Metabolizable Protein and Amino Acid Requirements of Growing Cattle'

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1 Published December 11, 2014 Metabolizable Protein and Amino Acid Requirements of Growing Cattle' V. A, Wilkerson, T. J. Klopfenstein, R. A. Britton, R. A. Stock, and P. S. Miller Department of Animal Science, University of Nebraska, Lincoln ABSTRACT: Metabolizable protein and amino acid metabolizable protein flow using weighted regression requirements for growing cattle were estimated using analysis (r2 =.69, n = 45) to determine the data from 11 research trials. A total of 543 steers were metabolizable protein requirements for maintenance individually fed a high-roughage diet supplemented (3.8 x BW.75 g/d, where BW is expressed in kilowith protein at several levels above a urea supplement grams) and growth (305 gkg of live weight gain). control. The mean weight for all animals was 253 kg, Calculated metabolizable amino acid requirements as with a range in mean initial to final weights of 200 to a percentage of metabolizable protein for a 316 kg, respectively. Daily gain ranged from -.04 to 253-kg animal gaining.49 kg/d were as follows:.89 kg. Metabolizable protein for each treatment group methionine, 3.0%; total sulfur amino acids, 5.8%; was calculated at the point at which the protein lysine, 8.0%; tryptophan, 1.0%; threonine, 5.2%; varequirement was met. The sum of dietary escape line, 5.7%; isoleucine, 5.6%; leucine, 6.9%; phenylalaprotein (basal and supplemental) and calculated nine, 3.9%; and histidine, 1.6%. The proposed requiremicrobial protein represented metabolizable protein ments were based on live animal gain and intake of supplied per test protein source analyzed in each trial. metabolizable protein and should represent the needs Daily gain was regressed against calculated of the growing beef animal. Key Words: Beef Cattle, Metabolizable Protein, Amino Acids, Protein Requirements J. Anim. Sci : Introduction Protein available to the ruminant for nutritional needs is supplied by microbial and dietary sources. Metabolizable protein (the quantity of true protein or amino acids absorbed) requirement can be met with knowledge of dietary escape protein and microbial protein production. Metabolizable protein systems (Burroughs et al., 1974; ARC, 1984; NRC, 1985) define the animal's requirement using estimates of available microbial and dietary escape protein and are potentially more accurate than the CP systems are. Escape protein values of commonly used supplemental protein sources have been documented (NRC, 19851, whereas microbial production has been related to dietary energy (Burroughs et al., 1974; ARC, 1984; NRC 1985). The metabolizable protein requirements that are currently in use were established from empirical formulas using N loss and tissue accretion. The purpose of the present research was 'Published with the approval of the director as paper no , Journal Ser., Nebraska Agric. Res. Div. Received October 5, Accepted May 21, first to estimate metabolizable protein requirements and evaluate the efficiency of metabolizable protein use with a database containing individual animal metabolizable protein intake and weight gain during normal growing periods. A second objective was to derive metabolizable amino acid requirements based on the predicted metabolizable protein requirements and small intestinal flow of amino acids calculated at the animal's metabolizable protein requirement. Metabolizable Protein Materials and Methods Prediction equations were developed from a database of 11 research trials conducted between 1978 and 1990 using primarily Hereford x Angus crossbred steers. Each trial was conducted as described by Klopfenstein et al. (1985). A total of 543 animals were used and feeding periods lasted between 85 and 120 d (Table 1). A variety of protein sources supplemented a roughage-based diet deficient in metabolizable protein. The source of roughage varied and included corncobs, corn stalks, corn silage, sorghum silage, and alfalfa hay. The roughage fraction 2777

