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Isoleucine requirement of 80- to 120-kilogram barrows fed corn-soybean meal or corn-blood cell diets^1,2,3

D. W. Dean^*, L. L. Southern^*,4, B. J. Kerr and T. D. Bidner^*

^* Department of Animal Sciences, Louisiana State University Agricultural Center, Baton Rouge 70803-4210 and and USDA-ARS-SOMMRU, National Swine Research and Information Center, Ames, IA 50011

	Abstract

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Six experiments were conducted to validate an Ile-deficient diet and determine the Ile requirement of 80- to 120-kg barrows. Experiment 1 had five replications, and Exp. 2 through 6 had four replications per treatment; all pen replicates had four crossbred barrows each (initial BW were 93, 83, 85, 81, 81, and 88 kg, respectively). All dietary additions were on an as-fed basis. In Exp. 1, pigs were fed a corn-soybean meal diet (C-SBM) or a corn-5% blood cell (BC) diet with or without 0.26% supplemental Ile (C-BC or C-BC+Ile) in a 28-d growth assay. On d 14, pigs receiving the C-BC diet were taken off experiment as a result of a severe decrease in ADFI. Growth performance did not differ for pigs fed C-SBM or C-BC + Ile (P = 0.36) over the 28-d experiment. In Exp. 2, pigs were fed the C-BC diet containing 0.24, 0.26, 0.28, 0.30, or 0.32% true ileal digestible (TD) Ile for 7 d in an attempt to estimate the Ile requirement using plasma urea N (PUN) as the response variable. Because of incremental increases in ADFI as TD Ile increased, PUN could not be used to estimate the Ile requirement. In Exp. 3, pigs were fed the C-BC diet containing 0.28, 0.30, 0.32, 0.34, or 0.36% TD Ile. Daily gain, ADFI, and G:F increased linearly (P < 0.01) as Ile increased in the diet. Even though there were no effects of TD Ile concentration on 10th rib fat depth or LM area, kilograms of lean increased linearly (P < 0.01) as TD Ile level increased. In Exp. 4, pigs were fed a C-SBM diet containing 0.26, 0.31, or 0.36% TD Ile. There were no differences in ADFI or ADG; however, G:F increased linearly (P = 0.02), with the response primarily attributable to the 0.31% Ile diet. In Exp. 5, pigs were fed 0.24, 0.27, 0.30, 0.33, or 0.36% TD Ile in a C-SBM diet. There were no differences in growth performance; however, average backfat, total fat, and percentage of fat increased quadratically (P < 0.10) with the addition of Ile. In Exp. 6, pigs were fed a 0.26% TD Ile C-SBM diet with or without crystalline Leu and Val to simulate the branched-chain AA balance of a C-BC diet. There were no differences in ADFI or ADG, but G:F increased (P = 0.09) when Leu and Val were added. In summary, the Ile deficiency of a C-BC diet can be corrected by the addition of Ile, and because ADFI was affected by Ile addition, the PUN method was not suitable for assessing the Ile requirement. The TD Ile requirement for 80- to 120-kg barrows for maximizing growth performance and kilograms of lean is not < 0.34% in a C-BC diet, but may be as low as 0.24% in a C-SBM diet.

Key Words: Carcass • Isoleucine • Pig • Plasma Urea Nitrogen

	Introduction

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Crystalline Lys, Thr, and Trp have become economical for supplementation into many practical swine diets. To optimize these dietary AA additions, there is a need for an accurate estimation of the fourth-limiting AA, which is Ile according to NRC (1998) in corn-soybean meal (C-SBM) diets fed to late-finishing pigs. The NRC (1998) statement is supported by the results of Liu et al. (2000), who reported that a corn diet supplemented with Lys, Thr, Trp, and Met was deficient in Ile. The NRC (1998) requirement for Ile is based on ideal AA ratios (Wang and Fuller, 1989; Chung and Baker, 1992; Baker, 1997), but there are few empirical data in the literature estimating the actual true ileal digestible (TD) Ile requirement for late-finishing pigs. Early research did not consider the AA digestibility of ingredients, and research was based on small numbers of pigs over a wide weight range (Becker et al., 1963; Brown et al., 1974). More recent reports are inconsistent. Parr et al. (2004) concluded that the TD Ile requirement for 87- to 100-kg barrows was near the NRC (1998) estimate of 0.29% TD Ile for high-lean barrows. In contrast, Kendall et al. (2004) suggested that the TD Ile requirement of 90-kg barrows was 0.36%.

