J. Anim. Sci. 2005. 83:863-870
© 2005 American Society of Animal Science
Level of supplemental protein does not influence the ruminally undegradable protein value1
L. R. Legleiter,
A. M. Mueller and
M. S. Kerley2
Department of Animal Sciences, University of Missouri, Columbia 65211
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Abstract
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Two experiments were conducted to determine whether elevating the percentage of ruminally undegradable protein (RUP) in the diet would influence the RUP value of the protein feedstuff. A single-effluent, continuous-culture study was designed to test the effect of RUP inclusion rate in the diet on ruminal degradability of the protein. Treatments consisted (DM basis) of a control diet with no supplemental protein, control + 2.5% bloodmeal (BM-L), control + 5% bloodmeal (BM-H), control + 4.45% soybean meal (SBM-L), and control + 8.89% soybean meal (SBM-H). Proteolytic activity and total VFA concentration were not affected (P = 0.73 and P = 0.13) by treatment. Within protein source, dietary RUP value was not affected (P = 0.94) by level of inclusion. When corrected for control diet RUP flow, the RUP value of the blood meal (BM) protein was higher (P = 0.01) than soybean meal (SBM); however, level of supplementation did not affect (P = 0.07) the RUP value of BM or SBM. In Exp. 2, 32 British x Continental crossbred steers (276 ± 26.3 kg) were fed for 72 d to examine the effects of balancing the AA:energy ratio, using BM as a RUP source, on ADG, G:F, and lean tissue deposition. Diets were formulated to provide increasing levels of arginine, while ruminally degradable protein and energy were held constant. Four dietary treatments provided 0.5, 1, 1.5, and 2x the required amount of arginine, whereas the control diet had no BM included. Daily DMI averaged 7.6 kg/steer and did not differ (P = 0.71) among treatments. Steers gained an average of 1.9 kg/d and average G:F was 0.260, with no differences (P = 0.60 and P = 0.97, respectively) among treatments. There was no difference (P = 0.48) in the change in 12th-rib fat depth during the study; however, change in LM area was affected quadratically as the level of BM increased in the diet, with the greatest increase in LM area occurring in steers fed the 1x and 1.5x required arginine treatments. Balancing the AA:energy ratio did not affect G:F, DMI, or ADG; however, it increased deposition of lean in the LM quadratically. Level of dietary inclusion of BM as an RUP source does not affect its RUP value or efficacy of providing postruminal AA in growing steers.
Key Words: Amino Acid:Energy • Bloodmeal • Feed Efficiency • Ruminally Undegradable Protein
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Introduction
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Numerous ruminant nutrition studies have focused on feeding ruminally undegradable protein (RUP), with the goal of increasing AA flow to the small intestine. Experiments designed to test the effects of feeding RUP to cattle have provided mixed results. Research has shown increased N and AA flows to the small intestine (Cecava and Parker, 1993
; Coomer et al., 1993; Zinn and Owens, 1993), as well as increased growth due to RUP addition (Sindt et al., 1993; Zinn and Owens, 1993; Lehmkuhler, 2001). However, others have reported no responses to feeding RUP, decreases in total N flow to the small intestine, and/or decreased microbial protein (MP) synthesis (Loerch and Berger, 1981; Plegge et al., 1983; Ludden et al., 1995).
Numerous reasons have been hypothesized for the lack of positive responses to feeding RUP (Santos et al., 1998). The RUP value of proteins has been shown to deviate from published values (Erasmus et al., 1988; Titgemeyer et al., 1989; Lardy et al., 1993), which is one of the plausible reasons for the lack of consistent responses. Previous work in our laboratory suggested that the RUP value decreased as the dietary inclusion level of a protein source increased. Lehmkuhler (2001) reported a decrease in RUP as levels of blood meal (BM) increased from 3 to 12% of DM. In an effort to explain why the RUP value of a protein may vary and possibly explain some inconsistencies observed when feeding RUP, our primary objective was to determine the ruminal proteolytic activity and RUP value of two protein sources (high and low RUP) when added to the diet at two levels (2.5 and 5%). The levels of inclusion were chosen for two reasons. First, Lehmkuhler (2001) observed a decrease in RUP between the 3 and 6% inclusion levels of BM. Second, most RUP sources are fed at levels of 5% or less to meet growth-limiting AA requirements. We hypothesized that as dietary inclusion rate of RUP increased, microbial adaptations would occur and result in more extensive protein degradation.
