J. Anim. Sci. 2004. 82:2333-2342
© 2004 American Society of Animal Science
Isoleucine requirements and ratios in starting (7 to 11 kg) pigs1
B. J. Kerr*,2,
M. T. Kidd
,
J. A. Cuaron
,
K. L. Bryant
,
T. M. Parr¶,
C. V. Maxwell# and
J. M. Campbell**
* USDA-ARS Swine Odor and Manure Management Research, Ames, IA 50011-3310;
and
Mississippi State University, Mississippi State 39762-9665;
and
Centro Nacional de Investigacion en Fisiologia y Mejoramiento Animal, INIFAP, Queretaro, Mexico, 76020;
and
Akey, Lewisburg, OH 45338;
and
¶ University of Illinois, Urbana 61801;
and
# University of Arkansas, Fayetteville 72701; and
and
** APC, Inc., Ankeny, IA 50021
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Abstract
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Two experiments were conducted to refine the Ile needs in 7- to 11-kg pigs. In Exp. 1, 1,680 pigs were fed a 1.25% digestible Lys diet containing 7.5% spray-dried blood cells (as-fed basis) with supplemental crystalline Ile (0.06% increments) to generate seven levels of apparent digestible Ile (0.47 to 0.83%). There were 12 replicates of each treatment with 20 pigs per pen, and treatments were imposed at an initial BW of 7 kg and continued for 16 d. Responses in ADG, ADFI, G:F, and plasma urea nitrogen (PUN) were quadratic (P < 0.01) over the 16-d period. Data were fitted to both a single-slope broken line and a quadratic fit, and when the quadratic response curve was superimposed on the broken line, the points at which the quadratic curve first intersected the plateau of the broken line occurred at 0.70, 0.73, 0.66, and 0.65% digestible Ile for ADG, ADFI, G:F, and PUN, respectively. Using the ADG and ADFI obtained at this intersection point resulted in an estimate of 9.1 mg of digestible Ile per gram of weight gain. In Exp. 2, 1,840 pigs were fed similarly composed diets, except that digestible Lys was lowered in six diets to 1.10% by decreasing soybean meal. Crystalline Ile was supplemented at 0.09% increments to generate six levels of digestible Ile (0.37 to 0.83%). A seventh diet contained 1.25% digestible Lys by supplementing the 0.83% digestible Ile diet with 0.19% L-Lys•HCl to verify that 1.10% digestible Lys was deficient for these pigs. There were 12 replicates of each treatment with 22 pigs per pen, and treatments imposed at an initial BW of 7 kg and continued for 16 d. Supplementation of Lys to the 0.83% digestible Ile diet (1.10 vs. 1.25% digestible Lys) did not affect ADG (260 vs. 264 g/d, P = 0.60) and ADFI (359 vs. 343 g/d, P = 0.20), whereas G:F (725 vs. 774 g/kg, P < 0.01) was improved by increasing dietary Lys. Responses in ADG, ADFI, and G:F to the first six diets were quadratic (P < 0.01) over the 16-d period. The points at which the quadratic curve first intersected the plateau of the broken line occurred at 0.686, 0.638, and 0.684% digestible Ile for ADG, ADFI, and G:F, respectively. Using the ADG and ADFI obtained at this intersection point results in an estimate of 9.9 mg of digestible Ile per gram of weight gain. These results suggest that although the percent digestible Ile requirement and digestible Ile:Lys ratio for starter (7 to 11 kg) pigs may be higher than 1998 NRC recommendations, the requirement may be lower than current recommendations when taking gain and feed intake into account.
Key Words: Blood Cells • Isoleucine • Starting Pigs
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Introduction
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Research evaluating Ile requirements of pigs is dated, sparse, and difficult to interpret (Table 1
). Mitchell et al. (1968a,b) utilized a casein-based diet and Brown et al. (1974) used a gelatin-based diet to estimate the Ile requirement using nitrogen retention or plasma Ile, but neither reported performance data. In papers reporting pig performance, semipurified diets containing large amounts of cornstarch with either blood flour (Brinegar et al., 1950; Becker et al., 1957, 1963; Bravo et al., 1970; Oestemer et al., 1973) or herring meal (Henry et al., 1976) have been utilized, with only four papers utilizing more practical ingredients (Taylor et al., 1985; Bergstrom et al., 1997; Lenis and van Diepen, 1997; James et al., 2001).
