Influence of Levels of Supplemental Urea on Characteristics of Digestion and Growth Performance of Feedlot Steers Fed a Fat-Supplemented High-Energy Finishing Diet

The Professional Animal Scientist 10:5-10

Influence of Levels of Supplemental Urea on Characteristics of Digestion and Growth Performance of Feedlot Steers Fed a FatSupplemented High-Energy Finishing Diet R. A. ZINN, PAS, J. L. BORQUEZ, and A. PLASCENCIAl Imperial Valley Agricultural Center University of California EI Centro, CA 92243

can be efficiently utilized as the sole source of supplemental nitrogen in fat-supplemented highenergy finishing diets for feedlot cattle. Furthermore, feedlot cattle growth performance may be enhanced by supplementing diets with urea at levels beyond that necessary for maximal microbial growth. (Key Words: Cattle, Urea, Dry Lot Feeding, Metabolism.)

Abstract

Four Holstein steers (382 kg) with cannulas in the rumen and proximal duodenum were used in a 4 x 4 Latin square experiment. Treatments consisted of 90% concentrate (steam-flaked cornbased) diet supplemented with either 5.5% soybean meal (SBM) or .8, 1.2, or 1.6% urea (dry matter basis). All diets contained 4% tallow. There were no treatment effects (P>.10) on ruminal pH and ammonia concentrations. Ruminal digestion of organic matter, acid detergent fiber, and starch tended to increase with supplemental urea level. There was a quadratic component (P<.10) to urea level effects on total tract dry matter and organic matter digestibility, being maximal with 1.2% supplemental urea. Total tract acid detergent fiber and starch digestion increased (P<.05) with urea level. Passage of nonammonia nitrogen, microbial nitrogen, and feed nitrogen were similar (P>.10) across treatments. Nonammonia nitrogen entering the small intestine/ nitrogen intake decreased linearly (P<.05) with increasing level of urea supplementation. Thirty-two Holstein steers were used in a 4 x 4 Latin square experiment to evaluate treatment effects on feedlot growth performance. Experimental periods were 35 d in duration. There were no treatment effects (~.10) on daily weight gain, dry matter intake, and diet net energy. Observed:expected net energy increased (P<.10) with increasing urea level. Urea

Introduction

Factors controlling the efficient use of supplemental urea in finishing diets for feedlot cattle are not well defined. Based on early reviews of the literature, it was proposed that supplemental NPN could be effectively utilized by cattle provided it consisted of not more than one-third of the total N or 1% of total diet OM (5, 20). However, there was very little empirical or economical basis for these highly generalized recommendations. More recently (17), recommendations of upper limits for NPN utilization have been factorialized on the basis of fermentation potential. This approach is more appealing from a conceptual standpoint. However, it too has very limited empirical support in practical growing-finishing diet formulations, not only from the standpoint of ruminal N efficiency, but also with respect to effects of levels of supplementation on diet acceptability and feedlot growth performance. The latter is of particular concern in fat-supplemented diets. In several studies (2, 8, 11, 22) negative associative effects on growth performance have been observed between supplemental fat (3

l lnstituto de Investigaciones en Ciencias Veterinarias UABC Mexicali, Mexico. ' , Reviewed by H. W. Essig, Z. Johnson, and L. W. Luther.

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ZINN ET AL.

TABLE 1. Composition of experimental diets fed to steers, Trials 1 and ~.

