J. Anim. Sci. 2005. 83:2151-2161
© 2005 American Society of Animal Science
Supplementing a ruminally undegradable protein supplement to maintain essential amino acid supply to the small intestine when forage intake is restricted in beef cattle1
E. J. Scholljegerdes2,
T. R. Weston,
P. A. Ludden and
B. W. Hess3
Department of Animal Science, University of Wyoming, Laramie 82071
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Abstract
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Twelve Angus crossbred cattle (eight heifers and four steers; average initial BW = 594 ± 44.4 kg) fitted with ruminal and duodenal cannulas and fed restricted amounts of forage plus a ruminally undegradable protein (RUP) supplement were used in a triplicated 4 x 4 Latin square design experiment to determine intestinal supply of essential AA. Cattle were fed four different levels of chopped (2.54 cm) bromegrass hay (11.4% CP, 57% NDF; OM basis): 30, 55, 80, or 105% of the forage intake required for maintenance. Cattle fed below maintenance were given specified quantities of a RUP supplement (6.8% porcine blood meal, 24.5% hydrolyzed feather meal, and 68.7% menhaden fish meal; DM basis) designed to provide duodenal essential AA flow equal to that of cattle fed forage at 105% of maintenance. Experimental periods lasted 21 d (17 d of adaptation and 4 d of sampling). Total OM intake and duodenal OM flow increased linearly (P < 0.001) as cattle consumed more forage; however, OM truly digested in the rumen (% of intake) did not change (P = 0.43) as intake increased. True ruminal N degradation (% of intake) tended (P = 0.07) to increase linearly, and true ruminal N degradation (g/d) decreased quadratically (P = 0.02) as intake increased from 30 to 105%. Duodenal N flow was equal (P = 0.33) across intake levels, even though microbial N flow increased linearly (P < 0.001) as forage OM intake increased. Total and individual essential AA intake decreased (cubic; P < 0.001) as forage intake increased because the supply of nonammonia, nonmicrobial N flow from RUP was decreased (linear; P < 0.001) by design. Total duodenal flow of essential AA did not differ (P = 0.39) across these levels of forage intake. Although the profile of essential AA reaching the duodenum differed (P 
0.02) for all 10 essential AA, the range of each essential AA as a proportion of total essential AA was low (11.1 to 11.2% of total essential AA for phenylalanine to 12.3 to 14.3% of total essential AA for lysine). Duodenal essential AA flow did not differ (P = 0.10 to 0.65) with forage intake level for eight of the 10 essential AA. Duodenal flow of arginine decreased linearly (P = 0.01), whereas duodenal flow of tryptophan increased linearly (P = 0.002) as forage intake increased from 30 to 105% of maintenance. Balancing intestinal essential AA supply in beef cattle can be accomplished by varying intake of a RUP supplement.
Key Words: Amino Acids • Beef Cattle • Digestion • Restricted Intake • Ruminally Undegradable Protein • Supplementation
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Introduction
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Studies evaluating nutrition-reproduction interactions in ruminants often use restricted feed intake to limit the supply of dietary energy (Schillo, 1992
). One problem inherent with this approach is that the quantity of microbial protein reaching the small intestine decreases when dietary energy is decreased (Asplund, 1994). Therefore, supply of energy and protein are confounded. One must supply an equal quantity of protein and/or essential AA to the small intestine across forage intake levels to separate the effects of dietary energy and protein on reproduction properly. Intestinal supply of essential AA can be increased by dietary supplementation with protein feeds selected for ruminal escape (i.e., ruminally undegradable protein; RUP). Nonetheless, a long ruminal retention time associated with restricted feed intake may increase extent of ruminal digestion (Riewe and Lippke, 1969) and thereby decrease the RUP value of feedstuffs (Meng et al., 1999; Schadt et al., 1999). Scholljegerdes et al. (2005) demonstrated that the RUP value of a supplement decreased as forage intake restriction became more severe. We hypothesized that RUP supplementation can be used to balance the intestinal supply of essential AA in cattle fed forage at levels below NRC (2000) recommendations for maintenance if appropriate RUP values are used for diet formulation. Our objectives were to evaluate the site and extent of digestion and the intestinal supply of essential AA in beef cattle consuming restricted amounts of forage plus varying quantities of an RUP supplement.