The use of alternative protein sources such as red blood cells, which are deficient in Ile, also created a need to know precisely the Ile requirement and to evaluate the interrelationships of the branched-chain AA (BCAA). Recently, Kerr et al. (2004b) in nursery pigs and Parr et al. (2003, 2004) in growing and finishing pigs suggested that a corn-based diet containing 5% red blood cells was deficient in Ile, and that the deficiency could be corrected by the addition of crystalline Ile.

We conducted six experiments to validate an Ile-deficient diet; to determine the TD Ile requirement for high-lean 80- to 120-kg barrows using growth, carcass traits, and minimal plasma area N (PUN) as response criteria; and to compare the BCAA relationship of a C-SBM diet to a corn-blood cell (C-BC) diet.

	Materials and Methods

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General

All methods used in these experiments related to animal care were approved by the Louisiana State University (LSU) Agricultural Center Animal Care and Use Committee. Yorkshire, Yorkshire x Landrace, or York-shire x Landrace x Duroc pigs from the LSU Agricultural Center were used in each experiment. Pigs were housed in a curtain-sided building with 1.5-m x 3.0-m pens and concrete slotted floors. Pigs were allotted to dietary treatment on the basis of BW and ancestry in randomized complete block designs. Treatments were replicated five times in Exp. 1 and four times in Exp. 2 through 6; all experiments had four barrows per replicate pen. Diets (Table 1) were formulated on an as-fed basis to meet or exceed the nutrient requirements (with the exception of Ile) of barrows gaining 350 g of lean/d (NRC, 1998) and formulated to contain 0.60% Ca and 0.50% P. Amino acid, mineral, and ME values for corn and SBM were based on NRC (1998). Blood cells (AP 301G; American Protein Corp., St. Louis, MO) were analyzed for total AA content (Table 2), and TD coefficients of 97.1, 95.4, 95.9, and 96.3% were used for Ile, Leu, Val, and Lys, respectively (Parr et al., 2003). Amino acid composition of the blood cells and basal diets was determined after acid hydrolysis (Method 982.30 E[a]; AOAC, 2000). Total sulfur AA content was determined after performic acid oxidation followed by acid hydrolysis (Method 982.30 E[b]; AOAC, 2000). Tryptophan content was determined after alkaline hydrolysis (Method 982.30 E[c]; AOAC, 2000). Treatment diets in mash form and water were provided ad libitum throughout all experiments. Amino acid additions were made on an as-fed basis.

Experiment 1 was conducted to validate a C-BC basal diet to be used subsequently in the Ile requirement studies. Sixty barrows with average initial and final BW of 93.3 ± 0.2 and 113.9 ± 2.9 kg were used. The three dietary treatments (Table 1) were 1) C-SBM; 2) C-BC diet containing 5% blood cells; and 3) C-BC with 0.26% supplemental Ile (C-BC + Ile).

At the beginning and end of the 28-d growth experiment, 10th rib backfat thickness and LM area were determined by ultrasound (Aloca 500 [12.5-cm and 3.5-MHz probe]; Coremetrics Medical Systems, Wallingford, CT). On d 14, pigs receiving the C-BC diet were removed from the experiment because of a decrease in ADFI.

Experiment 2

Experiment 2 was conducted to estimate the Ile requirement using PUN as the response variable (Knowles et al., 1997). Eighty barrows with average initial and final BW of 82.5 ± 0.8 and 84.3 ± 1.4 kg were used. At the start of the experiment, all pigs received the same late-finishing diet for 3 d and then were bled at 0900 the next day. They were then weighed and allotted to treatment. The C-BC diet containing 0.24% TD Ile (Table 1) was supplemented with increments of 0.02% crystalline L-Ile to provide treatment diets containing 0.24, 0.26, 0.28, 0.30, or 0.32% TD Ile. Pigs remained on treatment diets for 7 d and were bled and weighed at 0900 on d 10.