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Materials and Methods
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Continuous Culture
Experimental Design and Diets.
The continuous culture experiment was designed as a randomized complete block with five dietary treatments and two replications. Twenty-four single-effluent, continuous-culture fermenters were used in conjunction with four dietary treatments (n = 20) and one control diet (n = 4). The five diets (Table 1; DM basis) were control with no supplemental protein; control + 2.5% blood meal (BM-L); control + 5% blood meal (BM-H); control + 4.45% soybean meal (SBM-L), and control + 8.89% soybean meal (SBM-H). The BM-L and SBM-L diets and the BM-H and SBM-H diets were formulated to be isonitrogenous. The experiment consisted of two 10-d experimental periods, including a 7-d acclimation period followed by a 3-d sampling period. The treatments were fed twice daily at 0800 and 2000 at a rate of 30 g/feeding throughout the experimental period.
Ruminal fluid used to inoculate the fermenters was combined from two ruminally fistulated beef cows maintained on tall fescue (Festuca arundinacea) grass pasture and supplemented with 2.5 kg•d–1•cow–1 of a corn + corn gluten feed grain mix (approximately 12% CP). The strained ruminal fluid was mixed 1:1 with a buffered solution (Slyter, 1990) and then added to each fermenter up to the overflow port (approximately 1,460 mL).
Fermenters were continuously flushed with CO2 gas, stirred with magnetic stir plates, and immersed in a water bath maintained at 39°C using thermostatically controlled heaters (model 730, Fisher Scientific, Pittsburgh, PA). McDougall’s artificial saliva (Slyter, 1990) was continuously infused into fermenters using peristaltic pumps. Fermenter dilution rates were held constant at 4.5%/h for all treatments. The continuous culture system, control of dilution rate and operation conditions have previously been described in detail (Meng et al., 1999).
Sample Collection.
Samples were taken on d 8, 9, and 10 at 1200. For ammonia analysis, 20 mL of fermenter fluid was acidified with 1 mL of 6 N HCl and frozen. The effluent, collected in reservoirs immersed in ice-cooled water, was composited over the 3-d sampling period, preserved with 37% formaldehyde solution added at a rate of 2.5% of effluent volume, and stored at 4°C. For the quantification of proteolytic activity, 10 mL of fermenter contents was mixed with 20 mL of 20% glycerine salt solution and immediately stored at –80°C. Fermenter pH was measured daily at time of sampling.
At the termination of each 10-d experimental period, 37% formaldehyde solution was added to each fermenter at 2.5% of the fermenter contents. To release attached microorganisms from feed particles, contents were blended for 1 min before being strained through four layers of cheesecloth. To isolate the bacteria, the strained fraction was centrifuged at 1,000 x g for 5 min to remove feed particles. The supernatant fluid was recentrifuged at 30,000 x g for 30 min at 4°C. The pellet was washed once with 0.9% (wt/vol) saline solution and twice with distilled water, and recentrifuged at 30,000 x g for 30 min at 4°C to collect the bacteria. The resulting pellet was lyophilized (Virtis, Genesis 25XL, 5°C shelf, Gardiner, NY) and stored until analyzed.
Sample Analyses.
