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Growth, carcass traits, and plasma amino acid concentrations of gilts fed low-protein diets supplemented with amino acids including histidine, isoleucine, and valine^1,2

Department of Animal Science, University of Nebraska, Lincoln 68583-0908

⁴ Correspondence:
C206 Animal Sciences (phone 402-472-6423; fax: 402-472-6362; E-mail:
alewis2{at}unl.edu).

	Abstract

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Three experiments were conducted to determine the fifth-limiting amino acid for growing pigs in an 11% CP, corn-soybean meal diet. In each experiment, 36 gilts (initial weight 19.5, 21.9, and 21.0 kg, respectively) were penned individually and fed one of six diets in a randomized block design for 35 d. Diets containing 16, 12, and 11% CP were fed in each experiment. All 12 and 11% CP diets were supplemented with lysine, tryptophan, threonine, and methionine to provide the same total concentrations as those in the 16% CP diet. In Exp. 1, the 11% CP diet was supplemented with isoleucine, valine, or isoleucine + valine to concentrations equal to those in the 16% CP diet. In Exp. 2, the 11% CP diet was supplemented with histidine, histidine + valine, or histidine + isoleucine + valine. In Exp. 3, the 11% CP diet was supplemented with valine, histidine + valine, or isoleucine + valine. Gilts were allowed free access to feed and water. In all experiments, ADG and feed efficiency (G/F) were reduced (P 0.07) as dietary protein was reduced. Supplementation of isoleucine alone further reduced (P < 0.05) ADG, ADFI, G/F, and fat-free lean gain. In contrast, supplementation of valine alone resulted in numerical increases in ADG and ADFI in two experiments, although the differences were not significant (P > 0.05). Supplementation with histidine and valine together resulted in growth performance equal to or greater than that of pigs fed the 12% CP diet, but less than that of pigs fed the 16% CP diet. Supplementation of isoleucine and valine together resulted in better growth performance (P < 0.05) than supplementation of either amino acid alone. In two experiments (Exp. 1 and 3), supplementation of the 11% CP diet with isoleucine and valine together resulted in ADG that were not significantly different (P > 0.05) from those of pigs fed the 16% CP diet. Supplementation of all three amino acids (Exp. 2) did not improve performance over supplementation with histidine and valine. Plasma urea concentrations were reduced (P < 0.05) as dietary protein was lowered from 16 to 12%. Additions of crystalline amino acids did not affect plasma urea levels. Plasma amino acid concentrations reflected the dietary additions of crystalline amino acids, but did not assist in the identification of the sequence of limiting amino acids. These data suggest that valine is the fifth-limiting amino acid and that either histidine or isoleucine is the sixth-limiting amino acid in an 11% CP diet.

Key Words: Amino Acids • Crude Protein • Histidine • Isoleucine • Pigs • Valine

	Introduction

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In previous research (Figueroa et al., 2002), we have shown that the protein content of corn-soybean meal diets for 20- to 50-kg pigs can be reduced from 16 to 12% with little or no decrease in growth performance if the diet is supplemented with crystalline lysine, tryptophan, threonine, and methionine. Further reduction to 11% CP, however, decreased growth and feed efficiency, probably because other essential amino acids became limiting. The research indicated that the amino acids most likely to be limiting were histidine, isoleucine, and valine.

Russell et al. (1987) described two experiments with growing pigs fed diets supplemented with isoleucine and valine. They found that pig performance was improved when both isoleucine and valine were added to an 11% CP diet and that the performance was similar to that of pigs fed a 16% CP control diet. Supplementation with either isoleucine or valine alone did not result in an equal improvement in growth performance. In fact, isoleucine alone seemed to decrease performance and valine alone improved ADG but not feed efficiency. Mavromichalis et al. (1998) reported that valine (but not isoleucine) was as limiting as tryptophan, threonine, or methionine in a low-protein corn-soybean meal-whey-based diet for nursery pigs.

Based on NRC (1998) requirements, histidine is also marginally deficient in an 11% CP corn-soybean meal diet for growing pigs. Fuller et al. (1979) reported that histidine was the third-limiting amino acid in a barley diet for growing pigs. Brudevold and Southern (1994) suggested that histidine, isoleucine, tryptophan, and valine were equally third limiting (after lysine and threonine) in sorghum-soybean meal diets for 10- to 20-kg pigs.

Therefore, we conducted three experiments to determine whether histidine, isoleucine, and valine, either alone or in various combinations, increase growth performance when supplemented to an 11% CP diet containing lysine, tryptophan, threonine, and methionine.

