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ANIMAL NUTRITION |
,2
* Department of Animal Sciences, University of Illinois, Urbana 61801 and
and
USDA-ARS Swine Odor and Manure Management Research Unit, Ames, IA 50011-3310
Abstract
Three pig trials were carried out to determine the true digestible Ile requirement for maximal weight gain and minimal plasma urea nitrogen (PUN) of late-finishing (87 to 105 kg) pigs. In Exp. 1, an Ile-deficient basal diet was developed and confirmed to be markedly deficient in Ile, yet fully efficacious when fortified with surfeit Ile. This diet contained corn and dried red blood cells (RBC) as Ile sources, and was analyzed to contain 10.5% CP, 0.25% Ile, and 0.63% lysine; ME was calculated to be 3,475 kcal/kg. True digestibility of Ile in the basal diet was 88% based on previous digestibility trials in ileal-cannulated pigs and cecectomized roosters. Experiment 2 was a growth trial that involved five graded levels of crystalline Ile supplementation (0.02%) to generate five dose levels of true digestible Ile (0.25 to 0.33%), Diets 1 through 5, respectively. Gain and feed efficiency showed a linear response to incremental doses of Ile (P = 0.003 and 0.036, respectively), with an apparent plateau at 0.31% true digestible Ile. In Exp. 3, a replicated 5 x 5 Latin square, five barrows (Square 1) and five gilts (Square 2) were used in five 4-d feeding periods, with five levels of true digestible Ile (0.22 to 0.30%). Using feed intake as a covariate, a linear decrease in PUN occurred in gilts (8.9, 8.6, 8.0, 7.0, and 5.5; P = 0.004) and in gilts and barrows combined (9.5, 9.2, 9.2, 8.5, and 7.6; P = 0.006) as Ile increased incrementally. The PUN results for barrows (10.5, 10.0, 10.2, 9.9, and 9.7) were not affected by dietary Ile (P = 0.417). The results of these experiments suggest that the factorial requirement estimate of 0.30% true digestible Ile for high-lean, late-finishing pigs suggested by the NRC Subcommittee on Swine Nutrition is accurate.
Key Words: Blood Cells • Finishing Pigs • Isoleucine • Requirement
Introduction
Little information regarding Ile requirements of finishing pigs is available due to the misconception that it is adequate in common feedstuffs. Available research in this area is primarily older (Becker et al., 1963
; Brown et al., 1974), and although this in itself is not a deciding factor to making the data irrelevant, other factors cause the research to be less applicable to modern pig production. Earlier work used semipurified diets, tended to be less experimentally extensive, and was not based on digestible AA concentration. In addition, current genotypes are leaner and faster growing, but have a lower appetite than pigs 30 yr ago. Recent Ile recommendations by the NRC (1998) were arrived at factorially using ideal AA ratios (Wang and Fuller, 1989; Chung and Baker, 1992b; Baker, 1997).
Variability in finishing pigs makes interpretation of results difficult. Attempts to minimize variability by using short-term feeding assays have resulted in trials that use plasma urea nitrogen (PUN) as an indicator of the requirement (Liu et al., 2000a). Research by Liu et al. (1999) has shown that reducing CP level and adding AA does not affect growth performance of early- or late-finishing pigs. Their results, however, suggested that Ile is a limiting AA for late-finishing barrows fed AA-fortified, low-CP corn diets (Liu et al., 2000b). When diets containing 7.1% CP, consisting of corn and supplemental crystalline L-Lys, L-Thr, L-Trp, and DL-Met were fed to late-finishing pigs, growth responses to supplemental Ile were observed, suggesting that Ile may become a limiting AA for late-finishing pigs fed low-CP diets.
The objectives of our experiments were to develop an efficacious Ile-deficient diet based on dried red blood cells (RBC) that would show a marked growth response to supplemental Ile. Subsequently, we proceeded to determine the Ile requirement for finishing (87 to 100 kg) pigs based on maximal weight gain and minimal PUN.