2 2778 WILKERSON ET AL ranged from 75 to 93% of the diet. The range in the lowest initial to the highest final mean BW was 200 to 316 kg. Midtrial BW, ADG, and DMI ranged from 203 kg, -.04 kg/d, and 4.2 kgld to 288 kg,.89 kgld, and 7.5 kg/d, respectively. Daily gain was calculated as the difference between initial and final BW. When metabolizable protein exceeded the requirement for trial conditions daily gain reached a plateau, and in all trials in which a plateau was reached the maximum gain of steers was determined with a nonlinear plateau model. In trials in which maximal gain was not achieved, daily gain for the highest level of supplemental test protein was used. Table 1 provides a reference to each experiment evaluated and a summary of starting date, days on feed, protein source(s1, and number of animals per treatment. Single protein sources, such as feather meal, with obvious amino acid deficiencies were excluded. Microbial contribution to absorbed protein was determined from the equation of Burroughs et al. (1974) with g of microbial true protein produced per kilogram of TDN consumed. Estimates of dietary TDN (Table 2) were based on NRC (1984) and laboratory IVDMD (Marten and Barnes, 1980) values. Laboratory IVDMD was considered equivalent to TDN for roughage samples not represented in the NRC (1984). In vitro dry matter digestibility values for the roughages were consistent with published values (e.g., corn silage equaled 67%). The level of dietary supplement was fixed per trial and composed of a test protein source and(or) a nonprotein N (NF") source. Dietary minerals were balanced with each protein source. The metabolizable protein supplied by the supplement varied according to the ratio of the protein sources. The amount (kilograms/day) of test protein supplied by the supplement for each trial and protein source was calculated by dividing the maximum gain (kilograms1 day) with protein efficiency. Protein efficiency was calculated for the protein sources within each trial and represented a unit response in gain per unit of test protein consumed. The amount of supplement needed to supply the necessary test protein was then calculated and expressed as a percentage of diet DM. The difference between the percentage of test protein supplement needed to meet maximum gain and the total supplement in the diet was made up with the NPN source. The NPN source made up the total supplement supplied to the urea treatment (Table 1). The urea treatment for each trial was deficient in metabolizable protein, and the metabolizable protein supplied was calculated in the same manner as the test protein sources. Dietary escape protein was determined from the average DMI and total diet escape protein, percentage of DM. Ingredient escape protein, percentage of DM, was assigned based on NRC (1985) recommendations or in situ results (Table 2 1 from the authors' laboratory. In situ estimates of escape protein were similar to published values summarized by NRC (1985). In situ estimates of escape protein, expressed as a percentage of CP, for SBM, corn silage, and blood meal were 28, 27, and 83%, respectively. True digestibility of protein sources, calculated by difference using a NPN control, was provided by in vivo digestion studies (Abrams et al., 1983; Goedeken et al., 1990a; Nakamura et al., 1991) and was used to adjust supplemental escape protein values. Metabolizable protein was represented as the sum of microbial and dietary fractions. The data set was analyzed using the regression procedure of SAS (SAS, 1985). The regression analysis was weighted for the difference in observations per independent protein source. The y-intercept at zero gain was considered to represent the maintenance requirement for metabolizable protein, and the slope demonstrated the amount of metabolizable protein required per kilogram of live weight gain ( LWG). Metabolizable Amino Acids Because of the large quantity of protein provided by the microorganisms, their amino acid content is very important. Most analyses of microbial protein have been done on unattached microbes and the analyses for sulfur amino acids and tryptophan may be suspect due to degradation during hydrolysis. Therefore, Goedeken et al. (1990a) estimated the amino acids in microbial protein (Table 3 1 by feeding a diet containing only NPN and analyzing whole ruminal contents. Goedeken et al. (1990a,b) calculated the small intestinal flow of amino acids from ruminant microbial protein (Table 3 1, base diet, and supplemental protein for 10 protein sources or protein combinations in two