Recently, we developed an Ile-deficient diet using spray-dried blood cells (SDBC) in growing-finishing pigs (Kerr et al., 2002) that was shown to be an excellent model to evaluate the Ile requirements of growing-finishing pigs (Parr et al., 2003, 2004). Likewise, we evaluated SDBC in starting pigs (Kerr et al., 2004) and showed that up to 7.5% SDBC can be supplemented with no adverse effects on pig performance provided that the requirement for Ile is met. With the commercial availability of Lys, Trp, Thr, and Met, dietary CP levels can be dramatically decreased to lower the excretion of nitrogen into the environment. To lower dietary CP, however, a clear understanding of the Ile requirement or Ile:Lys ratio is essential to formulate to these minimal protein levels. In addition, utilization of alternative protein ingredients, largely blood products, becomes possible because their Ile limitation can be corrected by crystalline Ile supplementation.
Our objectives were to use a previously determined efficacious Ile-deficient diet and refine the Ile requirement and Ile:Lys ratio for starting (7 to 11 kg) pigs based on pig performance and plasma urea nitrogen (PUN).
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Experimental Procedures
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All experimental procedures followed the Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching (FASS, 1999).
Experiment 1
Sixteen hundred and eighty pigs (Line 326 sire x C22 dams; Pig Improvement Co., Franklin, KY) at an industry research location were used to define the apparent digestible (dig) Ile requirement for 7- to 11-kg pigs through the use of SDBC in a corn-soybean meal-whey-fish meal-based diet. Pigs were weaned at approximately 17 d of age and fed a common Phase I diet containing 1.6% total Lys for 8 d. Pens were 1.65 m x 3.0 m and had floors that were 100% plastic slats. Feed was provided with a feeder with 0.76 m of trough space, and water was provided by two nipple waters. Pigs were allowed ad libitum access to feed and water. Pigs were offered the experimental diets for 16 d, with individual pig weights and feed disappearance (as-fed basis) recorded at the end of 16 d to calculate ADG, ADFI, and G:F.
Diets (Table 2) were formulated on an apparent digestible AA basis using previously analyzed CP and AA values of corn, soybean meal, whey, fish meal, and SDBC (Kerr et al., 2004), apparent AA digestibility estimates (Southern, 1991; personal communication for SDBC, APC, Inc.), and balanced relative to dig Lys, with Trp:Lys, Thr:Lys, and total sulfur amino acids (TSAA):Lys ratios of 0.19, 0.72, and 0.66, respectively. These values are higher than ratios according to Baker (1997) but were used to ensure no deficiencies, owing to the lack of peer-reviewed digestibility estimates for SDBC. Crystalline Ile replaced cornstarch in the diets containing SDBC and was supplemented to achieve dig Ile levels ranging from 0.47 to 0.83%. Diets were also formulated to contain (as-fed basis) 0.92% Ca, 0.57% available P, 6.8% lactose, and 3,340 kcal ME/kg. Dietary treatments were pelleted and imposed for 16 d with an average initial BW of 6.6 kg. Nitrogen analysis of ingredients and mixed diets were done by the macro-Kjeldahl procedure (AOAC, 1995) and CP was calculated (N x 6.25). Amino acid concentrations of ingredients and mixed diets were determined following acid hydrolysis, Trp concentrations following alkaline hydrolysis, and Met and Cys following performic acid oxidation (AOAC, 1995; 982.30 E[abc], Chp. 45.3.05) using a high-performance cation exchange resin column (Beckman Systems Inc., Fullerton, CA). Amino acid concentrations were not corrected for incomplete recovery resulting from hydrolysis.
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Table 2. Ingredients (as-fed basis) and chemical composition of isoleucine-deficient diets for starting (7 to 11 kg) pigs, Exp. 1
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Following weighing on d 16, two pigs from each pen were randomly selected and bled once via jugular venipuncture. The heparinized blood samples were stored on ice until blood collection from all pigs was complete. Samples were 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 utilized in determining 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).