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Materials and Methods
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General
Twelve ruminally and duodenally cannulated Angus-cross cattle (eight heifers and four steers; average initial BW = 594 ± 44.4 kg) were used in a triplicated 4 x 4 Latin square design (Neeter et al., 1990) experiment (heifers were used in a duplicate 4 x 4 Latin square, whereas steers were utilized in a single 4 x 4 Latin square conducted simultaneously) in accordance with an approved University of Wyoming Animal Care and Use Committee protocol. The four treatments included chopped (2.5 cm) bromegrass hay (9.4% ash, 11.4% CP, and 57% NDF on an OM basis) at 30, 55, 80, and 105% of the forage intake required for maintenance according to the NRC (2000) diet evaluation computer software. Initial BW (2-d average) was used to calculate maintenance hay requirement, and hay intake was not adjusted for changes in BW during a period of the Latin square. Actual data, which were obtained from previous work from our laboratory (Scholljegerdes et al., 2001, 2005), were used as inputs for animal descriptions, environmental conditions, as well as forage chemical and nutrient composition. Basal (forage only) flows of total essential AA for each animal at each experimental intake level were predicted using the equation reported by Scholljegerdes et al. (2004; total essential AA flow to the small intestine, g/d = [0.055 x g of OM intake] + 1.546). One specified level of an RUP supplement was provided to cattle fed each level of hay below maintenance intake. This RUP supplement was composed of 6.8% porcine blood meal, 24.5% hydrolyzed feather meal, and 68.7% menhaden fish meal; the quantity fed was formulated to provide postruminal supply of AA equal to that of cattle consuming forage at 105% of maintenance. The criteria for selecting these supplemental protein ingredients were their high potential for ruminal escape based on in situ analysis (Scholljegerdes, 2001) and capacity to supply essential AA in a profile closely matching the duodenal essential AA profiles reported by Scholljegerdes et al. (2004) for cattle fed restricted amounts of a similar forage (Figure 1). The quantity of RUP supplement delivered was adjusted for the expected decrease in RUP caused by restricted intake; anticipated RUP values for the supplements at the 30, 55, and 80% intake levels were 49.2, 56.2, and 59.1% as reported by Scholljegerdes et al. (2005). Orts were not observed from d 10 through 21 of the experimental periods. Each period of the Latin square lasted 21 d, with 17 d for diet adaptation to allow for adjustment of the digestive system to forage intake level and RUP supplement. Ruminal, duodenal, and fecal samples were collected for 4 d after the adaptation period. All forage and RUP supplements were provided in equal portions at 0600 and 1800 daily. As a marker for digesta flow, boluses of 5.0 g of Cr2O3 were dosed intraruminally at each feeding (total = 10 g of Cr2O3/d) from d 8 to 19 of each sampling period. Cattle had ad libitum access to water and trace-mineral salt (Champions Choice; Akzo Nobel Salt Inc., Clarks Summit, PA; guaranteed analysis [% of DM]: NaCl, 95 to 99; Co, Cu, I, Mn, Zn, and Fe, < 1%) until d 14 of each sampling period. On d 14 of each sampling period, to avoid any confounding effects of salt intake on water intake and fluid passage rate, trace mineral salt was no longer provided.
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Figure 1. Open bars represent the essential AA profile of duodenal digesta in beef cattle consuming restricted amounts of forage (Scholljegerdes et al., 2004), and the black bars represent the essential AA profile of a ruminally undegradable protein supplement that contained 6.8% porcine blood meal, 24.5% hydrolyzed feather meal, and 68.7% menhaden fish meal on a DM basis.