experiments. These flows were calculated at the point at which the metabolizable protein requirement was met. The CV for amino acids across all supplements was used as an indicator of the degree to which an amino acid was limiting growth and was used in determining the amino acid requirements. A low CV for an amino acid from several test protein sources would indicate a high correlation to gain by limiting growth in each case. Metabolizable Protein Results Prediction of metabolizable protein requirements for maintenance and mowth were based on ADG (Figure 1). Other variables considered in the prediction equation were the squared term of ADG, midtrial BW, and animal age; however, none was significant. Based on the 45 observations of individual protein sources, the requirements for maintenance (y-intercept) and gain (slope) were 242 k 17 g/d and 305 _t 31 gkg of BW gain (r2 =,691. The number of animals per protein source ranged from 3 to 30. The average

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4 ~~ ~ ~~ ~ 2780 WILKERSON ET AL. Table 2. List of total digestible nutrients and crude protein values for ingredients used in calculating metabolizable proteina TDN Escape Ingredient % Sourceb CP, 8 %,C Sourceb Alfalfa hay 60 NRC Lab Blood meal 65 NRC Lab Corncobs 48 Lab Lab Ammoniated corncobs 60 Lab -.5 Lab Ensiled corncobs 48 Lab -.5 Lab Corn dried distillers grain 86 Lab Lab Corn grain 90 NRC NRC Corn gluten meal 89 NRC NRC Corn silage 67 Lab Lab/NRC Ammoniated corn stalks 45 Lab Corn starch 100 Corn wet distillers grains 86 Lab Lab Feather meal 70 Lab Lab Glucose 100 Cane molasses 72 NRC NRC Sorghum silage 69 Lab Lab Soybean meal 84 NRC Lab Soybean meal 84 NRC Lab Soybean meal 84 NRC Lab Soybean meal 84 NRC Lab Soybean meal supplement 84 NRC Lab Soybean meal 84 NRC NRC Soybean meal 84 NRC Lab Supplement Lab NRC Supplement 32, liquid 47 Lab Lab anutrient analyses are on a 100% DM basis. bnrc, 1984 or 1985; laboratory in vitro or in situ analysis. Escape protein expressed as a percentage of DM. midtrial weight adjusted by the number of observations per protein source was 253 kg. The maintenance requirement (grarndday) based on metabolic body size was, therefore, 3.8 x BW.75. Metabolizable Amino Acids The protein supplements varied in amino acid content, particularly in total sulfur amino acids, methionine, and lysine. One way to evaluate the need for a specific amino acid is with the CV (Table 4) for grams of amino acid flowing to the small intestine, when the metabolizable protein requirement was met with different protein supplements. This approach suggested that methionine was likely to be firstlimiting in most situations. It also suggested that tryptophan might be limiting. The low CV for threonine was due to the similar amount of threonine in the protein supplements and likely not due to its being limiting. The values for threonine, valine, leucine, and phenylalanine are overestimates because they were overfed so that the protein source(s) would supply the limiting amino acids. Therefore, requirements for these amino acids were estimated from the data of Fenderson and Bergen (1975) based on their ratios to the sulfur amino acids and lysine. The derived amino acid requirements (Table 4) were expressed as grams per day and as a percentage of the metabolizable protein flow (Table 5). Percentages may be useful for calculating the specific amino acid need from the metabolizable protein requirement. However, this assumes that the proportion of amino acids required is the same for maintenance and growth. Table 3. Amino acid profile of ruminal contents from steers fed solka floc and ureaa Amino acid % of CP Threonine 6.4 Valine 6.1 Cystine 2.3 Methionine 2.9 Isoleucine 5.5 Leucine 8.5 Tyrosine 2.9 Phenylalanine 5.2 Lysine 7.9 Histidine 1.4 Arginine 6.7 Tryptophan 1.0 Diaminopimelic acid 1.1 abased on Goedeken et al. 1990a

