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ANIMAL NUTRITION |
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* Department of Animal Sciences, University of Stellenbosch, Matieland 7602, South Africa;
and
Department of Animal and Range Sciences, New Mexico State University, Las Cruces 88003; and
Extension Animal Sciences and Natural Resources Department, New Mexico State University, Las Cruces 88003
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: lamb • limiting amino acid • nitrogen retention
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Ruminal microbial protein is a major source of MP, particularly when ruminants are fed diets low in ruminally undegradable protein (RUP). However, microbial protein may not supply adequate absorbable AA for the needs of rapidly growing animals (Merchen and Titgemeyer, 1992).
Approaches to identify limiting AA in sheep have varied. Nimrick et al. (1970) supplemented AA individually or in combinations with a semipurified diet and reported that Met, Lys, and Thr limited N retention of lambs fed a diet devoid of true protein. In contrast, Storm and Ørskov (1984) deleted AA from an essential AA mixture supplied to lambs maintained by intragastric nutrition and demonstrated that Met, Lys, His, and Arg were limiting in ruminal microbial protein. These discrepancies in limiting AA also exist for cattle. Richardson and Hatfield (1978) reported that postruminal infusion of Met, Lys, and Thr increased N retention of steers fed a semipurified diet, whereas Greenwood and Titgemeyer (2000) and Löest et al. (2001) utilized a deletion approach to determine that Met, His, Lys, Leu, and Val limited N retention of steers fed a diet low in RUP.
Because sheep maintained by intragastric nutrition may experience atrophy of gastrointestinal tissues and altered AA metabolism (Ørskov et al., 1979), we hypothesized that feeding lambs a more typical diet low in RUP would reveal different limiting AA than those reported by Storm and Ørskov (1984). Therefore, our objective was to identify, using a deletion approach, which essential AA limit N retention of lambs fed a soybean hull-based diet low in RUP.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Ruminally cannulated Rambouillet wethers housed in individual metabolism crates in a room with continuous lighting and evaporative cooling were used in a diet characterization study (n = 2) and 3 N balance experiments (n = 6/experiment) to determine which essential AA are limiting. Experimental conditions utilized were similar to those used for evaluating limiting AA in growing cattle (Greenwood and Titgemeyer, 2000; Löest et al., 2001, 2002); lambs were limit-fed a soybean hull-based diet (Table 1) formulated to supply little RUP and were supplied with additional energy sources that were absorbable by the animal, but not readily fermentable by ruminal microorganisms. These experimental conditions created a model where the basal supply of absorbable AA (from ruminal microbial protein plus RUP) was potentially deficient relative to the animal’s requirements for AA to support the energy-allowable gain (see below).
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A preliminary evaluation of the ruminal fermentation characteristics of the soybean hull-based diet (Table 1
) for N balance Exp. 1 and 2 (see below) was conducted by measuring rumen protein degradability and ruminal pH, NH3-N, and VFA concentrations in 2 ruminally cannulated Rambouillet wether lambs (31.6 kg average BW). Lambs were limit-fed the soybean hull-based diet in equal portions twice daily to correspond with the first 2 N balance experiments (see below). After adapting lambs for 7 d to the diet, rumen protein degradability was determined using an in situ procedure. Dacron bags (25 x 80 mm; 53 ± 10 µm pore size; Ankom Technology Corp., Fairport, NY) were prepared in duplicate (20 contained 2 g of soybean hull-based diet ground to pass a 2-mm screen, and 20 were blanks) and sealed (Impulse heat sealer; National Instrument Corporation, Baltimore, MD). All Dacron bags were placed in large polyester mesh bags (200 x 350 mm), submerged in water (±39°C) for 20 min, and incubated in the rumen of each lamb for 0, 2, 4, 6, 8, 12, 16, 24, 48, and 72 h. After incubation, bags were washed (Maytag Corporation, Newton, IA) in cold water according to Mathis et al. (2001), dried at 55°C for 24 h, allowed to air-equilibrate, weighed, and analyzed for N by Kjeldahl analysis (AOAC, 2003). Blank bags were used to correct for N influx during ruminal incubation at each time point. Extent of ruminal protein degradation was separated into fractions A (N that disappeared at 0 h), C (N remaining after 72 h), and B (100% – A – C; NRC, 1985b). Rate of protein degradation (kd) for fraction B was calculated as the slope of the linear regression of the natural logarithm of N remaining against hour of incubation (Mathis et al., 2001).
Solid (kp) and liquid passage rates were determined by dosing 100 g of Yb-labeled soybean hull-based diet (labeled with 3.5 g of YbCl3 as described by Trater et al., 2001) and 150 mL of Cr-EDTA solution (2.77 g of Cr/L) into the rumen, and obtaining fecal samples directly from the rectum every 24 h for 120 h thereafter. Fecal grab samples were dried at 55°C for 72 h, ground (Wiley mill with 2-mm screen, Thomas Scientific, Swedesboro, NJ), and analyzed for Yb and Cr using an inductively coupled plasma optical emission spectrometer (Optima 4300 DV, PerkinElmer Instruments, Shelton, CT) as described by Munn (2004). Solid and liquid passage rates were calculated from Yb and Cr concentrations, respectively, by obtaining the slope of the linear regression as described for kd. Effective degradation of CP in the rumen was calculated according to Ørskov and McDonald (1979) with the equation A + B [kd/(kd + kp)].