There were 12 replicates of each treatment in a randomized complete block design. Blocks were formed on the basis of gender and weight. Assignment to treatment was done randomly from within blocks, with 20 pigs per pen with six replicates of gilts and six replicates of barrows. Data for each response criterion were analyzed by ANOVA using the GLM procedure of SAS (2001) with barn, replicate, gender, and dietary treatment included in the model. Estimates of requirements for performance and blood measurements were estimated by subjecting the pen means data to least squares broken-line methodology (Robbins et al., 1979). As described in our previous experiment (Parr et al., 2003), a second objective requirement estimate from the quadratic model was determined by establishing the first point where the quadratic line intersected the plateau of the broken line. The pen of pigs was used as the experimental unit for all data.
Experiment 2
Eighteen hundred and forty-eight pigs (Line 326 sire x C22 dams; Pig Improvement Co.) at the same industry research location were used to define the dig Ile:Lys ratio for 7- to 11-kg pigs using a diet similar to the one described above. In an effort to make the diet limiting in Lys such that Ile:Lys ratios could be assessed, dig Lys was decreased to 1.10% (88% of Exp. 1) by decreasing the level of soybean meal inclusion. This level was picked after consultation with the industry location indicated that the dig Lys requirement for 6- to 12-kg pigs at that location was approximately 1.25%. Diets (Table 3) were formulated on an apparent digestible AA basis, as described in Exp. 1, with the concentration of Thr, Trp, and TSAA kept at the same level as in Exp. 1. Consequently, the resultant ratios of Thr:Lys, Trp:Lys and TSAA:Lys were much higher than suggested (Baker, 1997), but when Lys was added to the final diet, no additional dietary adjustments would need to be made. Crystalline Ile replaced cornstarch to achieve dig Ile levels ranging from 0.37 to 0.83%, or dig Ile:Lys ratios ranging from 0.34 to 0.75. A seventh diet was similar to the highest Ile diet, except that 0.19% L-Lys•HCl was added in the place of cornstarch to increase dig Lys to 1.25%. This diet was designed to verify that 1.10% dig Lys was Lys-deficient so Ile:Lys ratios could be assessed. Diets were also formulated to contain (as-fed basis) 0.85% Ca, 0.56% available P, 6.8% lactose, and 3,370 kcal ME/kg. Dietary treatments were imposed for 16 d with an average initial BW of 6.6 kg.
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Table 3. Ingredients (as-fed basis) and chemical composition of isoleucine-deficient diets for starting (7 to 11 kg) pigs, Exp. 2
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Pigs were housed, managed, replicated, and allotted as described in Exp. 1, except that there were 22 pigs per pen. Data were analyzed as described previously, except that the PDIFF option of SAS was used to separate treatment means for any gender x treatment interaction, and an orthogonal contrast was used to verify that the seventh diet containing 1.25% dig Lys elicited a performance response compared with the diet containing 1.10% dig Lys when each diet contained 0.83% dig Ile. Subsequently, performance response curves were estimated by subjecting the pen means data for pigs fed the first six diets to least squares broken-line methodology and a quadratic model as described above.
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Results
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Experiment 1
There were no interactions between gender and treatment for ADG (P = 0.43), ADFI (P = 0.60), G:F (P = 0.77), or PUN (P = 0.61), such that only the effects of dig Ile level are reported. Based on past research (Kerr et al., 2004), we fully expected, and obtained, clear performance responses to graded levels of Ile (Table 4). Average daily gain, ADFI, and G:F increased, and PUN decreased, as dig Ile levels increased (quadratic, P < 0.01). Using single-slope, broken-line methodology (Robbins et al., 1979), minimal break points of 0.658, 0.672, 0.616, and 0.606% dig Ile were determined (Figures 1 through 4). The pen means data from Exp. 1 were also fitted to quadratic regression equations with the levels of dig Ile that maximized ADG, ADFI, and G:F (i.e., upper asymptote) calculated to be 0.754, 0.770, and 0.729%, respectively. The level of dig Ile that minimized PUN was calculated to be 0.720%. In addition, we calculated the first intercept X-value of the broken line (on the plateau) and the quadratic fitted line (Figures 1 through 4). These intercepts were calculated to be 0.702, 0.729, 0.659, and 0.645% dig Ile for ADG, ADFI, G:F, and PUN, respectively.