5 METABOLIZABLE PROTEIN AND AMINO ACID REQUIREMENTS I I U. m C W w 500 P - 0 n.- A n m o Gain, kgld Figure 1. Relationship between metabolizable protein and gain. The regression line describing this relationship is Y = 242 * 17 + (305 f 31 x X), where n = 45 and r2 = Burrow hs 2 cn 200 d m ' I TDN, kgid Burroughs, g/d = 104.4(TDN) x.8 NRC, g/d = (26.12UDN) ) x 6.25 x.8 x.8 Figure 2. Comparison of two methods: NRC, 1985 and Burroughs et al., 1974 for predicting digestible bacterial protein (DBP) from dietary energy. The regression lines are: Burroughs, DBP = x X; NRC, DBP = x X The protein supplements used in all trials supplied excessive amounts of some amino acids to meet limiting amino acid needs. Therefore, the derived requirements for metabolizable protein are higher than the animal's needs. The metabolizable protein requirements will supply sufficient amounts of essential amino acids. To avoid feeding excess amino acids, the optimal requirement profile should be derived. Discussion Microbial protein accounted for greater than half the metabolizable protein in these studies. This indicated a need for an accurate estimate of microbial protein synthesis. The equations of Burroughs et al. (19741, NRC (19851, and Rohr et al. (1986) were developed independently and relate microbial protein production to dietary energy intake. Burroughs et al. (1974) assumed that g of microbial true protein was produced per kilogram of TDN consumed. Burroughs et al. ( 1974) based their microbial protein equation on information available in the literature. The NRC (1985) summarized 16 experiments for cattle consuming > 40% of their diet as forage to develop a microbial protein production equation (MP [gldl = x TDN [kgldl ; r2 =.77). These experiments used diaminopimelic acid or RNA as microbial markers. The equation was developed primarily for dairy cows with high intakes. The negative intercept from this equation implies that a negative microbial production is possible at low energy intake, and this is not biologically realistic (Figure 2). However, the calculated average microbial yield, g of microbial true protein per kilogram of TDN, from the NRC (1985) data set (n = 1181, is similar to that of Burroughs et al. (1974). Rohr et al. (1986) summarized 22 experiments in which the f0rage:concentrate ratio varied between 1OO:O to 49:51 for the purpose of determining microbial protein production. Their prediction of microbial production is similar to that of Burroughs et al. ( 1974) when expressed on the same protein and energy basis: vs g of microbial true protein per lulogram of TDN intake, respectively. Sixteen of the 22 experiments evaluated by Rohr et al. (1986) included I5N as the microbial marker. They further concluded that diaminopimelic acid underestimated microbial protein production by 14% compared with estimates based on I5N. Because of the potentially low estimates in microbial protein from the NRC (1985) regression equation, the Burroughs et al. ( 1974) equation for predicting microbial protein production was used in this study and is recommended for growing beef cattle fed high-forage diets. Microbial production is related to ruminal digestible OM. Factors that affect fermentation include composition, physical form, and OM intake. The results of Stokes et al. (1991) and Hoover and Stokes ( 1991 ) suggest that degradable intake protein and nonstructural carbohydrates should be considered rather than TDN alone when predicting microbial protein production. Furthermore, available energy (TDN) from ensiled ingredients should be corrected for the organic acids that do not contribute to microbial protein production (ARC, 1984). The data set used here contains some studies that used ensiled ingredients and therefore microbial production may have been overestimated, resulting in higher requirements for metabolizable protein. The requirement for metabolizable protein was compared among several systems including the current requirement (Burroughs et al., 1974; ARC, 1984; NRC, 1985) for a 253-kg steer gaining.49 kgld and

6 ~ ~ 2782 WILKERSON ET AL. Table 4. Estimated grams of amino acid flow to the small intestinea Amino acidc Methionine TSAAd Lysine Tryptophan Threonine Valine Isoleucine Leucine Phenylalanine Histidine Protein source BM:FTH: SBM BM FTH:BM FTH BM FTH BMFTH BM:CGM CGM cv abased on Goedeken et al. (1990a,b). bsbm = soybean meal, BM = blood meal, FTH = feather meal, CGM = corn gluten meal. Gramdday of absorbed amino acids. dtotal sulfur amino acids consuming 3.18 kg/d of TDN from 5.5 kg of diet. The NRC system indicates the highest requirement at 498 g/d, followed by the proposed system at 390 g/d, NRC (1984) at 362 g/d, and Burroughs et al. (1974) at 275 g/d. The protein growth system (Klopfenstein et