Blood was collected via the anterior vena cava and placed in 4-mL tubes (Monoject; Sherwood Medical, St. Louis, MO) containing 10.0 mg of sodium fluoride and 8.0 mg of potassium oxalate. Samples were placed on ice before centrifugation at 1,500 x g at 4°C for 20 min. Plasma was collected after centrifugation, and samples were frozen until analysis for PUN by the methods of Laborde et al. (1995).

Experiment 3

Experiment 3 was conducted to estimate the TD Ile requirement using growth performance and carcass traits. Eighty barrows with average initial and final BW of 85.3 ± 0.2 and 117.8 ± 3.6 kg were used. Treatments were the C-BC diet containing 0.28% TD Ile (Table 1) supplemented with increments of 0.02% crystalline L-Ile to provide treatment diets containing 0.28, 0.30, 0.32, 0.34, or 0.36% TD Ile. The growth trial lasted for 32 or 60 d, depending on slaughter date.

Three pigs per pen from the two heaviest replications and three pigs per pen from the two remaining replications were selected randomly and killed by exsanguination after electrical stunning (on d 32 and d 60, respectively) at the LSU Agricultural Center Meats Laboratory. Linear carcass measurements and values from total body electrical conductivity (TOBEC; Model MQI-27; Meat Quality Inc., Springfield, IL) were determined as described by Matthews et al. (2001). Fat-free lean and percent fat-free lean were determined by NPPC (2000) equations, which compensate for unequal BW.

Experiment 4

Experiment 4 was conducted to determine whether a C-SBM diet, seemingly deficient in Ile, would respond to dietary supplementation of Ile. Forty-eight barrows with average initial and final BW of 81.0 ± 0.2 and 117.3 ± 3.0 kg were used. Pigs were fed a C-SBM diet containing 0.26% TD Ile (Table 1) supplemented with increments of 0.05% crystalline L-Ile to provide treatments of 0.26, 0.31, or 0.36% TD Ile. The growth trial lasted for 47 d. Pigs and feeders were weighed on d 29 and 47 for calculation of ADFI, ADG, and G:F.

Experiment 5

Experiment 5 was conducted to determine the TD Ile requirement in pigs fed a C-SBM diet. Eighty barrows with average initial and final BW of 80.7 ± 0.2 and 113.0 ± 2.6 kg were used. The basal diet was a C-SBM diet containing 0.24 TD Ile (Table 1), which was subsequently supplemented with increments of 0.03% crystalline L-Ile to provide treatment diets containing 0.24, 0.27, 0.30, 0.33, or 0.36% TD Ile. The growth trial lasted 42 or 49 d, depending on slaughter date. Carcass data were collected as in Exp. 3. Pigs were bled at 0900 via the anterior vena cava on d 42. Handling of the blood and analysis of PUN were the same as in Exp. 2.

Experiment 6

Experiment 6 was conducted to determine the effect of Val + Leu addition to a C-SBM diet. Thirty-two barrows with average initial and final BW of 88.3 ± 0.4 and 113.5 ± 2.8 kg were used. Pigs were fed a C-SBM diet containing 0.26% TD Ile (Table 1) with or without 0.45% crystalline Leu and 0.31% crystalline Val. The addition of Leu + Val resulted in an Ile:Leu + Val that was the same as in the C-BC diet containing 0.28% TD Ile. The growth trial lasted for 38 d.

Statistical Analyses

Data were analyzed by ANOVA procedures using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC) in a randomized complete block design. The statistical model included treatment and replication for all six experiments. Initial PUN was used as a covariate in the analysis of d-10 PUN for Exp. 2. In Exp. 2, 3, 4, and 5, orthogonal contrasts were used to determine linear and quadratic effects of TD Ile. The two-slope NLIN procedure of SAS (break-point analysis) was used to estimate a requirement based on G:F in Exp. 3. Treatment differences were considered significant at = 0.10. The pen of pigs was the experimental unit for all data.

	Results

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In the diet validation experiment, Exp. 1, pigs receiving the C-BC diet were removed from the experiment on d 14 because of a severe decrease in ADFI, ADG, and G:F (P < 0.001; Table 2). Gain:feed was increased (P = 0.10) in pigs fed C-BC + Ile at d 14 compared with pigs fed either the C-SBM or C-BC diets. Feed intake, ADG, G:F, 10th rib backfat, and LM area were not different for pigs fed C-SBM or C-BC + Ile for the overall 28-d period.