After completion of the in situ procedure, lambs were allowed 4 d to return to constant rumen conditions before collecting ruminal contents at 0, 2, 4, 6, 8, and 10 h after feeding. Ruminal contents were strained through 4 layers of cheesecloth to obtain 100 mL of rumen fluid, and the pH was immediately recorded (Hanna pH meter Model 9042 MP, Van Nuys, CA). A sample of rumen fluid (8 mL) was added to vials containing 2 mL of 25% (wt/vol) m-phosphoric acid solution and frozen (–20°C) until analysis. Rumen fluid samples were analyzed for NH3-N concentrations using the procedure of Broderick and Kang (1980) modified for a microplate reader (ELX 808 Ultra Microplate Reader, Bio-Tek Instruments Inc., Winooski, VT) and for VFA concentrations using capillary GLC (Varian 3400; Varian Inc., Walnut Creek, CA) and flame ionization detection according to the procedures outlined by May and Galyean (1996).
Nitrogen Balance Experiment 1
Animals and Diet.
Six ruminally cannulated Rambouillet wether lambs (36.9 ± 1.9 kg initial BW) were used to determine if postruminal supply of Met, Lys, His, and Thr altered N retention and plasma AA concentrations. Lambs were housed individually in metabolism crates with free access to fresh water and were fed 667 g of DM/d of the soybean hull-based diet (Table 1) in equal portions at 12-h intervals (0800 and 2000 h). The diet was formulated to be low in RUP and was limit-fed at approximately 1.8% of BW daily to potentially increase ruminal protein degradation such that the predominant source of AA entering the small intestine would be microbial protein. Lambs were adapted to the soybean hull-based diet for 14 d before initiation of treatments.
Design and Treatments.
The experimental design was a 6 x 6 Latin square. Experimental periods were 7 d, which allowed 3 d for adaptation to abomasally infused treatments and 4 d for collections. A 3-d adaptation period was considered sufficient because ruminants adapt rapidly to postruminal infusions of nutrients (Hovell et al., 1983; Moloney et al., 1998). Treatments (Table 2) were continuous abomasal infusions of a solution (500 mL/d) containing 1) no AA (CON), 2) a mixture of 10 essential AA (10EAA), 3) 10EAA with Met removed (–MET), 4) 10EAA with Lys removed (–LYS), 5) 10EAA with His removed (–HIS), and 6) 10EAA with Thr removed (–THR). The AA profile of 10EAA (Table 2) was determined based on the difference between estimated AA requirements and metabolizable AA supply from rumen microbial protein. Metabolizable AA requirements were calculated using net protein requirements (factorial equation of NRC, 1985a) for 150 g/d of energy-allowable gain, a biological value of 0.66 for MP (NRC, 1985a), and the whole-body essential AA composition of lambs (Liu and Masters, 2003). Metabolizable AA supply was estimated based on the AA composition of microbial protein (Storm and Ørskov, 1983) and the assumption that microbial protein synthesis was 13% of dietary TDN (NRC, 2000). Utilization of an AA deletion approach from abomasal infusions of 10EAA ensured that a single limiting AA could be evaluated without interference from other potentially co-limiting AA (Greenwood and Titgemeyer, 2000). All treatments, except CON, supplied 2 nonessential AA (Glu and Gly) to minimize the use of essential AA treatments for de novo synthesis. Additionally, Glu is an important link between metabolism of carbon and N (Reeds et al., 2000), and Gly may be needed for sulfur AA metabolism because of the link to 1-carbon metabolism (Lambert et al., 2004). To prevent energy from limiting responses to postruminal AA infusions and to minimize the need of essential AA for gluconeogenic precursors (Campbell et al., 1997), all lambs received infusions of VFA (41 g/d of acetate and 14 g/d of propionate) and dextrose (73 g/d). The acetate (LabChem Inc., Pittsburgh, PA) and propionate (Sigma-Aldrich Co., St. Louis, MO) were mixed with deionized water and continuously infused (500 mL/d) into the rumen, and dextrose (Archer Daniels Midland Co., Decatur, IL) was dissolved with AA treatments (Table 2) for continuous infusion into the abomasum. The VFA and dextrose infusions were used to supply additional energy to lambs without altering rumen microbial protein synthesis. Also, the supply of dextrose may increase uptake of AA for protein synthesis by peripheral tissues due to increased insulin secretion (Ahmed et al., 1983; Wester et al., 2000). The rumen of lambs was allowed to adapt to an increase in acid load by gradually increasing the VFA infusions (to 41 g/d of acetate and 14 g/d of propionate) during the last 3 d of the 14-d adaptation period. The AA treatments and energy sources were infused continuously using a pump (Manostat Cassette Pump, Manostat, NY) such that the intestinal supply would be similar to continuous passage of nutrients to the intestines of ruminants. Infusions into the rumen were achieved by placing flexible tubing (1.6 mm i.d.) through the rumen cannula, and abomasal infusions were achieved by extending a similar tube through the reticulo-omasal orifice of lambs. The tubes were secured in the abomasum with a rubber flange (3-cm diameter).