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Sampling
To account for feed composition throughout the trial accurately, feeds were sampled every day and composited within each period. Beginning at 0400 on d 18 of each sampling period, duodenal (200 mL) and fecal (50 mL) samples were taken every 4 h. On d 19, collection times were advanced 2 h, so that samples were collected to represent every 2-h segment of a 24-h period. Fecal samples were dried in a 55° C forced-air oven, ground (Wiley mill; 1-mm screen, Thomas Hill and Sons, Philadelphia, PA), and composited (equal dry-weight basis) within animal for each period. Duodenal digesta samples were immediately frozen and later composited within animal for each period. Duodenal digesta samples were lyophilized (Genesis SQ 25 Super ES Freeze Dryer; The VirTis Co., Gardiner, NY), ground to pass a 1-mm screen, and stored for subsequent analyses.
Just before the 0600 feeding on d 20 of each period, 200 mL of whole ruminal contents was collected, which represented the 0-h baseline measurement. Cattle were then dosed intraruminally with 200 mL of Co-EDTA (Uden et al., 1980) for measurement of fluid passage rate. Samples (200 mL) of whole ruminal contents were collected 3, 6, 9, 12, 15, 18, 21, 24, and 36 h after dosing. Samples collected at 24 and 36 h were used only for Co determination. Ruminal pH was measured immediately on whole ruminal contents using a combination electrode (Orion Research Inc., Boston, MA), and whole ruminal contents then were strained through eight layers of cheesecloth. Ten milliliters of the resulting ruminal fluid was acidified with 0.1 mL of 7.2 N H2SO4 and immediately frozen. The remaining unstrained sample of whole ruminal contents was placed in a blender (Hamilton Beach/Proctor Silex, Washington, NC) with an equal volume of 0.9% NaCl (wt/vol) solution and homogenized for 1 min to dislodge particulate-associated bacteria. The homogenized sample was strained through eight layers of cheesecloth and immediately frozen for subsequent bacterial isolation by differential centrifugation (Merchen et al., 1986). The resulting bacterial isolate was lyophilized and ground with a mortar and pestle for subsequent laboratory analyses.
Laboratory Analyses
Feed, fecal, duodenal, and microbial samples were analyzed for DM and ash (AOAC, 1990). Nitrogen content of feed, microbial, duodenal, and fecal samples was determined using a LECO FP-528 N analyzer (LECO Corp., Henderson, NV). Neutral detergent fiber content of feed, duodenal digesta, and feces was determined using the procedures of Goering and Van Soest (1970) as modified by Vogel et al. (1991) using an ANKOM 200 fiber analyzer (ANKOM Technology, Fairport, NY). Chromium concentration of duodenal and fecal digesta was determined (Hill and Anderson, 1958) by atomic absorption spectrophotometry (Model 210 VGP AA Spectrophotometer; Buck Scientific, Norwalk, CT) with an air-plus-acetylene flame. Duodenal and isolated bacteria samples were analyzed for purine concentration as described by Zinn and Owens (1986) and modified by Obispo and Dehority (1999). Duodenal NH3 N concentrations were determined by steam distillation over MgO (AOAC, 1990).
Acidified ruminal fluid samples were centrifuged at 10,000 x g for 10 min; a 2.5-mL aliquot of the supernatant fluid was added to 0.5 mL of 25% metaphosphoric acid containing 2 g/L of 2-ethyl butyric acid (Goetsch and Galyean, 1983). These samples were centrifuged for 10 min at 10,000 x g, and the supernatant fluid was analyzed for concentrations of VFA using a Hewlett-Packard 5890 GLC (Hewlett-Packard, Avondale, PA) equipped with a 15 m x 0.533 mm (i.d.) column (Nukol, Supleco, Bellefonte, PA) with a ramp temperature of 110 to 150° C, increasing at 8° C per min. Helium was used as the carrier gas with a column flow rate of 20 mL/min. Injector and detector temperatures were 250° C. A portion of the remaining acidified ruminal fluid was used for determination of ruminal NH3 and ruminal Co concentrations. Ruminal NH3 concentration was determined by the phenol-hypochlorite procedure (Broderick and Kang, 1980). Cobalt concentration was analyzed according to the procedures outlined by Varga and Prigge (1982) using atomic absorption spectrophotometry (Model 210 VGP AA Spectrophotometer) with an air-plus-acetylene flame (Hart and Polan, 1984).