In Exp. 2 (Table 3), ADFI, ADG, (P = 0.02), and G:F (P = 0.08) were linearly increased as crystalline TD Ile was increased incrementally from 0.24 to 0.32%. Plasma urea N was not affected by Ile level.

	Discussion

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The use of PUN as a response variable has been shown to be a quick and effective means of evaluating an AA requirement for pigs at various stages of growth (Knowles et al., 1997; Guzik et al., 2002). The objective of Exp. 2 was to estimate the Ile requirement at the beginning of the 80- to 120- kg growth period by using PUN as the response criteria. However, a linear increase in feed intake led to similar PUN values among pigs fed TD Ile levels from 0.24 to 0.32%. Parr et al. (2004) used feed intake as a covariate in the analysis of PUN and reported results that were inconclusive. Consequently, we believe that PUN is most likely not a good indicator of an AA requirement when feed intakes are different among treatments. Although pigs only received the treatment diets for 7 d in Exp. 2, the magnitude of change in feed intake as Ile increased seemed significant in evaluation of the requirement and led us to increase our range of Ile levels for Exp. 3.

The failure of a requirement estimate from PUN led us to conduct an experiment evaluating the TD Ile requirement of finishing barrows using growth and carcass measurements as response criteria. Again, we observed linear effects of TD Ile on ADFI, and subsequently, ADG and G:F increased as TD Ile increased from 0.28 to 0.36%. Objective requirement estimates based on G:F using two-slope break-point analysis resulted in a requirement of 0.34% TD Ile; however, numerically, ADFI and ADG were greatest for pigs fed the 0.36% TD Ile diet. Our estimate of 0.34% TD Ile is greater than the 0.29% estimate of NRC (1998) and Parr et al. (2004), but it agrees closely with the suggestion of Kendall et al. (2004) that the TD Ile requirement is approximately 0.36%. The carcass responses of an increase in carcass length and kilograms of lean in Exp. 3 are likely the result of an increase in final BW caused by increased feed intake and growth. There were no significant differences among measurements of carcass composition when lean and fat were expressed as a percentage of carcass weight.

Our greater-than-expected requirement estimate of 0.34% TD Ile in a C-BC diet suggested that a C-SBM diet formulated with crystalline sources of the first three limiting AA (Lys, Thr, and Trp) should respond to additions of Ile. Experiment 4 was conducted to compare the response of Ile in a C-SBM diet with the response previously observed in a C-BC diet. Gain:feed increased when TD Ile increased from 0.26 to 0.31%, with no further improvement to the 0.36% level. Previous experiments with the C-BC diet indicated we would expect to observe a decrease in ADFI in a diet containing 0.26% TD Ile. Several researchers have observed feed intake responses to dietary supplementation of Ile (Parr et al., 2003; Kendall et al., 2004; Kerr et al., 2004a); however, in Exp. 4, ADFI was numerically greatest with the C-SBM diet that contained 0.26% TD Ile. All pigs had high ADFI (>3.3 kg). The data from this experiment suggested that the TD Ile requirement was between 0.26 and 0.31% in a C-SBM diet.

Experiment 5 was conducted to evaluate more thoroughly the TD Ile requirement of late-finishing barrows fed a C-SBM diet. Data from Exp. 4 indicated that G:F was improved when TD Ile increased from 0.26 to 0.31%. In Exp. 5, there were no significant differences in growth performance when levels of TD Ile ranged from 0.24 to 0.36%. A quadratic response of average backfat and TOBEC estimates of total fat and percent fat seem to be related to differences in the final BW of pigs within level of Ile supplementation. Measures of lean did not differ because of Ile supplementation. The results of this experiment do not agree with the results of Exp. 4, which suggested a TD Ile requirement >0.26% TD Ile for maximizing G:F in a C-SBM diet. The results of this experiment also do not agree with the results of Exp. 3, which suggested a TD Ile requirement of at least 0.34% in pigs fed a C-BC diet.

