. Norton and U. S. Garrigus. 1967. Nitrogen metabolism in the ovine. I. Adjustment to a urea-rich diet. J. Anita. Sci. 26:595. Chalupa, W. 1968. Problems in feeding urea to ruminants. J. Anim. Sci. 27:207. Dougherty, R. W. 1955. Permanent stomach and intestinal fistulas in ruminants; some modifications and simplifications. Cornell Vet. 45: 331. Drennan, M. J., J. H. G. Holmes and W. N. Garrett. 1970. A comparison of markers for estimating magnitude of rumen digestion. Brit. J. Nutr. 24: 961. Egan, A. R. 1965. Nutritional status and intake regulation in sheep. II. The influence of sustained duodenal infusions of casein or urea upon voluntary intake of low-protein roughages by sheep. Australian J. Agr. Res. 16:451. Freitag, R. R., W. H. Smith and W. M. Beeson. 1968. Factors related to the utilization of urea vs. protein-nitrogen supplemented diets by the ruminant. J. Anita. Sci. 27:478. Gray, F. V., A. F. Pilgrim and R. A. Weller. 1958. The digestion of foodstuffs in the stomach of the sheep and the passage of digesta through its compartments. 2. Nitrogenous compounds. Brit. J. Nutr. 12:413. Hamilton, P. B. 1963. Ion exchange chromatography of amino acids. A single column, high resolving, fully automatic procedure. Anal. Chem. 32:2055. Hatfield, E. E. 1970. Selected topics related to the amino acid nutrition of the growing ruminant. Fed. Proc. 29:44. Helmer, L. G. and E. E. Bartley. 1971. Progress in the utilization of urea as a protein replacer for ruminants. A review. J. Dairy Sci. 54:25. Hill, K. J. 1965. Abomasal secretory function in the sheep. In R. W. Dougherty, R. S. Allen, W. Burroughs, N. L. Jacobson and A. D. McGilliard. (Ed.). Physiology of Digestion in the Ruminant. Butterworths, Washington, D. C. Hill, F. W. and D. L. Anderson. 1958. Comparison of metabolizable energy and productive energy determinations with growing chicks. J. Nutr. 64:587. Hirose, Y., R. S. Emery, C.-F. Huffman and G. H.
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reported for sheep (Phillipson, 1964) to cattle on the basis of metabolic size (BW "Ts ). In view of the continuous secretory nature of abomasal glands described b y Hill (1965) and the constant intake of animals in this study, it is unlikely that changes in abomasal secretions would account for increases in N o f the magnitude observed. Alterations in rate of passage o f the indicator and]or N could result in artifactual increases in N reaching the abomasum. However based on the large number of samples taken and the actual increases in N observed when N was expressed as mg N/g dry matter (A. W. Young, unpublished data), such a possibility appears remote. Certain workers (Drennan etaL, 1970) have obtained variable results using chromic oxide as an "indicator to estimate rumen digestion from abomasal samples. Others (Waldo et aL, 1971) have reported consistent results using this indicator. In any event, little evidence exists to refute the validity o f the indicator technique when comparing rations of similar composition. Results o f this and related studies indicate a certain rationale for reduced performance of feedlot c a t t l e starting on NPN rations. Hungate (1966) has reviewed evidence to suggest that the thermodynamics of the rumen would place limits on microbial protein synthesis. Potter et al. (1969) postulated that reduced feedlot performance o f cattle on NPN rations may reflect inadequate microbial protein synthesis. Recently, Iowa w o r k e r s (Burroughs e t a l . , 1971) have suggeste d t h a t young, growing feedlot cattle m a y undergo amino acid deficiencies when started on NPN rations. Thus, t h e finding of quantitatively less N reaching the abomasum of steers on the NPN ration is consistent with these reports. Although ration utilization measured as N reaching the abomasum improved on b o t h rations, no "compensatory adaptation" on the urea ration was apparent. Since total N reaching the abomasum consists o f microbial N, endogenous N and undegraded feed N, estimation o f the quality of this protein to m e e t the tissue needs would be difficult. However, the rapid improvement in the post-ruminal N status o f steers on the SBM ration may be indicative o f ruminal escape o f a portion of the high quality SBM. Improvements in the N status of the ruminant b y post-ruminal
781
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Y O U N G ET AL. Potter, G. D., C. O. Little, N. W. Bradley and G. E. Mitchell, Jr. 1971. Abomasal nitrogen in steers fed soybean meal, urea, or urea plus two levels of molasses. J. Anim. Sci. 32:531. Reid, J. T. 1953. Urea as a protein replacement. A review. J. Dairy Sci. 36:955. Satter, L. D. and R. E. Roffler. 1973. Using NPN in the dairy cow ration. Proc. 34th Minn. Nutr. Conf. p. 45. Schaadt, H., Jr., R. R. Johnson and K. E. McClure. 1966. Adaptation to and palatability of urea, biuret and diammonium phosphate as NPN sources for ruminants. J. Anim. Sci. 25:73. Skeggs, L. T. 1957. An automated method for colorimetric analysis. Amer. J. Clin. Path. 28:311. Smith, G. S., R. S. Dunbar, G. A. McLaren, G. C. Anderson and J. A. Welch. 