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Table 4. Graded levels of isoleucine for starting pigs in 1.25% digestible Lys diets containing spray dried blood cells, Exp. 1 (as-fed basis)a
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Figure 1. Fitted broken-line and quadratic plot of daily weight gain as a function of digestible dietary Ile (Exp. 1) with observed treatment mean values (n = 12 observations per treatment mean). The minimal digestible Ile requirement determined by broken-line analysis using least squares methodology was 0.658% (Y plateau = 314.3; slope below breakpoint = –926.7; R2 = 0.985). The pen means data from Table 4 were also fitted to a quadratic regression equation: Y = –2,298.3(Ile2) + 3,468.1(Ile) – 987.7 (R2 = 0.965). The digestible Ile level that maximized weight gain (i.e., upper asymptote) was calculated to be 0.754% of the diet with the first intercept X-value of the broken line (on the plateau) and the quadratic fitted line occurring at 0.702% digestible Ile.
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Figure 4. Fitted broken-line and quadratic plot of plasma urea nitrogen as a function of digestible dietary Ile (Exp. 1) with observed treatment mean values (n = 12 observations per treatment mean). The minimal digestible Ile requirement determined by broken-line analysis using least squares methodology was 0.606% (Y plateau = 24.4; slope below break point = 40.0; R2 = 0.965). The pen means data from Table 4 were also fitted to a quadratic regression equation: Y = 93.59(Ile2) – 134.8(Ile) + 72.4 (R2 = 0.956). The digestible Ile level that minimized plasma urea nitrogen (i.e., upper asymptote) was calculated to be 0.720% of the diet with the first intercept X-value of the broken line (on the plateau) and the quadratic fitted line occurring at 0.645% digestible Ile.
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Experiment 2
Considering all seven treatments, there was an interaction between gender and treatment for G:F (P = 0.015), with G:F values of 327g, 499f, 604d, 707bc, 711bc, 730bc, and 808a for gilts and 309g, 540e, 574de, 710bc, 697c, 721b, and 740b for barrows (means with a differing superscript letters differ, P < 0.05). This interaction, however, seemed to be due to the gilts having a greater response to the diet containing the higher level of dietary Lys relative to castrates. Although unexpected for pigs of this weight range, it is common knowledge that gilts have higher Lys requirements than barrows (NRC, 1998). There were no interactions between gender and treatment for ADG (P = 0.78) or ADFI (P = 0.82). Using an orthogonal contrast between the two diets containing 0.83% dig Ile, comparison of 1.10% dig Lys to 1.25% dig Lys showed that ADG (260 vs. 264 g/d, P = 0.60) and ADFI (359 vs. 343 g/d, P = 0.20) were not affected by supplementation of 0.19% L-Lys•HCl. In contrast, G:F (725 vs. 774 g/kg, P = 0.001) was improved with the supplementation of crystalline Lys. For the dig Ile:Lys ratio comparison (the first six treatments), the interactions between gender and treatment were not significant for ADG (P = 66), ADFI (P = 0.87), or G:F (P = 0.11), so data for gilts and barrow data were combined.
Similar to Exp. 1, we obtained clear performance responses to graded levels of Ile (Table 5). Average daily gain, ADFI, and G:F increased as dietary Ile levels were increased (quadratic, P < 0.01). Using single-slope, broken-line methodology, minimal break points of 0.613, 0.571, and 0.617% dig Ile were determined (Figures 5 through 7). The pen means data from Exp. 2 were also fitted to quadratic regression equations with the levels of total Ile that maximized ADG, ADFI, and G:F (i.e., upper asymptote) calculated to be 0.763, 0.725, and 0.762% dig Ile, respectively. In addition, we calculated the first intercept X-value of the broken line (on the plateau) and the quadratic fitted line (Figures 5 through 7). These intercepts were calculated to be 0.686, 0.638, and 0.684% dig Ile for ADG, ADFI, and G:F, respectively.