Feeds, duodenal digesta, and microbial OM were analyzed for AA content (AOAC, 1990) using a Beckman 6300 HPLC interfaced with Beckman System Gold (Model 6300, Beckman Instruments, Palo Alto, CA). The cationic HPLC system used postcolumn derivatization with ninhydrin, followed by UV detection. To permit values to be compared with results of Scholljegerdes et al. (2004) directly, AA concentrations were not corrected for incomplete recovery resulting from hydrolysis.
Calculations and Statistical Analyses
Flow of OM was calculated by dividing the amount of Cr dosed by the concentration of Cr in the respective duodenal sample. Duodenal flow of OM, N, NDF, and essential AA was calculated by multiplying the respective nutrient concentration in duodenal OM by duodenal OM flow. The essential AA profile of duodenal digesta and microbial OM was calculated by dividing flow of individual essential AA by flow of the total of all essential AA. Steers were used in a 4 x 4 Latin square run concurrent with heifers in a replicated 4 x 4 Latin square. All data were analyzed using the MIXED procedure of SAS (SAS Inst., Inc., Cary, NC) for a Latin square in a randomized complete block design experiment. In addition to the effects of experimental treatment and period, the effect of gender was included in the model as a block. Animal was used as the random effect. Additionally, the model used for analysis of time-course data (repeated measures) included time as well as treatment x time interactions. The three-way interaction, animal x period x treatment, was used to specify variation among animals using the RANDOM statement of SAS. An autoregressive covariance structure (AR1 of the SAS MIXED procedure) was determined to be most appropriate based on Akaike’s Information Criterion. Orthogonal polynomial contrasts were used to compare linear, quadratic, and cubic responses to level of forage intake (Steel and Torrie, 1980).
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Results and Discussion
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OM Intake and Digestion
Duodenal OM flow increased linearly (P < 0.001) from 30 to 105% of the forage intake required for maintenance (Table 1). This finding agrees with other research (Merchen et al., 1986; Sniffen and Robinson, 1987; Scholljegerdes et al., 2004) that indicated increased duodenal OM flow with an increasing level of intake. Microbial and nonmicrobial OM flow increased linearly (P < 0.001) as intake increased from 30 to 105% of the forage intake required for maintenance, which is consistent with our previous findings (Scholljegerdes et al., 2001) in which cattle were fed diets similar to those reported herein. Because OM intake and duodenal OM flow increased proportionally as forage intake was increased, true ruminal OM digestibility as a percentage of OM intake did not differ (P = 0.43) among treatments. Similar responses to restricted intake were noted in previous work from our laboratory (Scholljegerdes et al., 2004); however, these results are contrary to those reported by others (Merchen et al., 1986; Murphy et al., 1994; Hussein and Berger, 1995), who observed that OM digestibility increased when intake was restricted. It is important to note that these researchers fed diets formulated to be isocaloric in an effort to avoid a decrease in available nutrients to the animal and ruminal microbial population. Although the quantity of OM digested postruminally (g/d) did not differ (P = 0.25) when expressed as a percentage of duodenal flow, postruminal OM digestibility decreased linearly (P = 0.001) as dietary forage increased. Total tract OM digested (g/d) increased (P = 0.001), whereas total tract OM digestibility (% of intake) decreased linearly (P < 0.001) as inclusion of RUP decreased. The increase in amount of postruminal digestion for animals consuming the RUP suggests that the RUP ingredients, which are present at higher levels with hay restriction, were readily degraded postruminally. Overall, absolute quantities of OM digested in the rumen seemed to be a function of OM supplied and not digestibility of the diet per se; however, as digesta reached the duodenum, the quantity of OM that escaped ruminal degradation was readily digested postruminally. The high proportion of ruminal escape protein in the RUP supplement provided the restricted intake treatments with a highly digestible OM source postruminally. Therefore, despite the decrease in ruminal OM digested (g/d) with restricted intake, the inclusion of RUP increased the overall OM digestibility of the diet.