1960. Measurement of the adaptation response to urea nitrogen utilization in the ruminant. J. Nutr. 71:20. Snedecor, G. W. 1956. Statistical Methods. (5th Ed.). Iowa State University Press, Ames. Steel, R. G. D. and J. H. Torrie. 1960. Principles and Procedures of Statistics. McGraw-Hill Book Co., New York. Thornton, R. F. 1970. Urea excretion in ruminants. I. Studies in sheep and cattle offered the same diet. Australian J. Agr. Res. 21:323. Tucker, R. E. and J. P. Fontenot. 1970. Nitrogen metabolism in lambs fed urea as the only nitrogen source. J. Anim. Sci. 30:330. (Abstr.). Vercoe, J. E. 1969. The transfer of nitrogen from the blood to the rumen in cattle. Australian J. Agr. Res. 20:191. Virtanen, A. 1. 1966. Milk production of cows on protein-free feed. Science 153:1603. Waldo, D. R. 1967. Symposium: Nitrogen utilization by the ruminant: Nitrogen metabolism in the ruminant. J. Dairy Sci. 51:265. Waldo, D. R., J. F. Keys, Jr. and C. H. Gordon. 1971. Corn starch digestion in the bovine. J. Anim. Sci. 33:305. Welch, J. A., G. C. Anderson, G. A. McLaren, C. D. Campbell and G. S. Smith. 1957. Time, diethylstilbestrol and vitamin B t ~ in the adaptation of lambs to NPN utilization. J. Anim. Sci. 16:1034. Young, A. W., J. A. Boling and N. W. Bradley. 1973. Performance and plasma amino acids of steers fed soybean meal, urea or no supplemental nitrogen in finishing rationsl J. Anim. Sci. 36: 803.
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Conner. 1960. Effect of protein status on salivary urea secretions. J. Dairy Sci. 43:874. Houpt, T. R. 1959. Utilization of blood urea in ruminants. Amer. J. Physiol. 197:115. Houpt, T. R. and Katherine A. Houpt. 1968. Transfer of urea nitrogen across the rumen wall. Amer. J. Physiol. 214:1296. Hungate, R. E. 1966. The Rumen and Its Microbes. Academic Press, New York. Lewis, D. 1960. Ammonia toxicity in the ruminant. J. Agr. Sci. 55:111. Lewis, D., K. J. Hill and E. F. Annison. 1957. Studies on the portal blood of sheep. 1. Absorption of ammonia from the rumen of the sheep. Biochem. J. 66:587. Little, C. O., N. W. Bradley and G. E. Mitchell, Jr. 1966. Plasma amino acids in steers fed urea supplements. J. Anim. Sci. 25:260 (Abstr.). Ludwick, R. L., J. P. Fontenot and R. E. Tucker. 1971. Studies of the adaptation phenomenon by lambs fed urea as the sole nitrogen source: digestibility and nutrient balance. J. Anim. Sci. 33:1298. Masson, M. 1950. Microscopic studies of the alimentary micro-organisms of the sheep. Brit. J. Nutr. 4:Proc. VIII. McDonald, I. W. 1948. The absorption of ammonia from the tureen o f t h e sheep. Biochem. J. 42:584. McLaren, G. A. 1964. Symposium on microbial digestion in ruminants: nitrogen metabolism in the rumen. J. Anim. Sci. 23:577. McLaren, G. A., G. C. Anderson, L. I. Tsai and K. M. Barth. 1965. Level of readily fermentable carbohydrates and adaptation of lambs to all-urea supplemented rations. J. Nutr. 87:331: N. R. C. 1970. Nutrient Requirements of Domestic Animals. No. 1754. Nutrient Requirements of Beef Cattle. National Research Council, Washington, D. C. Oltjen, R. R. 1969. Effects of feeding ruminants non-protein nitrogen as the only nitrogen source. J. Anim. Sci. 28:673. Phillipson, A. T. 1964. The digestion and absorption of nitrogenous compounds in the rurdinant. In H. N. Munro and J. B. Allison. (Ed.). Mammalian Protein Metabolism. Academic Press, New York. Potter, G. D., C. O. Little and G. E. Mitchell, Jr. 1969. Abomasal nitrogen in steers fed soybean meal or urea. J. 6nim. Sci. 28:711.
Summary Abomasal cannulated steers were used to quantitate various nitrogenous components reaching the abomasum during a 68-day adjustment period to either soybean meal (SBM) or urea supplemented ear corn rations (11% crude protein, CP). Mean estimates showed that total nitrogen (N) reaching the abomasum per day amounted to 64.3 g and 71.7 g for steers fed the urea and SBM rations, respectively ( P < .05). Signigicant ( P < .05) increases in the peptide N fractions, boundamino N and free-amino N, accounted for the 11.5% increase in total N on the SBM ration. Following introduction of urea into the basal corn ration, abomasal N decreased for 14 days and then increased linearly (P < .06). For steers on the SBM ration, abomasal N decreased for 8 days and then increased linearly (P < .05) at a daily rate of 250 mg N greater (P < .05) than was observed on the urea ration. No significant differences in plasma amino acids were detected; however, a significant (P < .05) linear response to time was noted for most of the essential amino acids on the SBM ration. Plasma urea-N levels (mg/100 ml) were significantly ( P < .01) elevated for steers fed the urea ration (7.80 vs 6.47). Increases in plasma urea levels were associated with
adjusting steers to both rations ( P < .05). Throughout the trial daily increases in plasma urea amounted to .08 mg/100 ml and .03 mg/100 ml for the urea and SBM rations, respectively (P < .05). Results of this study showed that quantitatively more N reached the abomasum of steers fed the SBM ration over the 68-day trial. Increased N post-ruminally should be consistent with improved performance of cattle started on natural protein supplements as any "compensatory adaptation" to the urea ration was not apparent.