Item

Soybean meal control .8%

Urea level 1.2%

1.6%

5.00 5.00 78.14

5.00 5.00 77.74

5.00 5.00

77.34

5.00 4.00 1.56 .50

.80 5.00 4.00 1.56 .50

1.20 5.00 4.00 1.56 .50

1.60 5.00 4.00 1.56 .50

Nutrient composition (OM basis) NE, Mcallkg C tv1aintenance 2.34 Gain 1.64 Crude protein, % 11.3 Ether extract, % 7.4 AOF,% 8.3 CalCium, % .72 Phosphorus, % .31 Sulfur, %C .17

2.31 1.62 10.6 7.5 9.1 .70 .28 .15

2.30 1.61 11.7 7.5 8.8 .70 .28 .15

2.29 1.60 12.8 7.5 9.3 .70 .28 .15

Ingredients, % (OM basis) Sudangrass hay Alfalfa hay Steam-flaked corn Soybean meal Urea Cane molasses Tallow Limestone TM saltb

5.00 5.00 73.44 5.50

kg/L (21 Ib/bushel). Chromic oxide was added to the diets as a digesta marker. Dry matter intake was restricted to 6.3 kg/d. Diets weie fed at 0800 and 2000 daily. Experimental periods consisted of a 10-d diet adjustment period followed by a 4-d collection period. During the collection period, duodenal and fecal samples were taken from all steers, twice daily as follows: d 1, 0750 and 1350; d 2, 0900 and 1500; d 3, 1050 and 1650; and d 4, 1200 and 1800. Individual samples consisted of approximately 500 mL of duodenal chyme and 200 g (wet basis) of fecal material. Samples from each steer and vvithin each col-

lection period were composited for analysis. During the final day of each collection period, ruminal samples were obtained from each steer at approximately 4 h postprandial via the ruminal cannula. Ruminal fluid pH was determined on fresh samaChromic oxide (.4% OM basis) was added to diets in Trial 1 as a ples. Samples were then strained through four laydigesta marker. ' . ers of cheese cloth. Ten milliliters of strained rumibTrace mineral salt: CoS0 4, .068%; CUS04' 1.04%; FeS04' 3.57%; nal fluid was centrifuged at 5,000x for 10 min and ZnO, .75%; MnSo 4, 1.07%; KI, .052%; and NaCI, 93.4%. ammonia N concentration in supernate was deterCBased on tabular values of nutrient composition of individual diet ingredients (NRC, 1984), with the exception of tallow that was assigned mined according to Fawcett and Scott (6). Two NEm and NEg values of 6.03 and 4.88 Mcal/kg, respectively (25); and SBM, milliliters of freshly prepared 25% (wtlvol) which was assigned NEm and NEg values of 2.70 and 1.96 Mcallkg, respectively (27). metaphosphoric acid was added to 8 mL of strained ruminal fluid. Samples were then centrifuged (17,000x for 10 min) and supernatant fluid to 6%, DM basis) and high levels of supplemental stored at -20'C for VFA analysis. Upon completion urea (1.25 to 2%, DM basis). The objective of this of the trial, ruminal fluid was obtained from all study was to evaluate the effects of levels of supsteers and composited for isolation of ruminal bacplemental urea on digestive function and feedlot teria via differential centrifugation. Samples were subjected to all or part of the following analysis: DM growth performance of steers fed a fat-sup--------IJ-IplemeA~~~~---tl+ettt.------\-'-(oLJlv=en.--rlr¥ing at 105'C IJntii no further weight loss) ~ ash, Kjeldahl N, ammonia N (1); purines (30); VFA concentrations of ruminal fluid (31); chromic oxide Materials and Methods (10); and starch (26). Microbial organic matter (MOM) and N (MN) leaving the abomasum were Trial 1. Four Holstein steers (382 kg) with 'T' calculated using purines as a microbial marker (30). cannulas in the rumen and proximal duodenum (31) Organic matter fermented in the rumen (OMF) was were used in a 4 x 4 Latin square experiment to considered equal to OM intake minus the differevaluate the effects level of urea supplementation ence between the amount of total OM reaching the has on characteristics of ruminal and total tract duodenum and MOM reaching the duodenum. digestion of dietary OM, ADF, starch, and N. TreatFeed N escape to the small intestine was considments consisted of 90% concentrate diet supered equal to total N leaving the abomasum minus plemented with either soybean meal (S8M) or .8 ammonia Nand MN and, thus, includes any en(U-.8), 1.2 (U-1.2), or 1.6% urea (U-1.6; DM basis). dogenous contributions. Methane production was Composition of experimental diets is shown in Tacalculated based on the theoretical fermentation ble 1. The steam-flaked corn was prepared by steaming corn at atmospheric pressure (steam balance for observed molar distribution of VFA and chest set at 103'C) before rolling to a density of .27 OM fermented in the rumen (24). The trial was