Feeds, effluent, and isolated bacteria were analyzed for DM, OM (AOAC, 1984), and N (model FP-428, LECO, St. Joseph, MI). Isolated bacteria were analyzed for purine content according to Zinn and Owens (1986). Total microbial N flowing to the effluent was calculated by the N:purine ratio of isolated bacteria and the purine content of effluent. Microbial efficiency (MOEFF) was expressed as grams of microbial N per kilogram of OM truly digested. True fermenter digestibility of DM and OM was calculated as the differences of DM and OM intakes and effluent residues corrected for microbial DM or OM contribution. The samples for ammonia analysis were thawed at room temperature, centrifuged at 20,000 x g for 20 min, and the supernatant fluid was analyzed by the phenol-hypochlorite procedure (Broderick and Kang, 1980), with a DU-65 spectrophotometer (Beckman, Palo Alto, CA). The VFA concentration of the fermenter fluid was determined using gas chromatography (model 3400, Varian, Walnut Creek, CA) as outlined by Grigsby et al. (1992). Proteolytic activity was determined using the protease assay described by Brock et al. (1982) and Cotta and Hespel (1986). The samples were thawed at room temperature and strained through four layers of cheesecloth. Cotta and Hespell (1986) reported no detectable loss of proteolytic enzyme activity when samples were frozen for up to 2 wk, and only a 20% loss when frozen for up to 5 mo. For comparative purposes and assay accuracy, strained fermenter fluid was used. Samples were incubated with 1% (wt/vol) azocasein (Sigma Chemical Co., St. Louis, MO) for 2 h, and the resulting absorbance was determined at 450 nm using a DU-65 spectrophotometer (Beckman, Palo Alto, CA). One percent (wt/vol) azocasein was a saturating concentration of substrate for the protease activity of cell extracts (Brock et al., 1982).
Growth Study
Thirty-two crossbred steers were randomized among five experimental treatments (control, n = 6; Treatment 0.5x, n = 7; Treatment 1x, n = 7; Treatment 1.5x, n = 6; Treatment 2x, n = 6; Table 2) and fed for 72 d in a completely randomized design growth study. Steers were weighed on arrival and assigned randomly to treatments. After 1 wk of dietary and environmental acclimation, the steers were weighed on two consecutive days to determine average starting weight. Ultrasound measurements were taken for LM area (LMA) and 12th-rib fat estimation. The University of Missouri Animal Care and Quality Assurance Committee approved animal care procedures.
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Table 2. Ingredient composition and chemical analysis of diets fed in the growth study investigating increasing dietary blood meal concentration
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Steers were fed once daily and allowed ad libitum access to feed and water. Steers were housed in partially covered confinement feedlot pens with concrete flooring. To ensure barn differences had no effect, every 2 wk, cattle were rotated between the two barns, so that each pen of steers received equal exposure to the separate barns. Detailed individual animal feed intake data were collected using individual feed intake equipment (model 4000E, Growsafe Systems, Ltd., Alberta, Canada), which allowed for each animal to serve as an experimental unit. Individual animal intakes were calculated with Growsafe feed intake analysis software (v. 2.11).
Weights and ultrasound measurements were taken every 3 wk. Longissimus muscle areas and 12th-rib fat depths were recorded as an average of three images taken between the 12th and 13th ribs using an Aloka 500V ultrasound machine (Aloka Co., Ltd., Japan). Average LMA and 12th-rib fat depths were measured manually using AUSKEY animal ultrasound software (Animal Ultrasound Services, Ithaca, NY). Average fat thickness was measured at a point three-fourths of the length of the LM from the backbone end.
Using the Cornell Net Carbohydrate and Protein System v. 3.0 (O’Connor et al., 1993), arginine was determined to be the most limiting AA for 273-kg Continental crossbred steers gaining 2.1 kg/d. The Cornell model v. 3.0 estimated the arginine requirement to be approximately twofold that recommended by the newer version 4.0 adopted by NRC (1996). Previous research in our laboratory (Ludden and Kerley, 1998) suggested that arginine was the most limiting AA in corn-based diets. Diets (Table 2
) were formulated to provide increasing levels of arginine. The control diet consisted of corn, cottonseed hulls, soyhulls, and soybean meal (SBM) and had no supplemental BM. The other diets were formulated to provide 0.5, 1, 1.5, and 2x the required level of arginine (Treatments 0.5x, 1x, 1.5x, and 2x, respectively), while energy remained constant; therefore, the arginine:energy ratio was below balance and above the requirement (Figure 1). Spray-dried BM (purchased from American Feed and Farm Supply, North Kansas City, MO) was selected as the protein source to balance the AA:energy ratio. Based on in situ analysis of several protein sources (BM, fish meal, corn gluten meal, dried distillers grains with solubles, and two blended protein sources), BM had the highest RUP value and supplied the highest proportion of estimated limiting AA (data not shown). To account for variation between batches of feedstuffs, samples of the BM and SBM to be used in the growth study were analyzed via in situ degradation to ensure precise RUP and AA values to balance the AA:energy ratio. In situ analysis was conducted as a time-course incubation (incubation times = 0, 2, 4, 6, 12, 16, 24, 36, 48, and 72 h) using two ruminally cannulated steers. Dacron bags were inserted in reverse order, removed simultaneously, and washed thoroughly until rinse water was no longer discolored. Rate of degradation was calculated by regressing the percentage of potentially digestible N remaining on time of incubation. Using the rate of N digestion and estimated dilution rate, RUP was calculated as the sum of the fractions washed out of the rumen per hour plus the indigestible fraction (72 h).