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Table 5. Graded levels of isoleucine for starting pigs in 1.10% digestible (dig) Lys diets containing spray dried blood cells, Exp. 2 (as-fed basis)a
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Figure 5. Fitted broken-line and quadratic plot of daily weight gain as a function of digestible dietary Ile (Exp. 2) with observed treatment mean values (n = 12 observations per treatment mean). The minimal digestible Ile requirement determined by broken-line analysis using least squares methodology was 0.613% (Y plateau = 254.7; slope below break point = –861.1; R2 = 0.991). The pen means data from Table 5 were also fitted to a quadratic regression equation: Y = –1,422.7(Ile2) + 2,170.9(Ile) – 565.1 (R2 = 0.970). The digestible Ile level that maximized weight gain (i.e., upper asymptote) was calculated to be 0.763% of the diet with the first intercept X-value of the broken line (on the plateau) and the quadratic fitted line occurring at 0.686% digestible Ile.
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Figure 7. Fitted broken-line and quadratic plot of gain:feed as a function of digestible dietary Ile (Exp. 2) with observed treatment mean values (n = 12 observations per treatment mean). The minimal digestible Ile requirement determined by broken-line analysis using least squares methodology was 0.617% (Y plateau = 712.3; slope below break point = –1,505.6; R2 = 0.974). The pen means data from Table 5 were also fitted to a quadratic regression equation: Y = –2,586.3(Ile2) + 3,942.5(Ile) – 774.5 (R2 = 0.981). The digestible Ile level that maximized gain:feed (i.e., upper asymptote) was calculated to be 0.762% of the diet with the first intercept X-value of the broken line (on the plateau) and the quadratic fitted line occurring at 0.684% digestible Ile.
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Discussion
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There were minor discrepancies between calculated and analyzed concentrations of CP and amino acids, even though we took extreme care in feed preparation, sampling, and laboratory analysis. In our previous experiment, where the 7.5% SDBC diet was validated (Kerr et al., 2004), we did not see these discrepancies as calculated CP, Lys, and Ile levels were very close to analyzed values. Although the dried whey, fish meal, and SDBC were of the same batch in these experiments, different batches of corn and soybean meal were used and not reflected in the original AA analysis. In addition, AA analysis is variable (Albin et al., 2000; Fontaine and Eudaimon, 2000), especially for Trp (Sato et al., 1984).
Because our previous research (Kerr et al., 2004) validated the basal diet to be efficacious and adequate in all nutrients except for Ile, we believe that the requirements estimated in Exp. 1 reflect levels needed for today’s leaner, lower appetite pigs. Although previous research used Ile-deficient diets (Table 1), only a few (Taylor et al., 1985; Bergstrom et al., 1997; Lenis and van Diepen, 1997; James et al., 2001) had any semblance of a positive control. In addition to our basal diet being validated and using many ingredients typically fed to nursery pigs, the entire range of the growth curve (linear and plateau portions) was reflected in the dose levels of Ile, which provided us with an excellent response curve for all variables measured. In Exp. 1, the gain response of pigs to supplemental Ile was dramatic (approximately twofold) and closely reflects that noted in our validation study (Kerr et al., 2004). Others (Brinegar et al., 1950; Becker et al., 1957, 1963; Bravo et al., 1970; Oestemer et al., 1973; James et al., 2001) also reported a large response in ADG when crystalline Ile was supplemented to their Ile-deficient diet. We did not observe any decrease in ADG with Ile levels above the estimated requirement as noted by Henry et al. (1976). Effects of dietary Ile on ADFI, G:F, and PUN, although not as dramatic as that noted for ADG, still provided an excellent response range for regression analysis.