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Table 1. Effects of restricted forage intake and supplemental ruminally undegradable protein on site and extent of OM intake and digestibility in beef cattle consuming brome-grass haya
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NDF Intake and Digestion
Intake, duodenal flow, and fecal output of NDF increased linearly (P < 0.001) as forage intake increased from 30 to 105% of the forage intake required for maintenance (Table 2). Additionally, ruminal NDF digestion (g/d) increased linearly (P < 0.001) as forage intake increased; however, ruminal NDF digestibility tended to decrease (P = 0.08), and total tract NDF digestibility decreased linearly (P < 0.001), as more hay was fed. The trend toward decreased ruminal NDF digestibility with more hay could be due to the increase in fluid passage rate (linear; P < 0.001; Table 3), which decreased time for ruminal digestion. The quantity of NDF exiting the rumen depends partly on fluid passage rate from the rumen (Poppi et al., 1981). A similar trend for a greater ruminal NDF digestibility with restricted hay intake was observed in previous work (Scholljegerdes et al., 2004) in which beef cattle were fed forage at levels similar to those reported herein. Additionally, Merchen et al. (1986) reported that total tract NDF digestibility decreased from 63.9 to 60.5% when wethers were fed 1.6 or 2.6% of BW.
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Table 2. Effects of restricted forage intake and supplemental ruminally undegradable protein on site and extent of NDF intake and digestibility in beef cattle consuming brome-grass haya
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Table 3. Effects of restricted forage intake and supplemental ruminally undegradable protein on ruminal VFA, NH3, fluid passage rate and pH in beef cattle consuming brome- grass haya
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Ruminal Fermentation Patterns
The increase in OM truly digested (g/d) observed as forage intake increased resulted in a quadratic increase (P = 0.04) in total ruminal VFA concentration (Table 3
). No treatment x sampling hour interactions were detected (P = 0.10 to 0.99) for VFA or pH data; therefore, only main effects of diet were reported. The increase in ruminal NDF digested with increased forage intake may explain why the proportion of ruminal acetate increased (quadratic; P = 0.002). Molar proportions of propionate in ruminal fluid decreased quadratically (P < 0.001) as intake increased from 30 to 105% of maintenance. Reports are lacking on ruminal responses to restricted intake plus supplemental RUP, but mild nutrient restriction (60% of ad libitum) increased the proportion of ruminal acetate and decreased ruminal butyrate with no effect on the proportion of ruminal propionate (Firkins et al., 1986). Molar proportions of valerate, isovalerate, and isobutyrate increased (P < 0.02) with RUP inclusion, which likely reflect ruminal degradation of specific branch-chained AA (MacFarlane et al., 1992) from the RUP supplement. Increased ruminal degradation of the RUP supplement as forage intake restriction became more severe was noted in an in situ experiment using the same dietary regimens (Scholljegerdes et al., 2005).