Introduction Utilization of nonprotein nitrogen (NPN) involves its conversion to ruminal ammonia and synthesis into microbial protein (McDonald, t948). Such utilization may improve with length of time fed (Reid, 1953; McLaren et al., 1965; Virtanen, 1966; Ludwick et al., 1971) and amount to .2% increase in the daily absorbed N retained (Smith e t al., 1960). Several recent reviews of N metabolism (McLaren, 1964; Chalupa, 1968; Oltjen, 1969; Helmer and Barfley, 1971) devote sections to discussion of possible adaptation responses to NPN. However, the bulk of the literature points to rather inconclusive elucidations of the adaptation phenomenon, particularly as to the area or site o f such adaptations. The study reported herein was instigated to measure the importance of adjusting steers to different N sources as reflected bY (1)quantitative and qualitative changes in nitrogenous constituents reaching the abomasum, (2) changesin plasma
1This paper (74-5-94) is published with the approval of the Director of the Kentucky Agricultural Experiment Station. 2Present Address: Department of Meat and Animal Science, University of Wisconsin, Madison. a Department of Animal Sciences. 775
JOURNAL
OF ANIMAL
SCIENCE, vol. 40, no. 4, 1975
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N I T R O G E N M E T A B O L I S M IN T H E BOVINE: A D J U S T M E N T TO N I T R O G E N SOURCE AS R E F L E C T E D BY C H A N G E S IN A B O M A S A L N I T R O G E N AND P L A S M A COMPONENTS 1
776
YOUNG ET AL.
urea levels, and (3) changes in plasma free amino acids. Experimental Procedure
TABLE 1. COMPOSITION OF RATIONS FED TO STEERS DURING RATION ADJUSTMENT EXPERIMENT
Ingredient
International reference numbera
Ration (%) Soybean meal Urea
G r o u n d ear
corn Soybean meal (44%) Urea 281 Ground limestone Dicalcium phosphate Salt Vitamin A, IU/kg Analyses: b Crude protein, % Gross energy, kcal/g Chromic oxide, g/kg feed
4-02-850
89.00
97.08
5-04-604 5-05~070
9.40 --
-1.21
6-02-632
.60
.50
6-01-080 6-04-152
-1.00
.20 1.00
7-05-143
2,200
2,200
10.84
11.02
3.96
4.00
2.15
2.20
aAtlas of Nutritional D a t a o n U n i t e d S t a t e s and Canadian Feeds. 1971. National A c a d e m y o f S c i e n c e s . W a s h i n g t o n . D. C. b A n a l y s e s are o n air-dry basis.
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Six yearling Hereford steers weighing from 250 to 300 kg were fitted with permanent abomasal cannulae as described by Dougherty (1955). Prior to and following surgery steers were fed 1.8 kg of ground ear corn and 2.7 kg of alfalfa hay daily. Three weeks after surgery the steers were paired on the basis of size and randomly assigned to either the urea or SBM ration shown in table 1. During the 7 days preceding the initiation of the experiment only ground ear corn (containing .22% chromic oxide) was fed to reduce possible carry-over effects of exogenous proteins other than corn. Steers were fed either 4 or 5 kg of their respective rations in equal morning and evening feedings. Rations fed during the experimental period were calculated to contain 11.0% CP, utilizing either SBM or urea as supplemental N sources. Rations were calculated to be isocaloric and isonitrogenous and analyzed essentially so. Each ration contained .22% chromic oxide as an indicator for quantitating N reaching the abomasum, and both rations were pelleted in an attempt to minimize sorting and feed refusals. Salt, steamed bonemeal and ground limestone were offered free-choice. Additionally, both rations surpassed established N.R.C. (1970) requirements for total protein and TDN for growing steers of their mean trial body weight (300 kg) for no gain and approximated the requirements for .25 kg daily gain. Samples of abomasal contents were collected so that each 2-hr interval following the morning and evening feeding would be represented in the composited sample. To reduce stress to the animal these collections were made over a 3-day period so that 6 hr elapsed between each sample collection. These samples were immediately frozen, and subsequently all 12 samples from each steer were thawed and composited to one sample for that particular steer. This sampling procedure was repeated at predetermined intervals to give values representing mean days 2, 8, 14, 20, 26, 35, 44, 53 and 68. Nitrogenous components of abomasal fluid were fractionated by the technique presented by Potter et al. (1969). Total N analyses were
conducted on whole abomasal contents by macrokjeldahl procedures (A.O.A.C., 1960). Chromic oxide content of feed and abomsal contents was determined using the procedure of Hill and Anderson (1958). Results of these analyses were used to compute the total quantity of N reaching the abomasum by conventional N:Cr2 03 ratio techniques. Jugular blood samples were collected in heparinized tubes 0, 2, 4 and 6 hr after the morning feeding, and were taken on days 0, 7, 14, 21,28, 35, 42, 49 and 63. Plasma free amino acids were determined for individual steers on samples taken at the initiation of experimental period, and at 3, 6 and 9 weeks. To obtain a more representative sample of the plasma amino acid pool, a 2 ml sample of plasma from each of the 2-hr bleedings was composited for amino acid
777
NITROGEN METABOLISM IN THE BOVINE
N i t r o g e n intake,
Results
In agreement with earlier reports (Potter et al., 1969; Tucker and Fontenot, 1970; Potter et al., 1971), more N (P < .05) reached the abomasum of animals fed SBM-supplemented rations than from those fed urea rations (table 2). Only on mean day 2 was the reverse true (figure 1). It may be speculated that substantial
80.9
82.2
N i t r o g e n in abomasum, g/day a
g/day
71.7 • 3.7 b
6 4 . 3 • 3.1
N i t r o g e n in abomasum, % of intake
88.6
78.2
aRation differences axe significant (P < .05). bSEM.