7

EFFECT OF UREA ON DIGESTION AND GROWTH OF STEERS

analyzed as a 4 x 4 Latin square experiment (9). Treatment effects were tested for the following orthogonal comparisons: 1) SBM vs urea; 2) linear component of urea level; and 3) quadratic component of urea level. Trial 2. Thirty-two Holstein steers were used in a 4 x 4 Latin square experiment to evaluate treatment effects on feedlot growth performance. Steers were balanced by weight and assigned to four pens (eight steers per pen) equipped with automatic waterers and fence-line feed bunks. The trial was initiated March 20, 1991 and concluded August 7, 1991. Composition of dietary treatments is shown in Table 1. Steers were implanted with Synovex-S® (Syntex Corp., Des Moines, IA) upon initiation of the trial and then again on d 70. Experimental periods were 35 d in duration. Measures of steer performance were based on pen means. Assuming the primary determinant of energy gain was weight gain, the energy gain was calculated by the equation: EG = (.0557 W 75)g1.097, where EG is the daily energy deposited (megacalories per day), g is weight gain (kilograms per day), and W is the mean body weight (kilograms; NRC, 1984). Maintenance energy expended (megacalories per day, EM) was calculated by the equation: EM = .084W·75 (7). From the derived estimates for energy required for maintenance and gain, the NE for maintenance (NEm) and gain (NEg) of the diets were obtained by the process of iteration to fit the relationship: NEg = .877NEm . . :. .41 [derived from (17)]. The trial was analyzed as a 4 x 4 Latin square design experiment with contrasts as indicated for Trial 1. The protocol for this trial was approved by the University of California Animal Use and Care Administrative Committee. Results and Discussion

Influence of dietary urea level on ruminal pH, ammonia concentration, and ruminal VFA molar proportions 4 h after feeding is shown in Table 2. There was a quadratic component to urea level response on ruminal molar proportions of acetate (P<.05) and estimated methane production (P<.10); levels were lowest with U-1.2. The basis for this response is not certain. Dietary urea levels ranging from .6 to 1.8% did not influence ruminal VFA concentrations in lambs fed a wheat straw-based growing diet (23).

TABLE 2. Influence of urea level in a high-concentrate diet on ruminal pH, ammonia concentration, VFA molar proportions, and estimated methane production 4 h after feeding, Trial 1.

Item

Soybean meal control

pH 5.70 Ammonia, mg/100 mL 6.0 Ruminal VFA, moV100 mol Acetate a 53.4 Propionate 35.5 Butyrate 11.1 Methane productionb,c .42

Urea level .8%

1.2%

1.6%

SO

5.36 4.0

5.72 7.1

5.43 6.4

.31 3.3

53.7 36.2 10.1 .41

49.2 38.9 11.9 .37

57.2 32.4 10.4

.46

3.5 4.5 2.2 .06

aUrea level quadratic effect, P<.05. bMethane, mole per mole glucose equivalent fermented. cUrea level quadratic effect, P<.10.