Materials and Methods
General Procedures
All experimental procedures were approved by the University of Illinois Committee on Laboratory Animal Care. Crossbred pigs (Line 337 sires x Camborough 22 dams; PIC, Franklin, KY) obtained from the University of Illinois Swine Research Center were used. Following a 16-h period of feed deprivation (Exp. 1 and 2), pigs in growth trials were assigned to uniform blocks based on ancestry and BW. They were then randomly allotted from within blocks to pens and treatment diets. Pigs were housed in temperature-regulated barns containing pens (1.80 x 2.59 m) with partially slatted floors (Exp. 1 and 2) and in individual pens with totally slatted floors (1.83 x 2.13 m) for Exp. 3. Temperature was maintained at approximately 22°C, and lighting was regulated for a 16-h light:8-h dark cycle for the first two experiments; 24-h constant lighting was used for Exp. 3. Water and diets were provided for ad libitum consumption. At the termination of Exp. 1 and 2, pigs were again subjected to an overnight period of feed withdrawal, after which pigs and feeders were weighed to allow calculation of ADG, ADFI, and feed efficiency. Pigs in Exp. 3 continued to have ad libitum access to feed until the termination of the experiment.
Basal Diet and Isoleucine Digestibility
The basal Ile-deficient basal diet (Table 1) contained corn and dried RBC (AP-301G, American Protein Corp., Ames, IA). For Exp. 1 and 3, the basal diet was mixed at the University of Illinois Swine Research Center, whereas the basal diet used in Exp. 2 was mixed at United Feeds, Inc. (Gridley, IL). Following 24-h HCl hydrolysis, the basal diets were analyzed (Chung and Baker, 1992a) and found to contain 0.25% (Exp. 1 and 3) and 0.28% (Exp. 2) Ile. Amino acid concentrations were not corrected for incomplete recovery resulting from hydrolysis. Using the RBC digestibility values determined for Ile previously (Parr et al., 2003), the basal diet was determined to have an Ile digestibility of 88%. Lysine analysis of the basal diets indicated Lys levels of 0.63% for the basal diets used in Exp. 1 and 3 and 0.74% for the basal diet used in Exp. 2.
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), we formulated a basal diet with 0.25% Ile, 0.63% Lys, and 3,475 kcal ME/kg (Table 1
). The basal diet was formulated to meet or exceed NRC (1998) recommendations for all AA, with the exception of Ile. The corn-soybean meal positive-control diet contained a total Ile level that was in excess of the NRC (1998) estimated requirement (0.33%) for pigs in the weight category 80 to 120 kg (Table 1). Seventy-five pigs were used in this experiment. Five replicates of five pigs per pen (three pens of barrows and two pens of gilts) were assigned to one of three diets for a period of 14 d: 1) a positive-control corn-soybean meal diet, 2) negative control Ile-deficient RBC basal diet, and 3) the same as Diet 2 + surfeit (0.24%) Ile.
Experiment 2 (Growth Assay)
The objective of this study was to determine the Ile requirement for maximal growth of 87- to 100-kg late-finishing pigs fed graded doses of Ile. Six replicates of four pigs per pen (three pens each of barrows and gilts) were assigned to one of five treatments. The test diets included five graded levels of L-Ile, which were added to the Ile-deficient basal diet containing 0.74% Lys. Dose levels ranged from 0.25% (basal) to 0.33% true digestible Ile. Based on the current NRC (1998) estimate of 0.29 to 0.31% for the true ileal digestible Ile requirement of pigs in the 80 to120 kg weight category, we believed these concentrations would adequately represent both the linear and plateau regions of the growth curve.
Pigs were weighed and blocked within gender according to ancestry and BW. After an overnight period of feed withdrawal, pigs were offered ad libitum access to their test diet for a period of 16 d. Free access to water via a single nipple waterer was available. At the termination of the feeding period, pigs were again subjected to overnight feed withdrawal, after which pigs and feeders were weighed for calculation of daily gain, feed intake, and feed efficiency.
Experiment 3 (Latin Square Plasma Urea Nitrogen Assay)
The objective of this experiment was to determine the Ile requirement of pigs based on achievement of minimal PUN. We believed PUN analysis of the blood in a short-term feeding study might decrease the variability encountered in the growth trial (Exp. 2), and therefore result in a better estimate of the Ile requirement of the late-finishing pig.
Ten pigs (five barrows and five gilts) were used in a replicated 5 x 5 Latin square design, utilizing the five dietary levels of true digestible Ile (0.22 to 0.30% of the diet). Animals were individually penned and had ad libitum access to each test diet for a period of 4 d. At 0800 on d 5, animals were physically stimulated to awaken and consume feed. At 0900, each pig was bled once via jugular venipuncture. The heparinized blood samples were stored on ice until blood collection from all pigs was complete. Samples were then centrifuged at 3,000 x g for 15 min at 5°C, after which an aliquot of plasma from each sample was used for PUN analysis.