Table 6 summarizes the least squares break points, break point x quadratic curve intercept, and quadratic equation maximum for each of the variables evaluated in Exp. 1 and 2. Because the intercept of quadratic curve with the plateau value from the one-slope broken line has been suggested to be a more objective estimate of amino acid requirements (Parr et al., 2003), subsequent discussions will only focus on these values. There was good agreement with the growth data and PUN with intercepts calculated to be 0.702, 0.729, 0.659, and 0.645% dig Ile for ADG, ADFI, G:F, and PUN, respectively. On a percent basis, these values are substantially higher than the 0.60% dig Ile estimated by the 1998 National Research Council Subcommittee on Swine Nutrition. On an energy basis, our estimate is also higher, as the apparent dig Ile requirement from Exp. 1 is calculated to be 2.10 g/Mcal of ME, which compares with the NRC (1998) estimate of 1.78 g/Mcal of ME. We are less confident of this value, however, because the ME of our diet was from calculated values. In contrast, using the dig Ile level obtained at the break point x quadratic intersection point for ADG (0.702% dig Ile) and applying this dig Ile requirement estimate to the ADFI quadratic equation to estimate ADFI at 0.702% dig Ile resulted in an estimate of 9.1 mg of dig Ile per gram of weight gain, which is lower than the 10.6 mg of dig Ile per gram of weight gain that can be calculated from the NRC (1998).
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Table 6. Least squares broken line and quadratic equation summary of Ile requirement (Exp. 1) and Ile ratio (Exp. 2) assays
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The data from Exp. 2 are a bit more difficult to interpret. First, comparison of performance levels in Exp. 1 with those in Exp. 2 showed that pigs in Exp. 2 consumed less feed and grew slower. One could speculate that this is due to diets in Exp. 1 containing 6.75% soybean meal vs. 1.00% soybean meal in Exp. 2, but this effect was not evaluated. In Exp. 2, we elected to decrease dietary Lys by reducing the inclusion of soybean meal and not to change the inclusion level of SDBC, dried whey, and select fish meal. Second, the significant interaction between gender and treatment for G:F was seemingly due to gilts having a greater response to the higher level of dietary Lys relative to castrates. Although this response is expected for pigs of a higher BW, it would not be expected for pigs of this weight range as lean gain and gender inputs are not required in the current NRC (1998) model for nutritional requirements. It is essential to note, however, that there was no interaction between gender and the diets used to evaluate dig Ile:Lys ratios (the first six diets). Lastly, when the diet contained 0.83% dig Ile, comparison of 1.10% dig Lys with 1.25% dig Lys showed that ADG and ADFI were unaffected by Lys supplementation, whereas G:F was improved. Feeding what we estimated to be 88% of the Lys requirement of these pigs, we expected some depression in growth rate. Even though this did not occur, the reduction in G:F suggests that we were marginally deficient in Lys for this group of pigs, and consequently discussion of dig Ile:Lys ratios are valid, although caution must be taken because of only a minor G:F depression.
Similar to the results of Exp. 1, we obtained clear performance responses to graded levels of Ile. There was good agreement between the performance variables measured, with intercepts calculated to be 0.686, 0.638, and 0.684% dig Ile for ADG, ADFI, and G:F, respectively. On a percent basis, these values were also higher than the NRC (1998) recommendation, which supports our findings in Exp. 1. On an energy basis, the dig Ile requirement is calculated to be 2.03 g/Mcal of ME, which compares with 2.10 g/Mcal for Exp. 1 and 1.78 g/Mcal from the NRC (1998). As stated previously, we are not highly confident of this value because of the lack of confidence in ME values. Using the dig Ile level obtained at the break point x quadratic intersection point for ADG (0.686% dig Ile) and applying this dig Ile requirement estimate to the ADFI quadratic equation to estimate ADFI at 0.686% dig Ile resulted in an estimate of 9.9 mg of dig Ile per gram of weight gain. This is higher than the 9.1 mg of dig Ile per gram of weight gain estimated in Exp. 1, but lower than the 10.6 mg of dig Ile per gram of weight gain estimated from the NRC (1998). Our estimates of 9.1 (Exp. 1) and 9.9 (Exp. 2) mg of dig Ile per gram of weight gain are slightly higher than the average of 8.7 mg of dig Ile per gram of weight gain calculated from Table 1. Assuming that we accept that the 1.10% dig Lys was marginally deficient in Exp 2, the calculated dig Ile:Lys ratios are 0.62, 0.58, and 0.62 for ADG, ADFI, and G:F, respectively. The average of 0.61 is higher than the current NRC (1998) estimate of 0.55. Because our diet containing 7.5% SDBC has been shown to be efficacious (Kerr et al., 2004), our data suggest the average apparent dig Ile requirement is 0.68% and the apparent dig Ile:Lys ratio is 0.61, both of which are higher than current recommendations.