A treatment x time interaction (linear; P = 0.03; data not shown) was noted for ruminal NH3 concentrations, which was a result of an increase in ruminal NH3 shortly after feeding in cattle given restricted amounts of forage and supplemental RUP. The linear increase in ruminal NH3 as forage intake decreased and supplemental RUP increased was associated with an increased supply of dietary N. Restricting forage intake increased ruminal degradability of the RUP supplement (Scholljegerdes et al., 2005), contributing to the increase in ruminal NH3. Although the provision of supplemental RUP has been reported (Titgemeyer et al., 1989; Waltz et al., 1989) to decrease ruminal NH3, those studies were designed to compare supplemental RUP with dietary protein sources that have a greater proportion of ruminally degraded protein. Furthermore, the linear decrease in fluid passage rate (P = 0.001) as forage intake decreased allowed for a greater ruminal residence time of the RUP supplement, thereby giving the ruminal microbes more time to degrade the RUP supplement. The tendency for ruminal NDF digestibility to increase with the inclusion of RUP could be attributed to both a higher concentration of ruminal NH3 and/or greater molar proportions of branch-chained VFA. The cellulolytic bacteria inhabiting the rumen require both of these substrates for normal growth and activity (Hoover, 1986; Russell et al., 1992). Yang (2002) reported that forage fiber degradation was generally increased with provision of branched-chain VFA and AA. Additionally, ruminal pH decreased (linear; P = 0.001) as forage increased from 30 to 105% of the intake required for maintenance. This response may be a result of greater ruminal NH3 for cattle fed supplemental RUP, as well as increased quantity of OM digested in the rumen and a subsequent increase in VFA concentrations within the rumen of cattle consuming greater quantities of forage.
N Intake and Digestion
Nitrogen intake increased linearly (P = 0.001) as the quantity of RUP increased in the diet (Table 4). Notwithstanding, total duodenal N flow did not change (P = 0.33) as forage intake increased, which was a result of replacement of RUP by microbial protein synthesized in the rumen. As forage intake increased and RUP supply decreased, microbial N flow increased (linear; P < 0.001), whereas duodenal NH3 N did not differ (linear, P = 0.12) and nonammonia nonmicrobial N flow decreased (linear; P = 0.001). Duodenal NH3 N flows in our experiment were greater than in previous reports (Garrett et al., 1987; Waltz et al., 1989), but cattle in those studies were fed RUP supplements with diets formulated to meet requirements. The relatively high duodenal NH3 N flows reported herein are likely associated with the quantity of N digested (g/d) in the rumen and subsequent increase in ruminal NH3 concentration as forage intake was decreased. When dairy cattle were fed blood meal or feather meal, Cunningham et al. (1994) reported that nonammonia, nonmicrobial N reaching the small intestine increased when fed at levels similar to those fed in the present experiment. Although the absolute quantity of true ruminal N digested (g/d) increased linearly (P = 0.001) with inclusion of RUP, when expressed as a percentage of intake, true ruminal N degradation tended (linear; P = 0.07) to be less as supplemental RUP increased. This difference in ruminal degradability was expected because of the escape value of the RUP supplement. Microbial efficiency did not differ (P = 0.61 to 0.71 for the orthogonal contrasts) across forage intake level. This finding agrees with our previous results (Scholljegerdes et al., 2004) in which animals were fed forage at levels similar to those in the present experiment.
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Table 4. Effects of restricted forage intake and supplemental ruminally undegradable protein on site and extent of N intake and digestibility in beef cattle consuming brome-grass haya
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The linear increase in fecal N flow (P = 0.001) with an increase in forage intake, combined with the lack of differences in duodenal N flow, contributed to an increase in postruminal N digestibility (linear; P = 0.001) for the restricted forage intake treatments. These results indicate that postruminal digestibility of supplemental RUP was greater than that of the forage. Titgemeyer et al. (1989) reported that postruminal disappearance for feedstuffs high in RUP (corn gluten meal, blood meal, and feather meal) was greater than that of a protein source extensively degraded in the rumen (soybean meal). Similarly, reference values for intestinal digestibility of RUP (NRC, 2001) indicate that menhaden fish meal RUP (90% intestinal digestibility) is more available in the small intestine than the RUP provided by bromegrass hay (60% intestinal digestibility) or mixed ruminal bacteria (80% intestinal digestibility). Based on published values (NRC, 2001) for intestinal RUP digestibility, the predicted intestinal digestibility of the RUP fed is 77.0, 71.9, 66.5, and 60.0% for the 30, 55, 80, and 105% treatments; these values closely parallel lower tract N digestibility measurements determined in our study. Total tract N digestibility was greatest (linear; P < 0.001) for the 30% treatment and decreased as forage intake increased. This finding suggests that the N of RUP supplements containing a combination of porcine blood meal, hydrolyzed feather meal, and menhaden fish meal were readily digested in the small intestine.