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ruminal washout of NPN to the abomasum occurred during the first few days on the urea ration. After this initial period, ureolytic activity of the rumen microorganisms was probably sufficiently high to permit rapid hydrolysis of the urea. Quantitative fractionations of the abomasal nitrogenous compounds revealed that the 11.5% more N on the SBM ration was attributable to a significantly (P < . 10) increased NPN fraction (table 3). Subfractionation of the NPN showed that the peptide N fractions, bound-amino N and free-amino N, were significantly (P < .05) higher in abomasal contents of steers fed the SBM ration. A somewhat different pattern of abomasal constituents was Yij = p + Ri + Lj + (RxL)ij +/~ (Xij -- X) previously reported (Potter et al., 1969) in that + eij increased protein N and similar NPN values were found for SBM - vs urea-supplemented Yij = dependent variable rations. Differences in the degree of gastric # = mean common to all observa- proteolysis could account for this difference tions since Masson (1950) has presented evidence that bacteria undergo digestion to varying Ri = effect of the ith ration degrees in the abomasum. Lj = effect of the jth level of feed Abomasal N reached its lowest value 8 days intake after the steers were exposed to the SBM ration (RxL)ij = interaction of ration (Ri) and and then increased rapidly (figure 1). However, level (Lj). following introduction of urea into the basal /3 = linear regression of Yij on time corn ration, abomasal N continued on a within Ri and Lj downward trend for 14 days and then increased at a slower rate. I f one considers the entire eij random errors, assumed to be feeding trial, total N reaching the abomasum independent and normally dis- increased for both rations (figure 1) and a significantly linear response was noted for the tributed. SBM ration ( P < .05) and urea ration (P< .06), as seen in table 4. As can be seen from the Where rations accounted for a significant portion of the partitioned sum of squares in the analysis of variance or for clarity of the results, TABLE 2. TOTAL NITROGEN IN THE ABOMASUM the ration was deleted from the above- OF STEERS FED SBM OR UREA (MEAN VALUES mentioned model and separate regressions ADJUSTED TO A COMMON TIME) established for each ration. Tests of homoRation geneity of these regressions were performed Item SBM Urea according to Steel and Torrie (1960). determinations. Amino acid analyses were similar to the ion-exchange chromatographic procedure outlined by Hamilton (1963). Plasma proteins were precipitated from the samples prior to analysis with 5-sulfosalicylic acid (5% W/V). Plasma urea nitrogen analyses were conducted on all plasma samples according to the procedure described by Skeggs (1957). Since the primary objective of this experiment was to study ration adjustment over the feeding period, a regression analysis was used in the statistical treatment of the data (Snedecor, 1956). The following model was applied to the variables studied.
778
YOUNG ET AL.