Differences in ruminal pH and ammonia concentrations were small and not affected by treatment (P>.10). Thomas et al. (21) also observed that beyond 3 h after feeding, ruminal ammonia concentrations were not affected by dietary urea level (diets contained .7 to 1.2% urea). According to prediction equation of Satter and Roffler (19), expected ruminal ammonia concentrations (average for feeding interval) for the control, U-.8, U-1.2, and U-1.6 are 1.5, 1.0, 1.8, and 3.1 mg/dL, respectively. Average ruminal ammonia concentration in Trial 1 was 5.9 mg/dL (Table 2). Whether or not higher ruminal ammonia levels are a characteristic of fatsupplemented diets is not certain, although Thompson et al. (22) observed increases in ruminal ammonia concentrations of 9 to 20% with supplementation of 5% tallow. The influence of dietary urea level on characteristics of ruminal and total tract digestion are shown in Table 3. Ruminal digestion of OM, ADF, and starch were not affected (P>.1 0) by supplemental urea level. There was a quadratic component (P<.10) to urea level effects on total tract OM and OM digestibility, being maximal with 1.2% supplemental urea. The increase in OM digestibility was related in part to increased total tract ADF and starch digestion (linear effect, P<.05). Differences in postruminal digestion of OM, ADF, and starch were small (P>.10). Thus, beneficial effects of increasing levels of supplemental urea on total tract digestion were primarily due to enhanced ruminal digestion. This is consistent with Mehrez et al. (15), who observed increased in situ ruminal digestion of barley with dietary urea levels presumably in excess of

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ZINN ET AL.

TABLE 3. Influence of urea level on characteristics of digestion of a highconcentrate finishing diet, Trial 1.

Variable Intake, g/d OM OM AOF N Nonurea N Starch Flow to duodenum, g/d OM AOF

Soybean meal control .8%

Urea level

1.2%

1.6%

6,309 5,983 525 114 114.0 3,214

6,266 5,971 570 106 83.6 3,270

6,265 5,952 550 117 83.2 3,223

6,264 5,938 580 128 82.8 3,1 59

2,659 270

2,972 286

2,378 228

2,437 237

670

"An

SO

503 67 262

supplemented diets. Passage of nonammonia N, microbial N, and feed N were similar (P>.10) across treatments. Similarity in passage of feed N for the S8M and urea treatment is indicative of high ruminal degradability of SBM. Assuming that corn protein has a ruminal degradability of 50% and that urea has a ruminal degradability of 100%, then ruminal degradability of SBM was 82%. This high ruminal N degradability for SBM is consistent with previous studies [76%, (16); 80%, (12); 83%, (28); 76%, (29); 80%, (27)].