Essential AA Intake
Intake of total and individual essential AA was greatest for the 30% treatment and decreased (cubic; P < 0.001) as hay intake increased from 30 to 105% of maintenance energy requirements (Table 5). As forage intake restriction became more severe, dietary essential AA was expected to increase because the supplemental RUP should normalize the intestinal supply of essential AA across all levels of forage intake. Nonetheless, the cubic effect indicates that the quantity of essential AA from the RUP supplement increased as intake restriction became more severe. Supplemental RUP was designed to supply more essential AA than predicted from RUP values reported by the NRC (2000) alone because Scholljegerdes et al. (2005) previously demonstrated that RUP values decreased as forage intake was more severely restricted.
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Table 5. Effects of restricted forage intake and supplemental ruminally undegradable protein on total essential AA intake by beef cattle consuming bromegrass haya
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Microbial Essential AA Profile
Microbial essential AA flow increased linearly (P = 0.001) as forage intake increased from 30 to 105% of the intake required for maintenance (Table 6). This was expected because of the linear increase in microbial OM and N flowing to the duodenum as forage intake increased. Microbial RNA:N (quadratic; P = 0.08) and N concentration (quadratic; P = 0.09) tended to differ among treatments (data not shown). Microbial N and RNA:N may be influenced by energy intake (Cecava et al., 1988) but not by supplemental RUP (Erasmus et al., 1994). Although it seems that dietary treatment influenced (P < 0.05) microbial AA composition for eight of the 10 essential AA; the greatest difference among treatments was 0.6% units for microbial lysine content. Relatively small differences in microbial AA profile associated with changes in diet have been observed previously (Martin et al., 1996; Shabi et al., 2000; Scholljegerdes et al., 2004).
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Table 6. Effects of restricted forage intake and supplemental ruminally undegradable protein on microbial essential AA in beef cattle consuming bromegrass haya
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Duodenal Essential AA Flow
With the exception of arginine and tryptophan (linear; P = 0.001 and P = 0.001, respectively), intestinal supply of essential AA did not differ (P = 0.09 to 0.63) across forage intake levels (Table 7). Duodenal flow of arginine decreased, whereas tryptophan increased, as forage intake was increased from 30 to 105% of NRC (2000) maintenance energy requirements. Greater duodenal flow of arginine for cattle receiving supplemental RUP was expected because of to the high arginine content of the supplement (Figure 1). Support for this contention is provided by the lack of response (P = 0.41; data not shown) noted for duodenal arginine flow when the 105% of maintenance treatment was removed from the statistical analysis. We also attempted to account for the differences in duodenal flow of tryptophan by removing the 105% treatment from the statistical analysis; this revealed that duodenal supply of tryptophan continued to differ across treatments (linear; P = 0.01; data not shown). When Scholljegerdes et al. (2004) fed an all-forage diet at levels similar to this study, true ruminal digestibility of tryptophan decreased from 62.4 to 47.6% of essential AA intake as forage intake increased from 30 to 120% of maintenance. Duodenal tryptophan supply in the current experiment ranged from 13 to 18 g/d, far exceeding the NRC (2000) suggested requirement of 3.4 g/d (assuming an intestinal digestibility of 80%). Under certain conditions, high dietary intakes of tryptophan can cause acute bovine pulmonary edema and emphysema (Carlson, 1988), but none of the animals in the present experiment exhibited symptoms of acute bovine pulmonary edema and emphysema.