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) was prepared by first dissolving the branched-chain AA (L-Leu, L-Ile, L-Val) in 250 g of deionized water containing 12 g of 6 M HCl, followed by L-Met, L-His, L-Lys, L-Thr, L-Phe, L-Arg, L-Trp, and Gly. In a separate container, L-Glu was dissolved in 80 g of deionized water containing 10 g of a 50% (wt/wt) NaOH solution. Once the AA had dissolved, the 2 solutions were mixed together, and dextrose was added and allowed to dissolve. The volume of the final solution was increased to 500 mL with deionized water; the final pH was approximately 3.8. The solutions for all other AA treatments were prepared similar to 10EAA, with the exception that the AA of interest (e.g., Met for –MET) was omitted from the mixture. Collections and Analysis. From d 4 to 7 of each period, total fecal output was collected daily into fecal collection pans, and total urinary output was collected into 1-L bottles containing 50 mL of 6 M HCl to minimize NH3 loss. The weight of total fecal and urinary output was recorded daily. All the feces and a representative sample (2%) of urine were frozen (–20°C) and later composited by period for each lamb. Also, dietary samples and feed refusals (if present) were collected, composited by period for each lamb, and frozen. Composite samples of the diet, feed refusals, and feces were dried at 55°C for 72 h in a forced-air oven, allowed to air-equilibrate, weighed to determine moisture loss, and then ground to pass a 2-mm screen. These samples were then analyzed for DM (105°C for 24 h), OM (500°C for 8 h), and NDF using an Ankom 200 Fiber Analyzer (Ankom Technology Corp., Fairport, NY) without heat-stable amylase addition. Also, dietary, orts, fecal, and urinary samples were analyzed for N using a Leco FP-528 N Analyzer (Leco Corporation, St. Joseph, MI) to calculate N balance.
Blood samples were collected from the jugular vein of lambs into vacuum tubes (10 mL; Fisher Scientific, Pittsburgh, PA) containing sodium heparin at 3 h after the second 12-h feeding on d 7 of each period. Blood samples were immediately chilled on ice and then centrifuged (Sorvall RT6000B, Heraeus Instruments, South Plainfield, NJ, H-100B rotor) at 1,500 x g and 5°C for 20 min. Plasma was decanted into 7-mL plastic vials using disposable transfer pipettes and frozen (–20°C) until analysis of free AA by capillary GLC (Varian 3800; Varian Inc., Walnut Creek, CA) and flame ionization detection according to the procedure of Chen et al. (2002).
Nitrogen Balance Experiment 2
Six ruminally cannulated Rambouillet wether lambs (35.1 ± 1.4 kg initial BW) were used in a 6 x 6 Latin square to determine if postruminal supply of all 3 branched-chain AA (Leu, Ile, plus Val), Arg, Phe, and Trp altered N retention and plasma AA concentrations. Lambs were fed 667 g of DM/d of the soybean hull-based diet (Table 1) in equal portions at 12-h intervals. With the exception of treatments, all experimental procedures were similar to N balance Exp. 1. Treatments (Table 3) were continuous abomasal infusions of a solution (500 mL/d) containing 1) CON, 2) 10EAA, 3) 10EAA with Leu, Ile, and Val removed (–BCAA), 4) 10EAA with Arg removed (–ARG), 5) 10EAA with Phe removed (–PHE), and 6) 10EAA with Trp removed (–TRP).
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Upon discovering that at least 1 branched-chain AA limited N retention (see results for N balance Exp. 2), 6 ruminally cannulated Rambouillet wether lambs (46.0 ± 1.3 kg initial BW) were used in a 6 x 6 Latin square to determine which branched-chain AA (Leu, Ile, or Val) altered N retention and plasma AA concentrations. Lambs were fed 798 g of DM/d of the soybean hull-based diet (Table 1) in 2 equal portions daily. All experimental procedures were similar to N balance Exp. 1 and 2. Treatments (Table 4) were continuous abomasal infusions of a solution (500 mL/d) containing 1) CON, 2) 10EAA, 3) –BCAA, 4) 10EAA with Leu removed (–LEU), 5) 10EAA with Ile removed (–ILE), and 6) 10EAA with Val removed (–VAL).
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Each of the N balance experiments were analyzed as a Latin square by the MIXED procedure (release 8.02; SAS Inst. Inc., Cary, NC). The statistical model included effects of period and treatment, with lamb as a random variable. When the overall F-test was significant (P < 0.10) for treatment, pairwise t-tests were used to compare 10EAA to CON, and to compare the removal of individual AA with 10EAA and with CON. Differences with P < 0.10 were considered significant, and differences with 0.10 < P < 0.15 were considered a tendency toward significance. Because the diet characterization study was only to quantify nutritional variables in the rumen of lambs fed the soybean hull-based diet when no treatments were assigned, these data were not analyzed statistically.
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The CP fractional pool size in the rumen of lambs fed the soybean hull-based diet (Table 1) averaged 27, 58, and 15% for fractions A, B, and C, respectively. Rate of CP degradation for fraction B averaged 5.0%/h, and rate of passage averaged 2.6 and 4.4%/h for particulate (Yb-labeled) and liquid (Cr-EDTA), respectively. Effective degradation of CP in the rumen of lambs averaged 65.1% for the soybean hull-based diet.