8O
~ 7s
/
/
Dependent Rationb variable (y)a SBM Urea Total N .55 + .17 c .30 • .15 d es . l / -''J \ ; ssM Protein N .33 -+ .06 c .21 • .16 d Nonprotein N .22 • .11 .09 + .08 eo~ ",1 Bound amino N .12 + .05 .08 + .04 Free amino N .02 • .01 .01 • .01 D A Y OF T R I A L Purine-pyrimiFigure 1. Total nitrogen reaching the abomasum of dine N - - . 0 1 • .01 .02 • .01 steers at intervals t h r o u g h o u t t h e experiment. Undetermined N .06 • .02 - - . 0 2 -+ .02 ..I
regression coefficients, the rate of increase in total N to the abomasum was 250 mg/day aExpresscd as g/day reaching the a b o m a s u m . greater for the SBM ration over the 68-day trial. bRegzession coefficient (~) + SE~ expressed as Protein N increased in a linear manner on both change per day (g) of the feeding period. and dbinear response is significant P < .05 and rations. The increasing quantities of post- P < c.06. ruminal nitrogen observed in this study might explain the increased nitrogen retention asOne hypothesis concerning the reduced sociated with adjusting lambs (Welch et al., performance associated with feeding urea 1957; Smith et al., 1960; McLaren et al., 1965; rations is that microbial protein synthesis may Schaadt et al., 1966; Ludwick et al., 1971) or be insufficient to meet the amino acid needs of dairy cows (Virtanen, 1966) to urea diets. the tissues. If an adaptation to urea feeding Although NPN increased on both rations, large occurred, it might be postulated that plasma variability in the data refused establishment of amino acid patterns would be influenced trends. accordingly. At the near maintenance levels of intake in this study, it is doubtful that TABLE 3. N I T R O G E N F R A C T I O N S IN ABOMASAL increased quantities of amino adds presented to CONTENTS OF STEERS FED SBM OR the intestines for absorption would become UREA (G/DAY) accumulated in greatly increased concentrations in the plasma pool. However, the linear increase Rationa in most essential amino acids as a function of Fraction SBM Urea time on the SBM ration (table 6) is suggestive Protein N 2 0 . 2 -+ 1 . 7 20.8 + 1.7 that had the animals remained on their diets Nonprotein N b 51.5 + 3.4 4 3 . 4 + 2.5 longer or at greater levels of intake treatment Bound amino responses might have been apparent. This Nc 26.0 • 1.7 21.2 • 1.2 observation is confirmed in feedlot situations Free amino with these rations, i.e., that steers receiving Nc 7.0• .4 5.8• .3 SBM" supplements to ear corn finishing rations Purine-pyrimihave elevated plasma levels of certain essential dineN 9.6• .6 9.4• .5 amino acids (Little et al., 1966; Freitag et al., Undetermined N 9.0 • .9 7.1 • .7 1968; Young et aL, 1973). Plasma urea levels for the 6-hr period following feeding were significantly higher for aMean values -+ SEM adjusted to a c o m m o n time. b and CRation differences are significant P < .10 steers on the urea ration (table 5). Over the and P < .05, ~espectively. 68-day trial plasma urea levels increased linearly
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T A B L E 4. REGRESSION COEFFICIENTS F O R OBSERVING THE I N F L U E N C E OF LENGTH OF THE FEEDING PERIOD ON NITROGEN F R A C T I O N S IN ABOMASAL CONTENTS OF STEERS FED SBM OR U R E A RATIONS
NITROGEN METABOLISM IN THE BOVINE TABLE 5. PLASMA FREE AMINO ACID AND UREA NITROGEN CONCENTRATIONS OF STEERS FED SOYBEAN MEAL OR UREA RATIONS
SBM
Urea
A m i n o acid, u r n / 1 0 0 ml Total amino acids 178.4 • 13.6 T o t a l essential 84.6 • 5.9 Total nonessential 93.8 +- 7.8 Threonine 10.6-+ 1.3 Valine 16.4 • 1.1 Methionine 2.2 • .2 Isoleucine 8.0 -+ .7 Lettcine 12.5 • 1.3 Phenylalaline 4.6 • .4 Lysine 10.9 • .9 Histidine 7.4 • .7 Arginine 12.1 • 1.0 A s p a r t i c acid Serine G l u t a m i c acid Proline Glycine Alanine Tyrosine Urea n i t r o g e n , rag/100 mlb
1.7 12.3 11.5 7.4 37.2 19.0 4.5
• + • +• • +
6.47 -+
.2 1.3 1.0 .8 3.7 1.4 .4 .30
1 8 0 . 6 • 14.8 84.9 -+ 7.5 95.7 10.9 15.8 2.2 8.3 13.2 4.5 11.0 7.6 11.5
-+ • • • • • • • • •
8.3 1.7 1.5 .2 .9 1.4 .5 1.2 .7 1.1
1.9 13.3 12.8 7.6 35.7 20.0 4.4
• • • • • • •
.1 1.3 1.2 .7 4.0 2.0 .4
7.80 +
.36
aMean values adjusted to a common time • SEM. bRations differ significantly (P < .01).