Ruminal N efficiency (nonammonia N entering the smaii intestineiN intake) decreased iineariy AmmoniaN8 2.61 3.54 3.68 3.99 .46 (P<.05) with increasing level of urea supplementaNonammonia N 120 122 117 123 8 72.3 77.6 Microbial N 70.5 73.3 5.5 tion. This reflects ruminal microbial synthesis, and 44.8 Feed N 45.4 6.7 49.1 48.5 thus, nonammonia N passage to the small intestine Ruminal digestion, % 72.2 72.0 8.6 OM 67.3 62.5 was not enhanced by supplementing the basal diet 58.6 59.1 12.1 AOF 48.6 49.7 with more than .8% urea. Furthermore, providing a 89.5 89.2 Starch 84.4 79.5 8.1 ruminally degradable intact protein source (SBM) Feed Nb 56.7 55.0 61 .1 64.8 6.0 46.2 45.1 Nonurea feed NC 7.9 42.0 56.9 did not promote greater microbial synthesis than N efficiencyde 1.05 1.13 .06 1.02 .95 with supplemental urea alone. Postruminal digestion, % duodenal The NRC (17) puts forth two equations for deterOM 69.1 70.6 4.7 71.5 71.0 mining the urea fermentation potential (UFP) of the 20.3 AOF 15.5 7.6 15.5 16.1 Ne 1.9 76.0 76.2 78.0 79.8 diet. One was derived from Burroughs et al. (4) and 2.2 Starch 95.1 94.8 95.7 96.4 the other was derived from Satter and Roffler (19). Fecal excretion, g/d OM e! 827 1,061 105 Given that ruminal degradability of nonurea feed N 799 937 OMb 103 704 672 799 873 was 44% (Table 2) and that the basal diet conAOFe! 221 244 190 199 22 Nb tained 1.34% N, the expected UFP of the diet is 2.1 25.7 25.6 2.5 29.7 29.2 13.4 9.8 8.2 Starch e 18.5 24.8 and 2.0%, respectively. Clearly, both systems Total tract digestion, recommended by NRC (17) greatly exaggerated % OMe! 1.7 86.8 85.1 83.1 87.2 the amount of urea necessary to maximize OMb 1.7 88.7 88.1 86.6 85.4 microbial growth. Unfortunately, there are no meAOFeg 4.1 65.7 65.4 57.9 57.2 Nh 80.1 2.4 7.4.3--72.4 7+.7 rl--~~----1 taabeHsm--s-t\;leies--r-eJ30I'ted in the literature th-a-t- 99.7 2.5 Starch e 99.4 99.2 99.6 evaluate the UFP concept in cattle fed high-energy 8SBM vs urea, P<.01. finishing diets. However, based on protein supply to bUnear effect of urea level, P<.10. the small intestine and growth performance, Willms cSBM vs urea, P<.05. et al. (23) found that both systems overestimated dOuodenal nonammonia N/N intake. UFP of a wheat straw-based diet fed to lambs. the eUnear effect of urea level, P<.05. Thomas et al. (21) observed no differences in ADG !Quadratic effect of urea level, P<.10. and feed conversion among steers fed growinggSBM vs urea, P<.10. finishing diets in which urea was added in amounts hLinear effect of urea level, P<.05 . calculated (4) to be 75, 100, and 125% of UFP. Limitations of the UFP concept include the oversimplification of the relationship between net that needed for maximal microbial growth. Willms synthesis and the NE content of the diet, microbial et al. (23) also noted increased OM digestion with and failure to incorporate the contribution of recyincreasing level of urea supplementation up to 1.2% cled N to the ruminal N pool. Although it may be of diet DM. granted that over the broad range of potential diePassage of ammonia N to the small intestine tary NE some relationship between dietary NE and was greater (43%, P<.01) for urea- than for SBMStarch

.,,,, ...,,,

V'1V

""

~I

EFFECT OF UREA ON DIGESTION AND GROWTH OF STEERS TABLE 4. Influence of urea level in a high-concentrate diet on performance of feedlot steers and net energy value of the diet, Trial 2.

Variable Days on test Live weight, kg8 Initial Final Weight gain, kg/d DM intake, kg/d DM intake:gain, g:g Diet NE, Mcallkg Maintenance Gain Observed:expected NE b NE"b 9

Soybean meal control

35

Urea level

.8%

1.2%

1.6%

35

35

35

SD

355 404 1.41 7.26 5.26

356 404 1.38 7.26 5.38

357 406 1.39 7.21 5.32

356 406 1.43 7.15 5.16

1.1 1.6 .05 .12 .22

2.38 1.68

2.34 1.64

2.37 1.67

2.43 1.72

.07 .06

1.02 1.02

1.01 1.01

1.03 1.03

1.06 1.07

.03 .04

81nitial and final weights were reduced 4% to account for digestive tract fill. bLinear effect of urea level, P<.10.

microbial synthesis exists, within the more narrow limits of NE that would be considered practical for growing-finishing feedlot cattle, no such relationship has been demonstrated. Indeed, summarizing 12 metabolism trials conducted at this center involving steers fed diets ranging from 1.76 to 2.42 Mcal/kg of NEm, dietary NEm accounted for only 2.4% of the variation in microbial N passage to the small intestine per kilogram of DM intake. With respect to N recycling, Bunting et al. (3) demonstrated that with low-protein growing diets (30% forage) as much as 40% of the microbial N may be derived from blood urea N without diminishing net synthesis. In the present study, net flux of blood urea N into ruminal microbial N was 19% in steers fed the lowest level of urea (.8%). Accordingly, net passage of N to the small intestine may have been maximized with as little as .31 % supplemental urea. The influence of urea level on growth performance of feedlot steers is shown in Table 4. Treatment effects on ADG, DM intake, and diet NE were small (P> .10), although there was a slight trend for increased ADG and decreased DM:gain with increasing urea level. Because urea, which has an NEm of 0 was substituted in the basal diet for steam-flaked corn with an NEm of 2.38 Mcal/kg (17), increasing the urea level is expected to decrease the NE of the diet proportionately. Nevertheless, observed:expected diet NE increased linearly (P<.10) with increasing urea level. Lofgreen et al. (13) also observed increased diet NE as urea level