al., 1985) used to measure metabolizable protein requirements collectively includes all conversions, uses, and products of endogenous protein. Several of the protein treatments evaluated were at or near zero growth and were considered to represent maintenance conditions for metabolizable protein use. Therefore, the metabolic losses of protein are accounted for in the proposed requirement. Verite (1987) and Jarrige (1989) discussed the French metabolizable protein system and justified the maintenance requirement as a single function of body size (3.25 x BW.75) based on N balance studies with nonproducing animals. The system of Burroughs et al. (1974) based maintenance metabolizable protein on the Smuts (1935) equation using a 47% efficiency of metaboliza- ble to net protein. The NRC (1985) and ARC (1984) used the factorial approach to determine the maintenance metabolizable protein requirement. Endogenous protein losses are merged into one estimate as a function of metabolic fecal losses to indigestible DMI and endogenous urinary losses. Surface protein losses are a small part of the maintenance protein requirement with little difference between the NRC (1985) and the ARC (1984) systems. The efficiency of metabolizable to net protein differed between the NRC (1985) and ARC (1984) for maintenance,.67 vs.8, respectively. The difference in the efficiencies for metabolizable to net protein for the NRC (1985) and ARC (1984) systems does not bring the two systems together in estimating the maintenance requirement for metabolizable protein. The systems of Burroughs et al. (19741, ARC (19841, and NRC (1985) estimate metabolizable protein for gain from separate predictions of the protein content in empty body gain. Differences in the Table 5. Suggested maintenance plus growth metabolizable amino acid requirements for British-type growing beef steersa Amino acid Grams per davb Percentage of MPC Methionine TSAAd Lysine Tryptophan Threonine Valine Isoleucine Leucine Phenylalanine Histidine Ie 27.3 (22.3) (26.9) ) 6.1 abased on Goedeken et al. (1990a,b), adjusted to.49 kgid gain. bfor a steer weighing 253 kg and gaining.49 kgld. Requirement, gramsiday: maintenance, 3.8 x BW75; gain, 305 x LWG. dtotal sulfur amino acids. Values in parentheses estimated from Fenderson and Bergen (1975) (5.2) 7.0 (5.7) (6.9) 6.0 (3.9) 1.6

7 METABOLIZABLE PROTEIN AND AMINO ACID REQUIREMENTS I d Figure 3. Changes in protein accretion in the gain as rate of gain increases while body weight remains constant at 253 kg. Rate of gain represents the observed range in gain for 45 observations in 11 trials. Body weight represents the mean midtrial weight for the data set. The NRC line (solid) indicates the protein accretion from retained protein predictions (NRC, 1985). The calculated line (dashed] indicates the protein accretion using a metabolizable protein requirement of 305 g/kg of live weight gain and a constant efficiency of 50% for metabolizable protein to net protein. The efficiency of the proposed gain requirement to meet the NRC protein accretion is 59% at.11 kgld and 52% at.89 kgld gain. would decline from 59% at.ll kgld to 52% at.89 kgld gain. A similar comparison can be made for the changes in protein composition of gain as live weight increases (Figure 4). The range in midtrial weight from 203 to 288 kg represents the different experimental conditions used. Keeping the rate of gain constant at the average observed (.49 kgld) and varying BW across the midtrial range allows comparison of protein composition and metabolizable protein efficiency to NRC predictions. The efficiency of metabolizable protein used for the observed gain requirement to represent the expected composition of gain was calculated and declined from 60% at 203 kg BW to 53% at 288 kg BW. Jarrige (1989) reported that the efficiency of metabolizable protein use declines as live weight increases for both Holstein and Charolais bulls. Ainslie et al. (1993) reported similar results with Holstein steers. Oldham (1987) has pointed out two distinct parts to the efficiency of metabolizable protein use. These are amino acid pattern of a protein source in relation to an ideal pattern and the actual efficiency of an ideal amino acid pattern for protein deposition. In situations in which amino acid input limits performance, the observed