(P < .05) on both rations (table 6 and figure 2). However, the rate of increase was much greater (.08 mg/lO0 ml vs .03 mg/100 ml) on the urea ration. Discussion
Nitrogen arriving at the abomasum represents the net between dietary intake, preabomasal losses and extra-dietary (endogenous) influxes of N into the stomach compartments. At constant dietary intakes, increased quantities of abomasal N associated with length of exposure to the rations could have resulted from one or more of several factors. Improved ration utilization, as manifested in increased rates of microbial protein synthesis, w o u l d result in increased quantities of N available post-ruminally. It is generally agreed that ureolytic activity of fumen microorganisms far exceeds their protein synthetic capabilities. Further, any adaptation at the microflora level
is generally agreed to be quite rapid and, when measured as the ability of the microorganisms to assimilate ammonia, is largely accomplished in 7 (Lewis, 1960) to 19 days (Caffrey et al., 1967). Such a rapid adaptation would be consistent with increases in abomasal N observed in early phases of the trial. However, microbial adjustments fail to explain the prolonged and linear increases in post-ruminal N observed. Increasing the N recycled to the rumen should provide 'additional ammonia for microbial growth during periods of low rumen ammonia levels provided adequate carbohydrates are available. Most studies reveal an increase in urea returned to the rumen when ruminal ammonia levels, and ultimately plasma urea levels, are elevated (Lewis et aL, 1957; Houpt, 1959; Hirose et al., 1960). In studies with cattle, Vercoe (1969) and Thornton (1970) reported that maximal transfer of urea from blood to the rumen occurs at plasma urea levels of 12 mg/100 ml and 8 to 10 mg/100 ml, respectively. From the data of Vercoe (1969), it was estimated that 17 to 20 g N could be transferred to the rumen daily. Certain workers (Houpt and Houpt, 1968) suggest that transfer of urea across the rumen wall is a function of rumen ammonia concentration rather than blood urea concentration. However, most of the studies reviewed by Waldo (1967) show that total endogenous urea influx (salivary plus direct transfer) is largely governed by blood urea levels. Linear increases in plasma urea with time in our study would be consistent with increased urea return to the rumen. Data presented by Wisconsin workers (Satter and Roeffler, 1973) indicate average rumen ammonia levels for rations with this TDN and CP content would be fairly low (~3 mg/100 ml). Therefore, recycled urea would be utilized quite efficiently, resulting in increased microbial N reaching the abomasum. However, in the absence of urinary N excretion data, the relative magnitude of increased recycled N to the total N status of the animal cannot be ascertained. On the average (table 2), less N reached the abomasum than was consumed indicating urinary losses occurred. Additionally, such losses were probably greater on the urea ration with its correspondingly higher plasma urea levels. This observation agrees with the reciprocal relationship of plasma urea levels and
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Rationa Item
779
780
YOUNG ET AL.
TABLE 6. INFLUENCE OF LENGTH OF THE FEEDING PERIOD ON PLASMA AMINO ACIDS AND UREA FOR STEERS FED SOYBEAN MEAL OR UREA RATIONS Ration a
Dependent variable
Urea
30.09 16.46 13.63 2.24 3.42 .52 1.47 1.84 1.10 2.31 1.62 1.93
• + • +• ++ + + + ++
10.74 b 4.74 b 6.20 1.05 .86 b .16 b .53 b 1.03 .31 b ,79 b .53.b .76 b
20.22 7.84 12.37 1.26 1.08 .36 .45 .93 .57 .59 1.31 1.27
.64 .99 1.88 .87 4.43 3.55 1.26 .03
+ + + + + ++ +
.15 b 1.03 .83 b .60 2.89 1.08.b .35 I) .01 b
.61 1.83 1.66 1.27 4.34 2.06 .61 .08
Aspartic acid Serine Glutamic acid Proline Glycine Alanine Tyrosine Urea nitrogen, mg/100 ml cd
+ 11.69 • 5.94 • 6.55 + 1.36 + 1.21. + .16 b + .73 • 1.13 +.36 + .96 +.54b • .85 + ++ • + • • •
.10b 1.05 .93 .56 b 3.19 1.64 .29 .01 b
aRegression c o e f f i c i e n t s (~) + SEn expressed as change per 3-week period in units of the d e p e n d e n t variable. b Linear reaponse is significant (po< .05). CChange per day of the feeding period i n units of the d e p e n d e n t variable. d R a t i o n regressions differ significantly (P < .O5)
urinary N losses (Vercoe, 1969; Thornton, 1970). Recoveries of abomasal N in excess of dietary intake on the SBM ration during the later phases confirms the influx of endogenous N. Since our measurements were made on abomasal contents, it is apparent that endogenous N secretions into this organ accounted for a portion of the total N influx. Nitrogen influx via the gastric secretions and tissue slough probably represents a net increase in N since absorption of ammonia from this organ has been shown to be minimal (McDonald, 1948). Absorption of products of protein is generally assumed to be minimal in the monogastric stomach, and a similar situation probably exists in the abomasum (Gray et al., 1958). Relatively few studies are available quantitating gastric secretions in cattle, and the single reference to German studie s cited by Hill (1965) states that 30 to 35 liters are produced daily in the abomasum of the cow. Based on N contents reported for
sheep abomasal juices of 15 to 33 mg N/100 ml (Phillipson, 1964), from 4.5 to 11.6 g of endogenous N could be secreted daily in the abomasum of cattle. Similar estimates of influxes of 5.5 to 11.0 g N are obtained by extrapolating data on endogenous N secretions
.~ 10.0
s /
9.0 8.0
/ ~ 7.0 ~ 6,0 5.0 ~
~
UREA
~, 4.0 2
8
14
20
26
35
53
68
DAY OF TRIAL
Figure 2. Plasma urea concentrations measured throughout the experiment. Each value represents an average of samples taken at 0, 2, 4 and 6 hr after the morning feeding.