Ruminal pH of lambs fed the soybean hull-based diet averaged 6.7, 6.2, 6.4, 6.5, 6.6, and 6.7 at 0, 2, 4, 6, 8, and 10 h after feeding, and ruminal NH3 concentrations averaged 3.4, 4.6, 0.6, 0.4, 1.3, and 2.6 mM at 0, 2, 4, 6, 8, and 10 h after feeding. Ruminal concentrations of acetate, propionate, and butyrate averaged 49, 8, and 4 mM at 0 h, 81, 24, and 6 mM at 2 h, 64, 16, and 5 mM at 4 h, 63, 12, and 5 mM at 6 h, 52, 9, and 4 mM at 8 h, and 48, 8, and 3 mM at 10 h after feeding.
Nitrogen Balance Experiment 1
Effects of Met, Lys, His, or Thr removal from abomasal AA infusions on N balance and apparent diet digestibility of lambs are presented in Table 5. Total N intake was greater (P < 0.10) for lambs receiving 10EAA than CON because of additional N supply from abomasal AA infusions. Fecal N excretion was not different, and urinary N excretion was greater (P < 0.10) for lambs receiving 10EAA vs. CON. Also, lambs infused with 10EAA had greater (P < 0.10) apparent N digestibility and N retention (g/d, % of total N intake, and % of N digested) than CON lambs. Total N intake, fecal N excretion, and apparent N digestibility of lambs were not different between –MET and 10EAA. Removal of Met from 10EAA increased (P < 0.10) urinary N excretion and decreased (P < 0.10) N retention (g/d, % of total N intake, and % of N digested). Removal of Lys and His from 10EAA did not affect N balance and apparent N digestibility. Total N intake of lambs was less (P < 0.10) for –THR than 10EAA. Apparent N digestibility (expressed as % of N intake) was not affected, but grams of N digested and N retained (g/d) decreased (P < 0.10) for lambs receiving –THR vs. 10EAA. Removal of Thr from 10EAA did not significantly affect the retention efficiency of total N intake and digested N. Total tract apparent digestibility of OM and NDF was not affected by treatments.
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. Compared with CON, abomasal infusion of 10EAA increased (P < 0.10) plasma concentrations of total essential AA as well as all individual essential AA, except Ile, Phe, and Trp. Also, plasma concentrations of Asn and Orn were greater (P < 0.10) for 10EAA than CON. Removal of Met from 10EAA decreased (P < 0.10) plasma concentrations of Met and His, tended to decrease (P = 0.11) plasma Cys concentration, and increased (P < 0.10) plasma concentrations of Gly, Ser, and total nonessential AA. Removal of Lys from 10EAA decreased (P < 0.10) plasma Lys concentration and increased (P < 0.10) plasma Phe concentration. Removal of His from 10EAA decreased (P < 0.10) plasma His concentration, whereas Thr removal decreased (P < 0.10) plasma concentrations of Thr, Gly, and total nonessential AA.
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Effects of all 3 branched-chain AA (Leu, Ile, plus Val), Arg, Phe, or Trp removal from abomasal AA infusions on N balance and apparent diet digestibility of lambs are presented in Table 7. Total N intake numerically increased (not evaluated statistically due to no within treatment variation; no feed refusals) because of additional N from infused AA (10EAA vs. CON). Lambs receiving 10EAA had similar fecal N excretion, but greater (P < 0.10) urinary N excretion than CON lambs. Both apparent total tract N digestibility and N retention (g/d, % of total N intake, and % of N digested) were greater (P < 0.10) for lambs receiving 10EAA vs. CON. Removal of all branched-chain AA (–BCAA) from 10EAA decreased (P < 0.10) apparent N digestibility of lambs. Urinary N excretion was not different, but N retained (g/d) was less (P < 0.10) for lambs receiving –BCAA vs. 10EAA. Removal of branched-chain AA from 10EAA did not affect the efficiency with which total N intake and digested N was retained. Total tract apparent digestibility of N (expressed as % of N intake) was not affected by the removal of Arg, Phe, or Trp from 10EAA, but grams of N digested and N retained (g/d) were less (P < 0.10) for –ARG vs. 10EAA. Lambs receiving –TRP had greater (P < 0.10) urinary N excretion and less (P < 0.10) N retention (g/d) than 10EAA lambs. The retention efficiency of total intake N and digested N was not different for –ARG and –TRP compared with 10EAA. Abomasal infusions of treatments (with exception of –BCAA) did not affect total tract apparent digestibility of OM and NDF. Apparent NDF digestibility was less for –BCAA than CON.
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. Plasma concentrations of His, Leu, Lys, Met, Phe, Val, and total essential AA were greater (P < 0.10) for lambs receiving 10EAA than for CON lambs. Also, plasma Orn concentration was greater (P < 0.10), and plasma concentrations of Glu and Pro were less (P < 0.10) for 10EAA than CON. Removal of Leu, Ile, and Val (–BCAA) from 10EAA decreased (P < 0.10) plasma concentrations of Leu, Val, and total essential AA, but did not affect plasma Ile concentration. Also, removing the branched-chain AA from 10EAA increased (P < 0.10) plasma concentrations of His, Met, Thr, Ala, Asp, Glu, Gly, Orn, Pro, Tyr, and total nonessential AA. Removal of Arg from 10EAA increased (P < 0.10) plasma concentrations of His, Met, Gly, Pro, Tyr, and total non-essential AA, and decreased (P < 0.10) plasma Orn concentration. Removal of Phe from 10EAA decreased (P < 0.10) plasma concentrations of Phe, Glu, and Tyr, whereas –TRP increased (P < 0.10) plasma Asp and Pro concentrations compared with 10EAA, but did not affect plasma Trp concentration.