The enzymatic procedure used to determine PUN was based on the coupled urease/glutamate dehydrogenase reaction. When urea is hydrolyzed by urease, it forms carbon dioxide and ammonia. The ammonia released then reacts with 
-ketoglutarate and NADH in the presence of glutamate dehydrogenase to yield glutamate and NAD+. The decrease in absorbance due to the oxidation of NADH is then measured kinetically (Roche System Applications Manual, 2002).
Statistical Analysis
Data for weight gain, feed intake, and G:F (Exp. 1 and 2), and PUN (Exp. 3) were subjected to ANOVA procedures for randomized complete-block or Latin square designs (Steele and Torrie, 1980) using the GLM procedures of SAS (SAS Inst., Inc., Cary, NC). Experiment 2 was analyzed as a factorial with gender and diet as independent variables. Means were evaluated by orthogonal single-df comparisons. In some cases, specific preplanned nonorthogonal comparisons were also made.
Results
True digestibilities of Ile, Lys, Leu, and Val in the RBC product were determined previously (Parr et al., 2003). Using the predetermined 97.1% true Ile digestibility value for RBC (0.33% total Ile) together with NRC (1998) true Ile digestibility values of 87% for corn (0.28% total Ile) and 89% for soybean meal (2.25% total Ile), the estimated true Ile digestibility in the Ile-deficient basal diet was 88%. Therefore, the Ile-deficient pig assay diet shown in Table 1 contained 0.22% true digestible Ile in Exp. 1 and 3, and 0.25% true digestible Ile in Exp. 2.
In Exp. 1, weight gain and feed efficiency were decreased (P < 0.001) in pigs fed the Ile-deficient basal diet compared with pigs fed the corn soybean meal positive-control diet (Table 2). However, when the Ile-deficient basal diet was fully fortified with Ile, growth performance was restored to the same level as that occurring in pigs fed the positive-control diet. The results of this trial showed that the Ile-deficient diet did indeed cause a decrease in growth performance, and could therefore be used in further dose-titration studies to determine the Ile requirement of late-finishing pigs.
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. Daily weight gain showed an unexpected decrease in performance with the addition of the first increment of Ile. With the exception to Diet 2, however, daily weight gain and G:F increased linearly (P = 0.003 and 0.036, respectively) between 0.25 and 0.31% true digestible Ile in the diet, after which further responses did not occur. Our empirical estimate (0.31%) of the true digestible Ile requirement is similar to the NRC (1998) factorial estimate for pigs in the 80 to 120 kg BW weight category.
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). Gilts had lower (P < 0.01) PUN values than barrows, and displayed a linear (P = 0.004) decrease in PUN concentration across all levels of Ile fed. No apparent plateau in PUN was reached in gilts, even at the highest inclusion level of Ile (0.30% true digestible Ile), which suggests that the true digestible Ile requirement for late-finishing gilts is not less than 0.30%. The PUN results for barrows were not affected by dietary Ile (P = 0.417).
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It was imperative to the success of the studies herein that we develop an Ile-deficient diet that not only showed a depression in performance when compared with a corn-soybean meal positive-control diet, but that also demonstrated a restoration in performance when excess Ile was supplemented. Similar to the results of Kerr et al. (2002), pigs in Exp. 1 (Table 2) performed as expected, indicating that a diet with 5.0% RBC was deficient only in Ile, and that the diet was not only efficacious but was suitable for the dose-titration studies that were to follow. In addition, subsequent experiments suggest that potential branched-chain AA imbalances in our RBC-based diet are not a confounding factor in its use for Ile requirement assays (Parr, 2003).
Due to the aberrant gain results for the second treatment in Exp. 2 (Table 3), and the fact that weight gain between the first two treatments was not different, the gain results for these two treatments were evaluated together. Subsequently, a single-df contrast showed a significant difference between the first two treatment diets and the final three diets, and resulted in the growth response expected across all diets. Gain and G:F both seemed to plateau around 0.31% true digestible Ile. When barrows and gilts were evaluated separately (data not shown), gilts ate less and gained more slowly than did barrows. The barrows displayed growth responses that appeared to plateau around 0.31% true digestible Ile, with erratic feed efficiency data, whereas the gilts appeared to continue to gain and show an improvement in feed efficiency up to 0.33% true digestible Ile. However, because the gender x treatment interaction was not significant, the data for both genders were combined.