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Amino acid, pm/100 ml Total amino acids Total essential Total nonessential Threonine Valine Methionine Isoleucine Leucine Phenylalanine Lysine Histidine Arginine
SBM
NITROGEN METABOLISM IN THE BOVINE
infusions of several proteins have been reported (Hatfield, 1970). In the case of sheep fed rations low in N and infused per duodenum with casein, c o n c o m i t a n t improvements in appetite were observed (Egan, 1965). Importance o f increased appetite in adjusting feedlot cattle to rations cannot be over-einphasized. Literature Cited A.O.A.C. 1960. Official Methods of Analysis. (9th Ed.). Association of Official Agricultural Chemists. Washington, D. C. Burroughs, W., A. H. Trenkle and R. L. Vetter. 1971. Some concepts of protein nutrition of feedlot cattle (metabolizable protein and metabolizable amino acids). Vet. Med. Small Anim. Clin. 66:238. Caffrey, P. J., E. E. Hatfield, H. W. Norton and U. S. Garrigus. 1967. Nitrogen metabolism in the ovine. I. Adjustment to a urea-rich diet. J. Anita. Sci. 26:595. Chalupa, W. 1968. Problems in feeding urea to ruminants. J. Anim. Sci. 27:207. Dougherty, R. W. 1955. Permanent stomach and intestinal fistulas in ruminants; some modifications and simplifications. Cornell Vet. 45: 331. Drennan, M. J., J. H. G. Holmes and W. N. Garrett. 1970. A comparison of markers for estimating magnitude of rumen digestion. Brit. J. Nutr. 24: 961. Egan, A. R. 1965. Nutritional status and intake regulation in sheep. II. The influence of sustained duodenal infusions of casein or urea upon voluntary intake of low-protein roughages by sheep. Australian J. Agr. Res. 16:451. Freitag, R. R., W. H. Smith and W. M. Beeson. 1968. Factors related to the utilization of urea vs. protein-nitrogen supplemented diets by the ruminant. J. Anita. Sci. 27:478. Gray, F. V., A. F. Pilgrim and R. A. Weller. 1958. The digestion of foodstuffs in the stomach of the sheep and the passage of digesta through its compartments. 2. Nitrogenous compounds. Brit. J. Nutr. 12:413. Hamilton, P. B. 1963. Ion exchange chromatography of amino acids. A single column, high resolving, fully automatic procedure. Anal. Chem. 32:2055. Hatfield, E. E. 1970. Selected topics related to the amino acid nutrition of the growing ruminant. Fed. Proc. 29:44. Helmer, L. G. and E. E. Bartley. 1971. Progress in the utilization of urea as a protein replacer for ruminants. A review. J. Dairy Sci. 54:25. Hill, K. J. 1965. Abomasal secretory function in the sheep. In R. W. Dougherty, R. S. Allen, W. Burroughs, N. L. Jacobson and A. D. McGilliard. (Ed.). Physiology of Digestion in the Ruminant. Butterworths, Washington, D. C. Hill, F. W. and D. L. Anderson. 1958. Comparison of metabolizable energy and productive energy determinations with growing chicks. J. Nutr. 64:587. Hirose, Y., R. S. Emery, C.-F. Huffman and G. H.
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reported for sheep (Phillipson, 1964) to cattle on the basis of metabolic size (BW "Ts ). In view of the continuous secretory nature of abomasal glands described b y Hill (1965) and the constant intake of animals in this study, it is unlikely that changes in abomasal secretions would account for increases in N o f the magnitude observed. Alterations in rate of passage o f the indicator and]or N could result in artifactual increases in N reaching the abomasum. However based on the large number of samples taken and the actual increases in N observed when N was expressed as mg N/g dry matter (A. W. Young, unpublished data), such a possibility appears remote. Certain workers (Drennan etaL, 1970) have obtained variable results using chromic oxide as an "indicator to estimate rumen digestion from abomasal samples. Others (Waldo et aL, 1971) have reported consistent results using this indicator. In any event, little evidence exists to refute the validity o f the indicator technique when comparing rations of similar composition. Results o f this and related studies indicate a certain rationale for reduced performance of feedlot c a t t l e starting on NPN rations. Hungate (1966) has reviewed evidence to suggest that the thermodynamics of the rumen would place limits on microbial protein synthesis. Potter et al. (1969) postulated that reduced feedlot performance o f cattle on NPN rations may reflect inadequate microbial protein synthesis. Recently, Iowa w o r k e r s (Burroughs e t a l . , 1971) have suggeste d t h a t young, growing feedlot cattle m a y undergo amino acid deficiencies when started on NPN rations. Thus, t h e finding of quantitatively less N reaching the abomasum of steers on the NPN ration is consistent with these reports. Although ration utilization measured as N reaching the abomasum improved on b o t h rations, no "compensatory adaptation" on the urea ration was apparent. Since total N reaching the abomasum consists o f microbial N, endogenous N and undegraded feed N, estimation o f the quality of this protein to m e e t the tissue needs would be difficult. However, the rapid improvement in the post-ruminal N status o f steers on the SBM ration may be indicative o f ruminal escape o f a portion of the high quality SBM. Improvements in the N status of the ruminant b y post-ruminal