efficiency of amino acid utilization is related to both the ideal efficiency and the biological value of the absorbed amino acids from the protein source. efficiency of metabolizable protein to net protein also exist among these systems (.47 Burroughs et al., 1974;.67 NRC, 1985;.8 ARC, 1984). The proposed requirement for gain would seem to be high when considering the protein content of gain and efficiency values currently in the literature. Protein composition of gain decreases and fat composition increases with increases in rate of gain and BW (Byers, 1982). It has been proposed (NRC, 1985) that the change in protein composition due to body size and rate of gain results in a lower metabolizable protein requirement. Another approach, supported by this study, is that the metabolizable protein requirement for gain remains constant and that metabolizable protein utilization changes as body size and rate of gain vary. Protein composition of gain can be calculated using NRC ( 1985) equations. Figure 3 depicts a decline in the protein composition of gain with increases in LWG (NRC, solid line) for a constant BW. Animal weight is represented as the mean BW (253 kg) from the data set. The range of weight gains is representative of that observed in the data set. The composition of gain for the calculated line (dashed) indicates what occurs when a constant efficiency of gain is applied to the observed requirement (305 g/kg of LWG). The efficiency of metabolizable protein use for the observed requirement can be calculated to meet protein composition normally expected (NRC, 1985). The efficiency 19 ] Live weight, kg Figure 4. Changes in protein accretion in gain as body weight increases with a constant rate of gain 1.49 kgid]. Live weight represents the range for 45 observation in 11 trials. Live weight gain represents the mean for the data set. The NRC line (solid) indicates protein accretion from retained protein predictions (NRC, 1985). The calculated line (dashed) indicates the protein accretion using a metabolizable protein requirement of 305 glkg live weight gain and a constant efficiency of 50% for metabolizable protein to net protein. The efficiency of the proposed gain requirement to meet the NRC protein accretion is 60% at 203 and 53% at 288 kg of body weight.

8 2784 WILKERSON ET AL. Metabolizable amino acid requirements (Table 5 ) were established using the values from Table 4 and the observed metabolizable protein requirements. Threonine, valine, leucine, and phenylalanine were overfed in the diets and therefore the values for these amino acids overestimate their requirements. Methionine was determined to be the first-limiting amino acid, and this conclusion supported by reports indicating that microbial protein is limited in methionine, lysine, and threonine (Nimrick et al., 1970; Richardson and Hatfield, 1978). Burroughs et al. (1974) assigned amino acid requirements from the composition of beef protein. The proposed metabolizable amino acid requirements were similar to those suggested by Burroughs et al. ( 1974). Differences between the proposed requirements and those of Burroughs et al. (1974) may be due to metabolic efficiencies not accounted for when basing requirements on the amino acid composition of beef tissue. Implications Formulation of growing diets for beef steers using metabolizable protein and amino acids is possible. The metabolizable protein requirement of beef steers within the weight range of 203 to 288 kg is 3.8 x BW.75 (gramslday, where BW is expressed as kilograms) for maintenance and 305 gkg of live weight gain. Further research is needed to determine the requirements outside the current weight range and dietary nutrient inputs. Combinations of protein sources may provide the necessary profile of amino acids to meet metabolizable amino acid needs. A constant metabolizable protein requirement for gain may indicate a dynamic protein efficiency value based on rate of gain and BW. Literature Cited Abrams, S. M., T. J. Klopfenstein, R. A. Stock, R. A. Britton. and M. L. Nelson Preservation of wet distillers grains and its value as a protein source for growing ruminants. J. Anim. Sci. 57~729. Ainslie, S. J., D. G. Fox, T. C. Perry, D. J. Ketchen, and M. C. Barry Predicting amino acid adequacy of diets fed to Holstein steers. J. Anim. Sci. 71:1312. ARC The Nutrient Requirements of Ruminant Livestock. Suppl. 