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Table 7. Effects of restricted forage intake and supplemental ruminally undegradable protein on duodenal essential AA flow in beef cattle consuming bromegrass haya
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Duodenal Essential AA Profile
The profile of essential AA reaching the duodenum, calculated as the percentage of total essential AA, changed (P 0.02) as level of forage intake increased (Table 8). Although differences were highly significant, the greatest variation in essential AA proportions ranged from 11.0 to 11.2% for phenylalanine to 12.3 to 14.3% for lysine. Volden (1999) reported that duodenal profiles ranged from 6.3 to 6.4% of total AA for valine to 7.8 to 8.2% of total AA for leucine in dairy cattle that were fed at low (9.8 kg of DM/d during late lactation) or high (19.3 kg of DM/d during peak lactation) feed intakes. Similarly, Ludden and Kerley (1997) reported AA profiles (AA disappearing from the small intestine) where the AA differed by 0.01% (2.20 to 2.21% of total AA) for histidine to a difference of 0.49% units (0.74 to 1.23% of total AA) for tryptophan. Overall, altering the intake level of a given diet seems to have only minor effects on AA profile of duodenal digesta (Ludden and Kerley, 1997; Volden, 1999; Scholljegerdes et al., 2004).
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Table 8. Effects of restricted forage intake and supplemental ruminally undegradable protein on duodenal essential AA profile (% of total essential AA) in beef cattle consuming bromegrass haya
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If feed restriction is chosen as a method to evaluate the effects of dietary energy and protein on physiology of ruminants, the logical first step is to balance essential AA supply to the duodenum across the levels of feed intake being studied. In the current study, total essential AA flows at the duodenum did not differ (P = 0.40) across treatments and ranged from 561 to 512 g/d as forage intake increased from 30 to 105% of maintenance. By increasing the quantity of supplemental RUP delivered, as well as accounting for decreased RUP value of the supplement when forage intake was decreased (Scholljegerdes et al., 2005), we were able to balance the total essential AA supply to the small intestine. Nonetheless, we had predicted total essential AA flow to the small intestine to be 601 g/d for the 105% of maintenance treatment, an overestimate of 17%. Because of this over-prediction, total essential AA flow for the RUP-supplemented treatments was 110% of the forage-only treatment. Compared with our RUP calculations, the NRC (2000) estimates will account for changes in intake when calculating passage rate, but they do not alter the degradation constants for individual dietary ingredients. Therefore, the NRC (2000) values will overestimate the RUP value of a diet when dietary intake is restricted. Had we used the NRC (2000) equations unadjusted for ruminal protein degradability, total dietary supply of essential AA would have only been 85% of observed values, and, presumably, duodenal supply also would have been decreased.
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Implications
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Balancing the supply of essential amino acids to the duodenum of beef cattle consuming restricted quantities of forage can be accomplished by providing a ruminally undegradable protein supplement. Nonetheless, appropriate adjustments must be made to account for the more extensive ruminal degradation of supplemental protein that occurs when intake is restricted. Therefore, this approach to balancing essential amino acid supply to the small intestine should prove useful as an experimental model to avoid confounding effects of energy with protein status on the physiology of ruminants.
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Footnotes
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1 Research was supported by the USDA-NRI Competitive Grants Program (USDA-NRI #99-03628). 
2 Present address: USDA-ARS, NGPRL, Mandan, ND 58554.
3 Correspondence: P.O. Box 3684 (phone: 307-766-5173; fax: 307-766-2355; e-mail: brethess{at}uwyo.edu).
Received for publication December 6, 2004.
Accepted for publication May 26, 2005.
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Literature Cited
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AOAC. 1990. Official Methods of Analysis. 15th ed. Assoc. Offic. Anal. Chem., Arlington, VA.
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Abstract
Introduction