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Effects of removing branched-chain AA from abomasal AA infusions on N balance and apparent diet digestibility of lambs are presented in Table 9. Compared with CON, abomasal infusion of 10EAA increased (P < 0.10) total N intake, did not affect fecal N excretion, and increased (P < 0.10) urinary N excretion. Also, apparent N digestibility and N retention (g/d, % of total N intake, and % of N digested) was greater (P < 0.10) for lambs receiving 10EAA compared with CON. Removal of all branched-chain AA (–BCAA) from 10EAA increased (P < 0.10) fecal N excretion and decreased (P < 0.10) total tract apparent N digestibility. Urinary N excretion was not different between –BCAA and 10EAA, but N retention (g/d) decreased (P < 0.10) in response to –BCAA. Also, lambs receiving –BCAA had less (P < 0.10) efficiency with which total N intake was retained, but similar retention efficiency of digested N compared with lambs receiving 10EAA. Removal of Leu (–LEU), Ile (–ILE), and Val (–VAL) from 10EAA did not affect total N intake, fecal N excretion, and apparent total tract N digestibility, but increased (P < 0.10) urinary N excretion. However, the increases in urinary N excretion in response to –LEU and –ILE were not large enough to significantly alter N retention. In contrast, N retention (g/d, % of total N intake, and % of N digested) decreased (P < 0.10) when Val was removed from 10EAA. Treatments did not affect total tract apparent digestibility of OM and NDF.
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. Lambs that were abomasally infused with 10EAA had greater (P < 0.10) plasma concentrations of Leu, Lys, Met, Phe, Val, and total essential AA than CON lambs. Also, 10EAA infusions increased (P < 0.10) plasma concentrations of Asp, Asn, and Orn, and decreased (P < 0.10) plasma concentrations of Ala, Glu, Pro, and Ser compared with CON. Removal of all 3 branched-chain AA (–BCAA) from 10EAA decreased (P < 0.10) plasma Leu and Val concentrations, but did not affect plasma Ile concentration of lambs. Also, plasma concentrations of Met, Phe, Thr, Ala, Asp, Glu, Orn, Pro, and Ser increased (P < 0.10), and plasma Asn concentration decreased (P < 0.10) when all branched-chain AA were removed from 10EAA. Removal of Leu (–LEU) from 10EAA decreased (P < 0.10) plasma Leu concentration, and increased (P < 0.10) plasma concentrations of Ile, Phe, Thr, Val, total essential AA, Ala, Asp, Asn, Glu, Orn, and Pro. Removal of Ile (–ILE) from 10EAA tended to decrease (P = 0.12) plasma Ile concentration, and increased (P < 0.10) plasma concentrations of Thr and Glu, whereas –VAL decreased (P < 0.10) plasma Val and Asn concentrations and increased (P < 0.10) plasma Asp concentration compared with 10EAA.
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DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The approach of this study was to identify limiting AA in growing lambs when fed a diet low in RUP such that ruminal microbial protein was the major source of metabolizable AA. The ruminal CP degradability of the soybean hull-based diet was 65.1%, which is consistent with the value calculated by Greenwood and Titgemeyer (2000) for a similar soybean hull-based diet used to evaluate limiting AA in cattle. Also similar to Greenwood and Titgemeyer (2000), microbial protein was estimated to supply 58.5% of the MP based on the ruminal protein degradability of the diet and the assumption that microbial protein supply was related to observed digestible OM intake (TDN intake x 0.13; NRC, 2000). Based on the AA profile of microbial protein for sheep (Storm and Ørskov, 1983) and the AA profile of the RUP fraction of soybean hulls (NRC, 2000), the basal supply of absorbable AA was estimated to be (g/d): Arg (3.4), His (1.3), Ile (2.9), Leu (4.3), Lys (4.6), Met (1.0), Phe (2.8), Thr (2.9), Trp (0.7), and Val (3.3), when lambs were fed 667 g of DM/d of the soybean hull-based diet. These supplies of AA were predicted to be inadequate to support protein deposition for 150 g/d of energy-allowable (dietary + infused) gain (see Materials and Methods for calculations of metabolizable AA requirements); basal supplies were estimated to be (% of the requirement): Met (50), Arg (51), Trp (60), His (61), Thr (64), Leu (65), Lys (76), Phe (77), Val (83), and Ile (88).
Ruminal fermentation characteristics of the soybean hull-based diet illustrated that conditions were favorable to support ruminal microbial growth during most of a 12-h feeding interval. Ruminal pH decreased to a low of 6.2 at 2 h after feeding, but remained above the benchmark of 5.6 for subacute ruminal acidosis (Owens et al., 1998). Ruminal NH3 concentration averaged 2.1 mM and remained above 1.3 mM at all sampling times except at 4 h and 6 h after feeding. According to Satter and Slyter (1974), rumen NH3 concentrations of 1.2 to 2.9 mM (20 to 50 mg/L) are required to support growth of rumen bacteria. However, Hespell and Bryant (1979) suggested that NH3 concentrations as low as 0.1 mM are adequate to allow maximal growth rates for rumen bacteria. Rapid increases in ruminal VFA concentrations after feeding and high proportional levels of acetate are indicative of ruminal fermentation of fiber from the soybean hull-based diet.