1. Commonwealth Agricultural Bureaux, Slough, U.K. Blasi, D. A., T. J. Klopfenstein, J. S. Drouillard, and M. H. Sindt Hydrolysis time as a factor affecting the nutritive value of feather meal and feather meal-blood meal combinations for growing calves. J. Anim. Sci. 69:1272. Burroughs, W., A. Trenkle, and R. L. Vetter A system of protein evaluation for cattle and sheep involving metabolizable protein (amino acids) and urea fermentation potential of feedstuffs. Vet. Med. Small Anim. Clin. 69:713. Byers, F. M Protein growth and turnover in cattle: Systems for measurement and biological limits. in: F. N. Owens (Ed.) Protein Requirements for Cattle: Proc. Int. Symp. Oklahoma State Univ. MP-109, Stillwater. Cleale, R. M., IV, T. J. Klopfenstein, R. A. Britton, L. D. Satterlee, and S. R. Lowry Induced non-enzymatic browning of soybean meal Digestibility and efficiency of protein utilization by ruminants of soybean meal treated with xylose or glucose. J. him. Sci. 65:1327. Fenderson, C. L., and W. G. Bergen An assessment of essential amino acid requirements of growing steers. J. Anim. Sci. 41:1759. Gibb, D. J., T. J. Klopfenstein, and M. H. Sindt Combinations of rendered protein meals for growing calves. J. Anim. Sci. 70: Goedeken, F. K., T. J. Klopfenstein, R. A. Stock: and R. A. Britton. 1990a. Hydrolyzed feather meal as a protein source for growing calves. J. Anim. Sci Goedeken, F. K., T. J. Klopfenstein, R. A. Stock, R. A. Britton. and M. H. Sindt. 1990b. Protein value of feather meal for ruminants as affected by blood additions. J. h im. Sci. 68:2936. Hoover, W. H., and S. R. Stokes Balancing carbohydrates and proteins for optimum rumen microbia1 yield. J. Dairy Sci. 74: Jarrige, R Ruminant Nutrition: Recommended Allowances and Feed Tables. John Libbey Eurotext, Montrouge, France. Klopfenstein, T.. R. Stock, and R. Britton Relevance of bypass protein to cattle feeding. Prof. Anim. Sci. 1:27. Lewis, M., T. Klopfenstein, R. Britton, and T. Winowiski Nonenzymatic browning with sulfite liquor reduces rumen degradation of soybean meal. Nebraska Beef Cattle Rep. MP-53. Lewis, M., T. Klopfenstein, R. Britton, M. Sindt, and T. Winowiski Treated soybean meal for growing calves. Nebraska Beef Cattle Rep. MP-54. Marten, G. C., and R. F. Barnes Prediction of energy digestibility of forages with in vitro rumen fermentation and fungal enzyme systems. In: W. J. Pigden, C. C. Bolch, and M. Graham (Ed. 1 Standardization of Analytical Methodology for Feeds. pp International Development Centre, Ottawa, Canada. Nakamura, T., T. Klopfenstein, D. Gibb, and R. Britton Growth efficiency and digestibility of heated protein. Nebraska Beef Cattle Rep. MP-56. Nimrick, K., E. E. Hatfield, J. Kaminski, and F. N. Owens Qualitative assessment of supplemental amino acid needs for growing lambs fed urea as the sole nitrogen source. J. Nutr. 100: NRC Nutrient Requirements of Beef Cattle (6th Ed.). National Academy Press, Washington, DC. NRC Ruminant Nitrogen Usage. National Academy Press, Washington, DC. Oldham, J. D Efficiencies of amino acid utilization. In: R. Jarrige and G. Alderman (Ed.). pp Commission of the European Communities, Luxembourg. Richardson, C. R., and E. E. Hatfield The limiting amino acids in growing cattle. J. Anim. Sci. 46:740. Rohr, K., P. Lebzien, H. Schafft, and E. Schulz Prediction of duodenal flow of non-ammonia nitrogen and amino acid nitrogen in dairy cows. Livest. Prod. Sci. 14:29. SAS SAS User s Guide: Statistics (5th Ed.). SAS Inst. Inc., Cary, NC. Smuts, D. B The relation between the basal metabolism and the endogenous nitrogen metabolism, with particular reference to the estimation of the maintenance requirement of protein. J. Nutr. 9:403. Stock, R., T. Klopfenstein, D. Brink, S. Lowry, D. Rock, and S. Abrams Impact of weighing procedures and variation in protein degradation rate on measured performance of growing lambs and cattle. J. Anim. Sci. 57:1276. Stokes, S. R., W. H. Hoover, T. K. Miller, and R. P. Manski Impact of carbohydrate and protein levels on bacterial metabolism in continuous culture. J. Dairy Sci. 74:860. Vente, R Present situation of protein evaluation for ruminants in France: The PDI system. In: R. Jarrige and G. Alderman (Ed. ). pp Commission of the European Communities, Luxembourg.