The 1x treatment was formulated such that the AA profile of the residue complemented the AA provided by the MP, so that the AA requirement as predicted by the Cornell model (O’Connor et al., 1993) was met (Figure 1). The diets were formulated to be isoenergetic. Using SBM, the diets were also formulated to be equal in ruminally degradable protein so that ammonia N, AA, and/or peptides were not limiting in the rumen.
The AA requirements of the animals were estimated using the Cornell Net Carbohydrate and Protein System v. 3.0 (O’Connor et al., 1993). The AA supplied to the small intestine from the basal ingredients were calculated using values from the NRC (1996). Microbial protein flows and subsequent MP AA flows were estimated using a revised model of the Cornell Net Carbohydrate and Protein System that estimates microbial efficiency based on ruminal dilution rate (Mueller and Kerley, 2003). The difference between the required AA and the sum of the basal ingredient AA supply and MP AA supply equaled the deficient AA that needed to be supplied by BM RUP (Table 3).
Statistical Analyses
Data from both experiments were analyzed by AN-OVA using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC). Data from the continuous culture experiment were analyzed as a randomized complete block (run served as block), with treatment as the main effect. Differences among means were determined using single degree of freedom contrasts. Comparisons made were 1) BM vs. SBM; 2) high vs. low level of protein supplementation; 3) source x level interaction; and 4) control vs. others. Effects were considered significant at P < 0.05. Data from the growth study were analyzed as a completely randomized design and tested for linear and quadratic effects of arginine balance. Effects were considered significant at P < 0.10.
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Results
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Continuous Culture
The measured proteolytic activities and RUP values are reported in Table 4. Proteolytic activity, expressed as micorgrams of azocasein hydrolyzed/(h•g sample), was not affected (P = 0.73) by protein source or level. In continuous culture, the dietary RUP value, expressed as a percentage of CP, was higher when BM was added vs. SBM (P = 0.004). Within protein source, dietary RUP value was not affected (P = 0.94) by level of inclusion. The dietary RUP value for the control was lower (P = 0.03) than for protein-supplemented treatments. When corrected for control diet RUP flow, the RUP value of the BM protein was higher (P = 0.01) than SBM; however, level of supplementation did not affect (P = 0.07) the RUP value of BM or SBM.
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Table 4. Effect of ruminally undegradable protein dietary inclusion level on proteolytic activity and the ruminally undegradable value of the protein and diet in continuous culture
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The digestion, fermentation, and MOEFF results of the continuous-culture experiment are shown in Table 5. Dry matter digestibility decreased (P = 0.02) when BM and SBM were increased from 2.5 (BM-L) and 4.45% (SBM-L) to 5 (BM-H) and 8.89% (SBM-H). Similarly, OM digestibility was lower (P = 0.01) for BM-H and SBM-H than for BM-L and SBM-L. The control treatment had higher (P < 0.001) OM and DM digestibilities compared with the other treatments. There were significant source (P < 0.001), level (P = 0.002), and source x level (P = 0.02) effects for ammonia concentration. Additionally, the control treatment had a lower (P < 0.001) ammonia concentration compared with the other treatments. Although all diets contained 0.5% urea, ammonia concentrations for BM-L, BM-H, and control treatments were lower than the 5 mg/100 mL concentration recommended for optimal MOEFF (Satter and Slyter, 1974). Therefore, one might expect the lower ammonia concentrations to decrease MOEFF for the BM-supplemented treatments; however, there was no source effect (P = 0.08) for MOEFF. The high levels of BM and SBM increased (P = 0.008) MOEFF compared with the low levels of supplementation. Total VFA concentration was not affected (P = 0.13) by treatment. Acetate and isovalerate concentrations were lower (P = 0.03) in the fermenters receiving the control vs. the other treatments, whereas propionate and valerate concentrations were higher (P = 0.02) in control fed vs. others. In addition, acetate:propionate was lower (P = 0.004) for control vs. other treatments.