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Y O U N G ET AL. Potter, G. D., C. O. Little, N. W. Bradley and G. E. Mitchell, Jr. 1971. Abomasal nitrogen in steers fed soybean meal, urea, or urea plus two levels of molasses. J. Anim. Sci. 32:531. Reid, J. T. 1953. Urea as a protein replacement. A review. J. Dairy Sci. 36:955. Satter, L. D. and R. E. Roffler. 1973. Using NPN in the dairy cow ration. Proc. 34th Minn. Nutr. Conf. p. 45. Schaadt, H., Jr., R. R. Johnson and K. E. McClure. 1966. Adaptation to and palatability of urea, biuret and diammonium phosphate as NPN sources for ruminants. J. Anim. Sci. 25:73. Skeggs, L. T. 1957. An automated method for colorimetric analysis. Amer. J. Clin. Path. 28:311. Smith, G. S., R. S. Dunbar, G. A. McLaren, G. C. Anderson and J. A. Welch. 1960. Measurement of the adaptation response to urea nitrogen utilization in the ruminant. J. Nutr. 71:20. Snedecor, G. W. 1956. Statistical Methods. (5th Ed.). Iowa State University Press, Ames. Steel, R. G. D. and J. H. Torrie. 1960. Principles and Procedures of Statistics. McGraw-Hill Book Co., New York. Thornton, R. F. 1970. Urea excretion in ruminants. I. Studies in sheep and cattle offered the same diet. Australian J. Agr. Res. 21:323. Tucker, R. E. and J. P. Fontenot. 1970. Nitrogen metabolism in lambs fed urea as the only nitrogen source. J. Anim. Sci. 30:330. (Abstr.). Vercoe, J. E. 1969. The transfer of nitrogen from the blood to the rumen in cattle. Australian J. Agr. Res. 20:191. Virtanen, A. 1. 1966. Milk production of cows on protein-free feed. Science 153:1603. Waldo, D. R. 1967. Symposium: Nitrogen utilization by the ruminant: Nitrogen metabolism in the ruminant. J. Dairy Sci. 51:265. Waldo, D. R., J. F. Keys, Jr. and C. H. Gordon. 1971. Corn starch digestion in the bovine. J. Anim. Sci. 33:305. Welch, J. A., G. C. Anderson, G. A. McLaren, C. D. Campbell and G. S. Smith. 1957. Time, diethylstilbestrol and vitamin B t ~ in the adaptation of lambs to NPN utilization. J. Anim. Sci. 16:1034. Young, A. W., J. A. Boling and N. W. Bradley. 1973. Performance and plasma amino acids of steers fed soybean meal, urea or no supplemental nitrogen in finishing rationsl J. Anim. Sci. 36: 803.
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Conner. 1960. Effect of protein status on salivary urea secretions. J. Dairy Sci. 43:874. Houpt, T. R. 1959. Utilization of blood urea in ruminants. Amer. J. Physiol. 197:115. Houpt, T. R. and Katherine A. Houpt. 1968. Transfer of urea nitrogen across the rumen wall. Amer. J. Physiol. 214:1296. Hungate, R. E. 1966. The Rumen and Its Microbes. Academic Press, New York. Lewis, D. 1960. Ammonia toxicity in the ruminant. J. Agr. Sci. 55:111. Lewis, D., K. J. Hill and E. F. Annison. 1957. Studies on the portal blood of sheep. 1. Absorption of ammonia from the rumen of the sheep. Biochem. J. 66:587. Little, C. O., N. W. Bradley and G. E. Mitchell, Jr. 1966. Plasma amino acids in steers fed urea supplements. J. Anim. Sci. 25:260 (Abstr.). Ludwick, R. L., J. P. Fontenot and R. E. Tucker. 1971. Studies of the adaptation phenomenon by lambs fed urea as the sole nitrogen source: digestibility and nutrient balance. J. Anim. Sci. 33:1298. Masson, M. 1950. Microscopic studies of the alimentary micro-organisms of the sheep. Brit. J. Nutr. 4:Proc. VIII. McDonald, I. W. 1948. The absorption of ammonia from the tureen o f t h e sheep. Biochem. J. 42:584. McLaren, G. A. 1964. Symposium on microbial digestion in ruminants: nitrogen metabolism in the rumen. J. Anim. Sci. 23:577. McLaren, G. A., G. C. Anderson, L. I. Tsai and K. M. Barth. 1965. Level of readily fermentable carbohydrates and adaptation of lambs to all-urea supplemented rations. J. Nutr. 87:331: N. R. C. 1970. Nutrient Requirements of Domestic Animals. No. 1754. Nutrient Requirements of Beef Cattle. National Research Council, Washington, D. C. Oltjen, R. R. 1969. Effects of feeding ruminants non-protein nitrogen as the only nitrogen source. J. Anim. Sci. 28:673. Phillipson, A. T. 1964. The digestion and absorption of nitrogenous compounds in the rurdinant. In H. N. Munro and J. B. Allison. (Ed.). Mammalian Protein Metabolism. Academic Press, New York. Potter, G. D., C. O. Little and G. E. Mitchell, Jr. 1969. Abomasal nitrogen in steers fed soybean meal or urea. J. 6nim. Sci. 28:711.