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Implications
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The digestible isoleucine requirement of 7- to 11-kg pigs, as determined empirically in our studies, was higher than the NRC (1998) factorial estimate. Based on an average of all performance variables and plasma urea nitrogen, our results suggest the apparent digestible isoleucine requirement is 0.68% and a digestible isoleucine:lysine ratio of 0.61. Thus, low-crude protein diets with supplementation of multiple amino acids, certain experimental diets, or diets containing blood products may require isoleucine supplementation.
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Figure 2. Fitted broken-line and quadratic plot of daily feed intake (as-fed basis) as a function of digestible dietary Ile (Exp. 1) with observed treatment mean values (n = 12 observations per treatment mean). The minimal digestible Ile requirement determined by broken-line analysis using least squares methodology was 0.672% (Y plateau = 411.7; slope below break point = –781.7; R2 = 0.970). The pen means data from Table 4 were also fitted to a quadratic regression equation: Y = –1,845.2(Ile2) + 2,840.5(Ile) – 678.4 (R2 = 0.941). The digestible Ile level that maximized feed intake (i.e., upper asymptote) was calculated to be 0.770% of the diet with the first intercept X-value of the broken line (on the plateau) and the quadratic fitted line occurring at 0.729% digestible Ile.
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Figure 3. Fitted broken-line and quadratic plot of gain:feed as a function of digestible dietary Ile (Exp. 1) with observed treatment mean values (n = 12 observations per treatment mean). The minimal digestible Ile requirement determined by broken-line analysis using least squares methodology was 0.616% (Y plateau = 765.8; slope below break point = –1,416.7; R2 = 0.996). The pen means data from Table 4 were also fitted to a quadratic regression equation: Y = –3,320.1(Ile2) + 4,838.8(Ile) – 981.2 (R2 = 0.984). The digestible Ile level that maximized gain:feed (i.e., upper asymptote) was calculated to be 0.729% of the diet with the first intercept X-value of the broken line (on the plateau) and the quadratic fitted line occurring at 0.659% digestible Ile.
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Figure 6. Fitted broken-line and quadratic plot of daily feed intake (as-fed basis) as a function of digestible dietary Ile (Exp. 2) with observed treatment mean values (n = 12 observations per treatment mean). The minimal digestible Ile requirement determined by broken-line analysis using least squares methodology was 0.571% (Y plateau = 358.0; slope below break point = (1,033.3; R2 = 0.962). The pen means data from Table 5 were also fitted to a quadratic regression equation: Y = –1,727.7(Ile2) + 2,504.9(Ile) – 536.8 (R2 = 0.920). The digestible Ile level that maximized feed intake (i.e., upper asymptote) was calculated to be 0.725% of the diet with the first intercept X value of the broken line (on the plateau) and the quadratic fitted line occurring at 0.638% digestible Ile.
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Footnotes
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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. Appreciation is given to APC Inc. and BioKyowa Inc. for funding of this research. 
2 Correspondence: USDA-ARS-MWA-SOMMRU, National Swine Research and Information Center, NSRIC-2167, Ames, IA 50011 (phone: 515-294-0224; fax: 515-294-1209; e-mail: kerr{at}nsric.ars.usda.gov).
Received for publication July 30, 2003.
Accepted for publication March 16, 2004.
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Literature Cited
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Albin, D. M., J. E. Wubben, and V. M. Gabert. 2000. Effect of hydrolysis time on the determination of amino acids in samples of soybean products with ion-exchange chromatography or precolumn derivatization with phenyl isothiocyanate. J. Agric. Food Chem. 48:1684–1691.[Medline]
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Abstract
Introduction