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Table 5. Effect of ruminally undegradable protein dietary inclusion level on ruminal fermentation, digestion, and microbial efficiency in continuous culture
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Growth Study
The steers began the study averaging 276 ± 26.3 kg, with an average LMA of 41.06 ± 0.99 cm2 and 0.28 ± 0.02 cm 12th-rib fat (Table 6). Daily feed intake averaged 7.6 kg/steer and was not different (P = 0.71) among treatments. The ADG by steers across all treatments was 1.92 kg/steer with no differences (P = 0.60) between treatments. Likewise, G:F did not differ (P = 0.97) among treatments, averaging 0.26. There was no difference (P = 0.48) among treatments in the change in 12th-rib fat depth from the start of the study to the end; however, ending LMA tended (P = 0.15) to respond quadratically, with increases for the 1x and 1.5x treatments. There was a linear (P < 0.03) and quadratic (P < 0.01) effect of treatment for the change in LMA from the beginning of the study to the end. The change in LMA decreased between the control and 0.5x treatments, and then increased, with the highest changes for 1x and 1.5x, followed by a decrease at the 2x level.
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Table 6. Performance characteristics and composition of gain for steers receiving increased levels of arginine from bloodmeal
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Discussion
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We hypothesized that increasing the substrate available for proteolytic bacteria would increase protein degradation and subsequently decrease RUP percentage. If the RUP value of a feedstuff is affected by the dietary inclusion level, then the lack of response to feeding RUP would be partially explained and appropriate adjustments would need to be made when feeding RUP. Volden (1999) fed diets formulated to provide three levels of RUP, with tabular calculated postruminal protein flows of 69, 53, and 48 g/kg of DM. The measured RUP contributions when fed to cannulated lactating dairy cows at 19.3 kg of DM/d were 64.5, 51.5, and 45.0 g/kg of DM. The actual supplied RUP values closely matched the predicted, which suggested the RUP value of the diet remained constant as the percentage of RUP increased from 30 to 39% of CP. Likewise, Klusmeyer (1990) saw no change in nonammonia, nonmicrobial N flow to the duodenum as a percentage of nonammonia N when corn gluten meal was increased in the diet from 11 to 14.5%. However, in a continuous-culture experiment, Calsamiglia et al. (1992) found the partial replacement of SBM with expeller-processed SBM and fish meal did not decrease degradation of dietary CP, suggesting an unexpectedly low RUP value. It is important to note that all dietary treatments in the experiments referenced above contained a relatively high level of roughage (>40%). Little research has evaluated the effect increasing levels of RUP have on protein degradability in high-concentrate diets.
If dietary inclusion level of RUP affected its degradability by ruminal microbes due to the time of exposure or increased substrate availability, then we assumed the level of protease activity would be directly affected. Brock et al. (1982) reported that only 25% of the proteolytic activity in the rumen is present in the strained ruminal fluid (SRF), with the remainder being associated with the particulate feed fraction. Diet (forage vs. concentrate) may influence the relative level of proteolytic activity associated with the particulate feed fraction vs. SRF (Craig et al., 1981), with proportionally fewer microbes attached to the particulate fraction with diets high in nonstructural carbohydrates. Thus, for comparative purposes and accuracy of assay procedures, SRF was used to determine whether the dietary inclusion level of RUP affected the proteolytic activity within the rumen. In the continuous culture study, no difference was detected in proteolytic activity among treatments.