In the 3 N balance experiments, similar OM and NDF digestibilities for lambs receiving abomasal AA infusions compared with CON suggests that ruminal fermentation characteristics were not altered by postruminal AA infusions; postruminal AA infusions potentially could have affected ruminal fermentation through N recycling. Absorption of continuously infused crystalline AA by the gastrointestinal tract of lambs appeared to be near 100% because a 44% to 68% increase in total N supply due to abomasal AA infusions (10EAA treatment) in all 3 N balance experiments did not increase fecal N excretion above that observed for lambs receiving no AA infusions (CON). Consequently, greater apparent N digestibilities observed for 10EAA vs. CON lambs were attributed to greater absorption of abomasally infused AA compared with AA from the basal supply (intact proteins), and not due to an increase in N digestibility of the soybean hull-based diet.
Increases in N retention for lambs receiving 10EAA vs. CON in all 3 N balance experiments demonstrated that the basal supply (CON) of metabolizable AA was inadequate to support the lambs’ potential to retain N given the availability of other nutrients (e.g., energy) or the animal’s genetic potential for protein deposition or both. A simultaneous increase in urinary N excretion and plasma Orn concentration for lambs infused with 10EAA vs. CON is indicative of increased ureagenesis due to the supply of nonspecific N and suggest that the supply of some AA exceeded the animal’s requirement (Greenwood and Titgemeyer, 2000). Model predictions of postruminal supplies (dietary + infused) of individual essential AA to lambs receiving 10EAA were between 94 and 165% of the requirements. Greater plasma concentrations of all essential AA, except Ile and Trp, for lambs receiving 10EAA vs. CON (see all 3 N balance experiments) reflects both an increase in the absorption of these AA and that the AA supply (basal + infused) exceeded the lambs’ requirements. The fact that plasma Ile and Trp concentrations were not greater for lambs receiving 10EAA vs. CON suggests that additional supply of these AA by 10EAA infusions did not exceed the animals’ requirement, or that these AA were metabolized rapidly once absorbed. For example, catabolism of Ile may have been greater for lambs receiving 10EAA vs. CON due to stimulation of branched-chain AA oxidation by the additional supply of Leu (Block and Harper, 1984; Löest et al., 2001).
Essential AA supplied in excess of requirements generally are utilized less efficiently than those supplied below requirements (Titgemeyer, 2003). Therefore, increases in the efficiency with which digested N was retained by lambs receiving greater amounts of intake N (10EAA vs. CON) is likely because of greater absorption of crystalline AA compared with AA from the basal supply (intact proteins) or because the absorbable AA profile for lambs receiving 10EAA was a closer match to the lambs’ requirements than the AA profile of the basal supply (CON). Cole and Van Lunen (1994) and Schwab (1996) argued that the efficiency of protein utilization is not only dependent on the amount supplied, but also on the AA profile.
Limiting AA
The objective was to identify which essential AA limited protein deposition (N retention) of growing lambs. In N balance Exp. 1 and 2, a decrease in N retention (g/d) in response to the removal of Met or Trp from 10EAA was due to greater excretion of urinary N, whereas a decrease in N retention (g/d) in response to the removal of Thr or Arg from 10EAA was because of less total N intake. The decrease in N retention in response to the omission of Met and Trp from the abomasally infused AA mixture indicated that an inadequate supply of each of these AA limited protein deposition when lambs were fed a soybean hull-based diet. An increase in urinary N excretion when Met and Trp were omitted reflects AA catabolism and is indicative of an AA imbalance where their deficiency limited the use of other AA for protein deposition (Cole and Van Lunen, 1994).
The finding that Met limited N retention is not surprising considering that it was predicted to be the most limiting AA based on our estimates of metabolizable AA supplies and requirements (see above), and considering that many studies (Nimrick et al., 1970; Storm and Ørskov, 1984; Nolte and Ferreira, 2004) have reported that Met is limiting for growing sheep when microbial protein is the predominant source of absorbable AA. In addition to its direct role in protein accretion, Met may be transsulfurated to Cys (Finkelstein, 1990) for support of the high Cys content of wool (MacRae et al., 1993). Improvement of wool growth in sheep with supplementation of sulfur-containing AA is well documented (Reis et al., 1990; Sherlock et al., 2001). Also, Met may be needed for functions such as synthesis of polyamines (needed for wool fiber elongation; Hynd and Nancarrow, 1996) and methylation of phospholipids, proteins, nucleic acids, and other molecules (Finkelstein, 1990; Lobley, 1992; Chiang et al., 1996).