9

increased from 0 to 1.3%, and this notwithstanding the CP content of the diets averaged 14%! These responses to added urea seem to be primarily r~lated to enhanced ruminal and total tract digestion of OM and not to changes in protein flow to the small intestine, as shown previously (Table 3). The high ruminal degradability of SBM (Table 3) along with failure to detect statistically significant differences (P<.10) in feedlot performance (Table 4) between the SBM control and the urea-supplemented diets (Table 3) implies that SBM and urea are similar as N sources in finishing diets for feedlot cattle. This finding is consistent with some studies (14, 18). However, urea has been inferior as a supplemental N in less energy dense (less than 1.7 Mcal/kg of NEm) growing-finishing diets (21). In contrast with the results of this study and those of Lofgreen et al. (13), numerous studies (2, 8, 11, 22) have demonstrated a negative associative effect between urea compared with intact protein as sources of supplemental N in growingfinishing diets for cattle. The basis for these earlier findings are unclear. However, in all cases, energy density of the diets were below 1.6 Mcal/kg of NEm (dry matter basis). As indicated previously, urea has performed less favorably compared with intact protein in lower energy growing-finishing diets (21). Conclusions

The urea fermentation potential, as presently set forth (17) is invalid. Feedlot cattle growth performance may be enhanced by supplementing diets with urea at levels beyond that necessary for maximal microbial growth. Urea can be efficiently utilized as the sole source of supplemental nitrogen in fat-supplemented high-energy finishing diets for feedlot cattle. Literature Cited 1. AOAC. 1975. Official methods of analysis (12th Ed.). Association of Official Analytical Chemists, Washington, DC. 2. Buchanan-Smith, J. G., G. K. Macleod, and D. N. Mowat. 1974. Animal fat in low-roughage diets for ruminants: The effect of nitrogen source and an amino acid supplement. J . Anim. Sci. 38:133. 3. Bunting, L. D., J . A. Boling, and C. T. MacKown . 1989. Effect of dietary protein level on nitrogen metabolism in the growing bovine: I. Nitrogen recycling and intestinal protein supply in calves. J. Anim. Sci. 67:81 0. 4. Burroughs, W., D. K. Nelson, and D. R. Mertens. 1975. Protein physiology and its application in the lactating cow: The metabolizable protein feeding standard. J. Anim. Sci. 41:933.

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ZINN ET AL.