With no significant increases in proteolytic activity when dietary RUP inclusion increased, we expected the RUP value of the diet and proteins (BM and SBM) to be similar between the BM-L and BM-H treatments and likewise for the SBM treatments. In the continuous culture experiment, the dietary RUP value, as a percentage of CP, was similar between BM-L and BM-H with values of 49 and 55%, respectively. Likewise, dietary RUP values for the SBM treatments (45 and 39% for the SBM-L and SBM-H diets, respectively) were similar between inclusion levels but lower than the BM treatments. When corrected for control diet RUP flow, the RUP value of the BM protein was 91.5 and 83.0% for the BM-L and BM-H diets, respectively, and 37.6% for the SBM in the SBM-H diet. There was no level effect for BM RUP between BM-L and BM-H. Soybean meal RUP values for the SBM-L treatment are not shown due to inconsistent results. These inconsistencies in the SBM-L treatment were likely a function of the low quantity of SBM RUP above that of the basal RUP contribution, making them difficult to separate.
Our results indicate that the inclusion level of BM up to 5% of dietary DM did not affect its ruminal degradability. These results agree with those of Zinn and Owens (1993), who reported RUP value was not altered by level of inclusion when fed at levels of 2, 4, and 6%.
With the understanding that dietary inclusion level did not affect the percentage of RUP of the protein ingredient, an attempt was made to improve feed efficiency by feeding RUP to alter the AA:energy ratio. Bloodmeal was selected because it has a high RUP value and an AA profile that is well balanced with animal requirements. Specifically, BM has relatively high levels of arginine, lysine, and histidine, with adequate methionine. Increased inclusion of BM should increase the postruminal flow of arginine and other AA (Figure 1).
We hypothesized that balancing the AA:energy ratio by increasing the postruminal flow of limiting AA would result in increased feed efficiency as a result of maximizing the rate of lean tissue gain and minimizing fat deposition. Gain efficiency should be maximized at the point where no AA is limiting lean growth rate, or Treatment 1x, per our calculations (Table 3 and Figure 1). Therefore, validation that RUP value is constant would have been improved growth rate and/or gain efficiency in steers fed Treatments 1x, 1.5x, and 2x compared with steers fed the control or Treatment 0.5x. Lehmkuhler (2001) saw trends for increased feed efficiency and gain when a blend of BM and fish meal was included in feedlot diets; however, most of the improvement in feed efficiency was noted early in the study and diminished over the full length of the study. Some researchers have shown improved feed efficiency and increased gain when feeding RUP. Compared with urea-supplemented diets, Zinn and Owens (1993) observed a 13% improvement in ADG and an 8% increase in feed efficiency when a concentrate diet containing varying levels of a combination of BM, meat and bone meal, and feather meal was fed for 84 d. However, there have been numerous other researchers who have seen no improvement in gain or efficiency when RUP sources were fed (Merchen et al., 1987; Richards et al., 1998; Huntington et al., 2001).
Although unsuccessful, Lehmkuhler (2001) attempted to alter the composition of gain by increasing lean tissue deposition when feeding RUP. Earlier attempts by Epley et al. (1971) to alter composition of gain and carcass composition by balancing the digestible protein:digestible energy ratio of diets also were unsuccessful. However, in this experiment, the change in LMA was maximized in cattle fed treatments 1x and 1.5x, as the change in LMA behaved in a quadratic fashion relative to increasing dietary BM. We conclude from these data that balancing the AA:energy ratio by feeding RUP may have increased lean deposition in the LM of growing steers. One possible reason for the inability of Treatment 2x to promote greater lean tissue deposition in steers could have been the increased energetic cost of urea synthesis by calves fed the excessive protein level of this diet.
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Implications
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Ruminal microorganisms do not adaptively increase proteolytic activity toward ruminally undegradable protein sources. Thus, ruminal degradability of bloodmeal was not influenced by its inclusion level in the diet. Based on these results, when high-concentrate diets are fed, predicting postruminal flow of amino acids from ruminally undegradable protein does not need to be adjusted for level of dietary ruminally undegradable protein inclusion.
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Footnotes
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1 The authors gratefully acknowledge J. Porter, L. Mueller, A. Brockman, P. Davis, J. Christopher, and J. Kennemer for their assistance in this project. 
2 Correspondence: 111 ASRC (phone: 573-882-0834; fax: 573-884-7712; e-mail: kerleym{at}missouri.edu).
Received for publication May 7, 2004.
Accepted for publication December 22, 2004.
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Abstract
Introduction