A review of the literature on the Ile requirement of pigs (Table 10) resulted in eight requirement estimates from peer-reviewed journal articles (Becker et al., 1963; Oestemer et al., 1973; Taylor et al., 1985; Parr et al., 2003; Kerr et al., 2004a; Parr et al., 2004), four estimates from experiment station reports (Bergstrom et al., 1996; James et al., 2001), and two estimates from abstracts (Lenis and van Diepen, 1997; Kendall et al. 2004). Researchers used various techniques to estimate plateaus and break points in these studies. Thus, to evaluate data from these experiments more consistently, break-point analysis was performed on treatment means to estimate requirements when the authors used some other statistical method. Because many of these experiments were based on total Ile rather than on TD, a digestibility coefficient of 88% was used to calculate a TD Ile value when none was reported or when it was impossible to calculate the value from dietary ingredients. Growth performance values of pigs at or above the estimated requirement were used to calculate Ile intake and the milligrams of Ile required for 1 g of BW gain. The average estimate from Table 10 was 8.97 mg of Ile/g of BW gain. Kerr et al. (2004a) recently reported a similar review of the Ile requirement using some of the same data and had a slightly lower estimate of 8.69 mg of Ile/g of BW gain.

Our relatively high estimate of the TD Ile requirement of late-finishing barrows when using the C-BC diet may be a result of the BCAA balance. Red blood cells are extremely high in Leu (12.55%) and Val (8.25%), but comparatively low in Ile (0.26%). Numerous researchers have suggested an antagonism among the BCAA (Tannous et al., 1966; Allen and Baker, 1972; Edmonds and Baker, 1987). Papet et al. (1988) observed that excess Leu increases the activities of branched-chain aminotransferase in the liver and jejunum and the branched-chain keto-acid deaminase in the jejunum in preruminant lambs. Other researchers have suggested that an increased metabolic oxidation of Ile might occur when dietary levels of Leu are high (Tannous et al., 1966; Calvert et al., 1982; Edmonds and Baker, 1987). Edmonds and Baker (1987) demonstrated that plasma AA concentrations of Ile and Val were decreased when Leu was supplied in excess of 1 to 6%. It may be that the use of a C-BC diet overestimates the Ile requirement because of these metabolic conditions.

The calculated composition of our 0.24% TD Ile C-BC diet contained 0.57% more TD Leu than the 0.24% TD Ile C-SBM diet used in Exp. 4. This resulted in Leu:Ile of 5.58 and 3.51 for the 0.26% TD Ile C-BC and C-SBM diets (Table 11), respectively. Barbour and Latshaw (1992) reported no increase in the Ile requirement of broiler chicks when Leu and Val were increased in the diet; however, their proposed imbalanced diet had a Leu:Ile of 3.6, which is only slightly greater than in the C-SBM diets we used in Exp. 4 and 5. The ratio of Leu to Ile in our 0.36% TD Ile C-BC diet was 4.03, which approached the range calculated for our C-SBM diets. The Leu:Ile where performance begins to become impaired and the requirement for Ile increases in the pig is unknown, and it may vary depending on pig weight, genotype, and environment. Thus, it is difficult to determine whether our C-BC diet was responding to a deficiency of Ile or responding to a correction of the BCAA balance.

In summary, the TD Ile requirement for 80- to 120-kg barrows for maximizing feed intake, growth, feed efficiency, and kilograms of lean is not <0.34% or 12.90 mg of Ile/g of BW gain in a corn diet containing 5% blood cells. In contrast, the requirement may not be >0.24% in finishing barrows fed a C-SBM diet. Neither of these estimates agrees with the NRC (1988) suggestion of 0.29%. In addition, the PUN method is not suitable for assessing the Ile requirement of pigs because of tremendous changes in ADFI. Our data suggest that Ile is not a limiting nutrient in formulation of low-CP C-SBM diets for late-finishing barrows. Further research is needed to evaluate the optimal BCAA balance and the maximum level of red blood cells that can be fed to pigs.

	Footnotes

 
¹ Approved for publication by the director of the Louisiana Agric. Exp. Stn. as Manuscript No. 05-18-0111.

² The authors are thankful to R. L. Payne, J. L. Shelton, and the Louisiana State Univ. Agric. Center Swine Unit for assistance with data collection and analyses.

³ 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. Appreciation is given to BioKyowa, Inc. for partial funding of this research.

⁴ Correspondence—(phone: 225-578-3449; fax: 225-578-3604; e-mail: lsouthern{at}agctr.lsu.edu)

Received for publication March 1, 2005. Accepted for publication July 6, 2005.

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