5. Chalupa, W. 1968. Problems in feeding urea to ruminants. J. Anim. Sci. 27:207. 6. Fawcett, J. K., and J. E. Scott. 1960. A rapid and precise method for the determination of urea. J. Clin. Pathol. 13:156. 7. Garrett, W. N. 1971 . Energetic efficiency of beef and dairy steers. J. Anim. Sci. 32:451. 8. Hatch, C. F., T. W. Perry, M. T. Mohler, and W. M. Beeson. 1972. Effect of added fat with graded levels of calcium and urea-containing rations for beef cattle. J. Anim. Sci. 34:483. 9. Hicks, C. R. 1973. Fundamental concepts in the design of experiments. Holt, Rinehart and Winston, Inc., New York. 10. Hill, F. N., and D. L. Anderson. 1958. Comparison of metabolizable energy and productive energy determinations with growing chicks. J. Nutr. 64:587. 11. Jones, B. M., Jr., N. W. Bradley, and R. B. Grainger. 1961. Effect of fat and urea in the fattening rations for beef steers. J. Anim. Sci. 20:396. 12. Kropp, J. R., R. R. Johnson, J. R. Males, and F. N. Owens. 1977. Microbial protein synthesis with low quality roughage rations: isonitrogenous substitution of urea for soybean meal. J. Anim. Sci. 45:837. 13. Lofgreen, G. P., V. E. Mendel, and D. L. Mcilroy. 1968. Effects of kinds of milo, method of processing and level of urea on cattle performance. In: California Feeders Day Rep. p 28. Davis, CA. 14. Martin, J. J., F. N. Owens, D. R. Gill, D. E. Williams, R. J. Hillier, and R. A. Zinno1980. Protein sources for steers fed steam flaked, high moisture or whole shelled corn grain. In: Oklahoma State Univ. Anim. Sci. Res. Rep., MP-107. p 114. Stillwater, OK. i5. Mehrez, A. Z., E. R. iZirskov, and i. McDonald. i977. Rates of rumen fermentation in relation to ammonia concentration. Br. J. Nutr. 38: 447. 16. Merchen, N., T. Hanson, and T. Klopfenstein. 1979. Ruminal bypass of brewers dried grains protein. J. Anim. Sci. 49:192. 17. NRC. 1984. Nutrient Requirement of Beef Cattle (6th Rev. Ed.). National Academy of Sciences, Washington, DC. 18. Perry, T. W., W. M. Beeson, and M. T. Mohler. 1967. A comparison of high-urea supplements with natural protein supplements for growIng and fattening beef cattle. J. Anim . Sci. 26:1434.

19. Satter, L. L., and R. E. Roffler. 1975. Nitrogen requirements and utilization in dairy cattle. J. Anim. Sci. 58:1219. 20. Stangel, H. J. 1963. Urea and non-protein nitrogen in ruminant nutrition. Allied Chemical Corp., New York, NY. 21 . Thomas, E. E., C. R. Mason, and S. p, Schmidt. 1984. Relation of feedlot performance and· certain physiological responses to the metabolizable protein and urea content of cattle diets. J. Anim. Sci. 58:1285. 22. Thompson, J. T., N. W. Bradley, and C. O. Little. 1967. Utilization of urea and fat in meal and pelleted rations for steers. J. Anim . Sci. 26: 830. 23. Willms, C. L., L. L. Berger, N. R. Merchen, and G. C. Fahey, Jr. 1991. Effects of supplemental protein source and level of urea on inteS1in!i1 amino acid supply and feedlot performance of lambs fed diets based on alkaline hydrogen peroxide-treated wheat straw. J. Anim. Sci. 69: 4925. 24. Wolin, M. J. 1960. A theoretical rumen fermentation balance. J. Anim. Sci. 43:1452. 25. Zinn, R. A. 1988. Comparative feeding value of supplemental fat in finishing diets for feedlot steers supplemented with and without monensin. J. Anim. Sci. 66:213. 26. Zinn, R. A. 1990. Influence of flake density on the comparative feeding value of steam-flaked corn for feedlot steers. J. Anim. Sci. 68:767. 27. Zinn, R. A. 1993. Characteristics of ruminal and total tract digestion of canol a meal versus soybean meal in a high-energy diet for feedlot cattle. J. Anim. Sci. 71 :796. 28. Zinn, R. A., L. S. Bull, and R. W. Hemkin. 1981. Degradation of supplemental proteins in the rumen. J. Anim. Sci. 52:857. 29. Zinn , R. A., and F. N. Owens. 1983. Site of protein digestion in steers: predictability. J. Anim. Sci. 56:707. 30. Zinn, R. A., and F. N. Owens. 1986. A rapid procedure for purine measurement and its use for estimating net ruminal protein synthesis. Can. J. Anim. Sci. 66:157. 31 . Zinn, R. A., and A. Plascencia. 1993. Interaction of whole cottonseed and supplemental fat on digestive function in cattle . J. Anim. Sci. 71: 11.