The PUN results for gilts from Exp. 3 supported the growth data from Exp. 2 (Table 4). Ideally, PUN concentration should decrease in a linear fashion until the requirement is met. The data reflect this linear decrease, but no apparent plateau was reached, even at the highest inclusion level (0.30% true digestible Ile). The results, therefore, suggest that the true digestible Ile requirement for high-lean-gain, late-finishing gilts is not less than 0.30%, and may be higher. The PUN data obtained in barrows were not definitive in that the PUN decrease with increasing Ile was not significant. This suggests that either the true digestible Ile requirement for late-finishing barrows was below our lowest dietary level (0.22% Ile), however unlikely, or that there were some other peculiar and unexplained problems with the barrows used in Exp. 3. Therefore, our best estimate of the true digestible Ile requirement of late-finishing, high-lean-gain barrows is based on the growth study (Exp. 2), and is in agreement with the NRC (1998) requirement estimate.
The Ile assay diet in these studies (Table 1) contained an estimated 3,475 kcal of ME/kg. Using this value, together with the 0.31% estimated requirement value, the true digestible Ile requirement for Exp. 2 herein was calculated to be 0.89 g/Mcal of ME for all animals in the growth assay. Although the gender x treatment interaction was not significant in this trial, the results of the PUN trial lead us to believe that there may be a requirement difference between high-lean-gain barrows and gilts in the late-finishing stage of growth. If this is true, then based on the PUN data for high-lean-gain gilts in the late-finishing stage, the true digestible Ile requirement calculates to be no less than 0.89 g/Mcal of ME. Although NRC (1998) based their estimates of the true digestible Ile requirement on a diet containing 3,265 kcal of ME/kg, our data are in general agreement with their factorial estimate of 0.83 to 1.01 g/Mcal ME for late-finishing barrows and gilts having a lean gain potential of 325 to 350 g/d.
Implications
The true digestible isoleucine requirement of rapidly growing, high-lean-gain pigs from 87 to 120 kg as determined in our studies agrees with the NRC (1998) factorial estimate of 0.89 g/Mcal for barrows and gilts. Plasma urea nitrogen results suggested that the isoleucine requirement for late-finishing gilts might be higher than that for late-finishing barrows. Further research needs to be carried out to examine not only the possible gender difference in requirements, but also to determine how to remove the variability in data obtained from finishing pig experiments.
Footnotes
1 Mention of a trade name, proprietary product, or specific equipment does not constitute a guarantee or warranty by the USDA and does not imply approval to the exclusion of other products that may be suitable. 
2 Correspondence: USDA-ARS-MWA-SOMMRU, National Swine Research and Information Center, NSRIC-2167 (phone: 515-294-0224; fax: 515-294-1209; e-mail: kerr{at}nsric.ars.usda.gov).
Received for publication February 11, 2003. Accepted for publication December 22, 2003.
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Becker, D. E., I. D. Smith, S. W. Terrill, A. H. Jensen, and H. W. Norton. 1963. Isoleucine need of swine at two stages of development. J. Anim. Sci. 22:1093–1096.
Brown, H. W., B. G. Harmon, and A. H. Jensen. 1974. Total sulfur-containing amino acids, isoleucine and tryptophan requirements of the finishing pig for maximum nitrogen retention. J. Anim. Sci. 38:59–63.
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Liu, H., G. L. Allee, E. P. Berg, K. J. Touchette, J. D. Spencer, and J. W. Frank. 2000a. Amino acid fortified corn diets for late-finishing barrows. J. Anim. Sci. 78(Suppl. 2):45. (Abstr.)
Liu, H., G. L. Allee, K. J. Touchette, J. W. Frank, and J. D. Spencer. 2000b. Effect of reducing protein and adding amino acids on performance, carcass characteristics and nitrogen excretion, and the valine requirement of early-weaned finishing barrows. J. Anim. Sci. 78(Suppl. 2):45. (Abstr.)
NRC. 1998. Pages 3–15 in Nutrient Requirements of Swine. 10th rev. ed. Natl. Acad. Press, Washington, DC.
Parr, T. M. 2003. Branched-chain amino acid nutrition of grow-finish pigs and young chicks. Ph.D. Thesis, Univ. of Illinois, Urbana.
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