Our finding that Thr decreased N retention (g/d) of growing lambs is consistent with the work of Nimrick et al. (1970), and our finding that Arg decreased N retention (g/d) is in agreement with Storm and Ørskov (1984). Also, Ferreira et al. (1999) calculated that the duodenal supply of both Thr and Arg were inadequate for ram lambs fed a diet high in ruminally degradable protein. Altered N retention in response to Thr (Chalupa and Scott, 1976; Richardson and Hatfield, 1978) and Arg (Koenig et al., 1982; Davenport et al., 1990) have also been reported for cattle. In contrast, Greenwood and Titgemeyer (2000) did not observe a response to Arg when growing steers were limit-fed a diet similar to the soybean hull-based diet of our study, and they attributed their response (or lack thereof) to adequate supplies of Arg by the diet. In our study, both Arg and Thr were predicted to be deficient based on calculated requirements versus supplies. However, N retention, expressed as a percentage of total N intake, was not affected by the removal of Thr or Arg from 10EAA, and changes in grams of retained N appeared to be due to differences in total N intake. Other possible explanations for the responses to Arg include a potential increase in the need for Arg to support ureagenesis for excess N disposal (by design, the 10EAA infusions supplied AA in excess of that needed to support the energy-allowable gain), and an increased need for Arg for polyamine synthesis (D’Mello, 2003), which in turn is linked to wool growth (Hynd and Nancarrow, 1996). Also, it is possible that the excess supply of Lys in our model induced an Arg deficiency (D’Mello, 2003), although mammals appear to be less sensitive to Lys-Arg antagonism than avian species (Edmonds and Baker, 1987).
The finding that Trp is a limiting AA has not been reported previously for growing lambs. The N retention responses to Trp in our study agree well with our model predictions that Trp was the third most limiting AA. Also, the absence of a decrease in plasma Trp concentration when this AA was removed from 10EAA suggests that the absorbable Trp supply was below animal requirements. In contrast to our findings, Storm and Ørskov (1984) reported that Trp did not limit N retention of growing lambs maintained by intragastric nutrition. However, it is possible that AA requirements of lambs maintained by intragastric nutrition (Storm and Ørskov, 1984) were less than those in our study because of differences in growth rate. Also, atrophied gastrointestinal tissues could impact metabolism of AA in sheep maintained by intragastric nutrition (Titgemeyer, 2003). Although values in the literature for Trp are scarce because Trp is destroyed during acid hydrolysis of samples for AA analysis, limited data indicate that the Trp concentrations of ruminal microbial protein (Storm and Ørskov, 1983) are less than those in wool protein (Corfield and Robson, 1955), which may explain in part the deficiency in Trp.
In N balance Exp. 1 and 2, decreases in plasma concentrations of Met, Lys, His, Thr, and Phe when each of these essential AA were removed from 10EAA indicated that their metabolizable supply for lambs receiving 10EAA exceeded the animal’s requirement. A tendency for plasma Cys concentration to decrease (P = 0.11), and an increase in plasma Ser concentration in response to Met removal from 10EAA is typical for postruminal Met supplementation (Campbell et al., 1997; Löest et al., 2002) and corresponds with a reduction in the transsulfuration of Met to Cys (Finkelstein, 1990). An increase in plasma Gly concentration in response to the omission of Met from 10EAA is consistent with results of Campbell et al. (1997) and is likely because of the interaction of Gly with sulfur AA metabolism through methyl donation (Lambert et al., 2004). Additionally, the increases in plasma concentrations of nonessential AA can be a result of decreased protein accretion due to a Met limitation (Campbell et al., 1997). A decrease in plasma Gly concentration in response to the removal of Thr from 10EAA reflects a decrease in hepatic catabolism of Thr to Gly and acetyl-CoA via mitochondrial Thr dehydrogenase (Davis and Austic, 1994). When Arg was removed from 10EAA, plasma concentrations of His, Met, and several nonessential AA (Gly, Pro, and Tyr) increased, and N retention of lambs decreased, which typically is indicative of a limiting AA (Campbell et al., 1997). Greenwood and Titgemeyer (2000) observed increases in plasma non-essential AA concentrations in steers when Arg was omitted from abomasal infusions of essential AA, but N retention of the steers was not altered significantly. In our study, the decrease in N retention (g/d) in response to Arg omission appeared to a function of total N intake. Increases in plasma concentrations of Met and Gly due to the removal of Arg can be explained by interactions of these AA in biosynthesis of creatinine and polyamines (Gonzalez-Esquerra and Leeson, 2006). Decreases in plasma concentrations of Orn and Tyr due to the removal of Arg and Phe, respectively, are indicative of the metabolic relationship between these AA. Removal of Trp from 10EAA did not decrease its concentration in plasma and agrees with our conclusions that 10EAA did not supply sufficient Trp to exceed the animal’s requirements.
In both N balance Exp. 2 and 3, decreases in N retention in response the simultaneous removal of all 3 branched-chain AA from 10EAA indicated that the basal supply of at least 1 branched-chain AA limited protein deposition of growing lambs. Another consistency between N Balance Exp. 2 and 3 was that the decreases observed in N retention were in part because of decreases in N digestibility. The effects of branched-chain AA on N digestibility can be explained by their (particularly Leu) role in the regulation of AA transport systems (Peyrollier et al., 2000; Hyde et al., 2003) in intestinal epithelial cells. Branched-chain AA have not been identified previously as limiting AA for growing lambs, but Greenwood and Titgemeyer (2000) reported that N retention of growing steers fed a soybean hull-based diet similar to the diet used in our study responded to the removal of branched-chain AA from abomasal infusions of essential AA. It appears that diets based on soybean hulls may supply inadequate absorbable branched-chain AA for ruminants.
In N balance Exp. 3, a 15% reduction in N retention in response to the removal of Val from 10EAA indicated that this branched-chain AA was limiting for growing lambs, and the increase in urinary N excretion is a reflection of greater AA catabolism. Excretion of urinary N by lambs also increased when Leu and Ile were removed from 10EAA, but these responses were not large enough to significantly alter N retention. Predicted supplies of the branched-chain AA were inadequate based on protein deposition for 150 g/d of energy-allowable gain, but Val and Ile were predicted to be least limiting among the essential AA infused. Storm and Ørskov (1984) reported no differences in N retention of lambs maintained by intragastric nutrition when each of the branched-chain AA were omitted, but Löest et al. (2001) reported that the omission of the branched-chain AA, Leu or Val, from postruminal infusions of essential AA decreased N retention of growing steers. The branched-chain AA deficiencies observed in our study and in that of Löest et al. (2001) may be explained, in part, by similarities in the experimental procedures (e.g., in both experiments, a soybean hull-based diet was limit-fed and excess essential AA were abomasally infused). Also, it is possible that the role of Leu in branched-chain AA antagonism could have induced a Val deficiency in our study. Block and Harper (1984) demonstrated that excess Leu increases branched-chain AA catabolism through the activation of branched-chain keto-acid dehydrogenase.
In both N balance Exp. 2 and 3, removal of all 3 branched-chain AA from 10EAA decreased plasma concentrations of Leu and Val, but not Ile, likely because Ile supply by 10EAA infusate was not in excess of the animal’s requirement, or likely because the simultaneous removal of Leu alleviated a Leu-induced oxidation of Ile (Block and Harper, 1984). In N balance Exp. 3, decreases in plasma concentrations of Leu and Val and a tendency for plasma Ile to decrease when each of these branched-chain AA were removed individually from 10EAA are indicative that the 10EAA did supply excess amounts and that excess Leu appeared to stimulate Ile oxidation. Increased oxidation of branched-chain AA in response to excess Leu is, in part, supported by increases in plasma concentrations of both Ile and Val when Leu was removed from 10EAA. These increases in plasma Ile and Val concentrations due to the removal of an excess supply of Leu are consistent with other reports for sheep (Papet et al., 1988), cattle (Löest et al., 2001), and various monogastric species (Harper and Benjamin, 1984; Hargrove et al., 1988; Langer et al., 2000).
Another consistency between N balance Exp. 2 and 3 was an increase in some plasma essential AA (Met and Thr) and many nonessential AA (Ala, Asp, Glu, Orn, Pro) when all 3 branched-chain AA were omitted from 10EAA. Increases in these plasma AA concentrations are indicative of a decrease in their use for protein deposition likely because of one or more limiting branched-chain AA. Increases in plasma nonessential AA in response to branched-chain AA omission from abomasal infusions of a mixture of 10 essential AA have also been reported for growing cattle (Greenwood and Titgemeyer, 2000; Löest et al., 2001).
The lack of a N retention response to the removal of Lys, His, and Phe from 10EAA suggests that these essential AA were not limiting for growing lambs limit-fed the soybean hull-based diet. Nimrick et al. (1970) reported that Lys, but not His, was limiting for lambs fed a semipurified diet, whereas Storm and Ørskov (1984) reported that both Lys and His were limiting in ruminal microbial protein of growing lambs maintained by intragastric nutrition. Differences among studies for these essential AA could be explained in part by the experimental models used (semipurified diets vs. intragastric nutrition vs. soybean hull-based diet). For example, altered metabolism of absorbed AA due to gastrointestinal tissue atrophy in sheep maintained by intragastric nutrition (Ørskov et al., 1979) may affect responses to essential AA. Also, differences in growth rate (rate of N retention) of lambs among these studies might have affected which essential AA were limiting (MacRae et al., 1993). Storm and Ørskov (1984) initially observed a reduction in N retention when His was omitted, but their subsequent experiments with a stepwise reduction of His revealed no effect on N retention in sheep. Therefore, only few conditions may exist where His is limiting in sheep. Because both Nimrick et al. (1970) and Storm and Ørskov (1984) identified Lys as a limiting AA, we anticipated a similar response to the removal of Lys from 10EAA in lambs fed a soybean hull-based diet. However, the lack of a response to –LYS compared with 10EAA is supported by our model predictions that the metabolizable Lys supply from the soybean hull-based diet was one of the essential AA least likely to be limiting. It is possible that the soybean hull-based diet supplied sufficient metabolizable Lys. Greenwood and Titgemeyer (2000), who fed growing steers a soybean hull-based diet similar to the diet of the current study, observed a tendency only for a decrease in N retention when Lys was removed from abomasal AA infusions.
Conclusions
Nitrogen retention of lambs fed restricted amounts of a diet based on soybean hulls was limited by inadequate supplies of absorbable Met, Thr, Arg, Trp, and Val. This research indicates that several essential AA may limit performance, and implies that supplementation of these essential AA in a rumen-protected form could be beneficial to growing lambs fed diets low in RUP.
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2 Corresponding author: cloest{at}nmsu.edu
Received for publication December 3, 2007. Accepted for publication May 28, 2008.
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