	Materials and Methods

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A standard, 16% CP, corn-soybean meal diet and five low-protein (one 12% CP, four 11% CP) diets supplemented with crystalline lysine, tryptophan, threonine, and methionine were fed in each of three experiments. Low-protein diets were formulated by reducing the soybean meal content and increasing the corn content. The protein concentration in corn and soybean meal was determined before formulation of diets. In each experiment, diets formulated to contain 16, 12, and 11% CP constituted the first three diets. Additions of crystalline amino acids to the 12 and 11% CP diets were designed to achieve amino acid concentrations (on a total basis) equal to those in the 16% CP diets. The composition of these diets in Exp. 1 is presented in Table 1. The composition of diets in Exp. 2 and 3 differed slightly because of differences in the protein content of corn and soybean meal. The remaining three diets in each experiment were 11% CP and were supplemented with crystalline histidine, isoleucine, and valine, either alone or in various combinations. All diets were calculated to meet or exceed the NRC (1998) requirements for all nutrients other than certain essential amino acids. The amino acid additions and the analyzed protein and essential amino acid contents of each diet are shown in Table 2.

Gilts were weighed at the beginning of each 5-wk experiment and weekly thereafter. Feed intake was also measured weekly. Backfat thickness and longissimus muscle area (LMA) at the 10th rib were measured by real-time ultrasound on d 0 and 35, and fat-free lean weight and fat-free lean gain (FFLG) were calculated using the NPPC (1991) equation. Blood was collected from the anterior vena cava at approximately 1000 on d 14 and 35 (feed was not restricted before bleeding). Analysis of diet samples for protein and amino acids and blood plasma samples for urea and amino acids was as described by Figueroa et al. (2002).

Data from each experiment were analyzed as a randomized complete block design (six blocks) using PROC GLM of SAS (SAS Inst. Inc., Cary, NC). Individual pig was considered the experimental unit for all variables. Linear and quadratic comparisons of protein level (the 16, 12, and 11% diets) were tested using coefficients appropriate for the unequal spacing. Differences between individual treatment means were examined using Fisher’s least significant difference test. Differences among individual means were used to compare the responses of pigs fed diets supplemented with histidine, isoleucine, and valine (individually or in various combinations) with those of pigs fed the three "control" diets (16, 12, and 11% CP). In most cases, significant (P < 0.05) differences among individual means were only observed when the overall treatment effect was significant (P < 0.05); however, in a few cases, differences were observed when the individual comparisons were not "protected" by a significant overall treatment effect.

	Results

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Experiment 1
Gilts fed the 16% CP diet had excellent ADG (780 g/d; Table 3), but this gain was not maintained when the protein concentration was reduced to 12 and 11% (P < 0.05). Feed intake did not differ among gilts fed different protein concentrations; therefore, the differences in ADG were reflected in differences (P < 0.05) in feed efficiency (gain:feed; G/F). Supplementation of the 11% CP diet with isoleucine reduced ADG, whereas supplementation with valine had no effect. Supplementation with isoleucine + valine together resulted in ADG similar to those of pigs fed the 12% diet. The negative effect of isoleucine supplementation on ADG was mediated primarily by an effect on ADFI, which was lower (P < 0.05) in this group than in any other group of pigs. However, G/F was also reduced (P < 0.05) when diets were supplemented with isoleucine. Effects on FFLG were similar to those on ADG. The FFLG of pigs fed the 16% diet (313 g/d) was similar to the 315 g/d predicted by the NRC (1998) for pigs from 20 to 50 kg.

Reducing dietary protein concentration from 16 to 12% resulted in a large reduction (P < 0.05) in plasma urea concentration on both d 14 and 35 (Table 4). However, further reduction to 11% CP or additions of isoleucine and/or valine did not affect plasma urea.

Plasma concentrations of isoleucine and valine decreased (P < 0.05) in pigs fed diets not supplemented with isoleucine and valine as dietary protein concentration decreased from 16 to 11%. As expected, concentrations of these two amino acids in plasma increased (P < 0.05) when they were supplemented. Valine concentrations increased eightfold.

Concentrations of the remaining essential amino acids (except leucine) decreased (P < 0.05) as dietary protein decreased. The only other effect on this group of amino acids was a reduction in plasma leucine when isoleucine was supplemented.

Effects on nonessential amino acids were variable; most increased in plasma as dietary protein was reduced, although concentrations of ornithine decreased (P < 0.05), as did concentrations of the two conditionally dispensable amino acids, cystine (P < 0.05) and tyrosine (not significant). In addition, concentrations of cystine, glycine, and serine were affected (P < 0.05) by isoleucine and/or valine supplementation.

Experiment 2
Performance of gilts fed the 16% CP diet was excellent (Table 5), exceeding the NRC (1998) standards for 20- to 50-kg pigs in terms of ADG, G/F, and FFLG. However, as in Exp. 1, all of these traits were reduced (P < 0.05) when the dietary protein concentration was reduced. Also consistent with Exp. 1 was the fact that ADFI was unaffected by dietary protein concentration. Supplementation with histidine alone did not improve any of the growth traits. However, supplementation of histidine + valine or histidine + isoleucine + valine increased ADG and G/F to levels that were intermediate to levels achieved by gilts fed the 12 and 16% CP diets. Growth performance was equal or better when the diet was supplemented with histidine + valine than when it was supplemented with histidine + isoleucine + valine, suggesting that the inclusion of isoleucine was not beneficial in this experiment.

Effects on plasma urea concentration (Table 6) were similar to those observed in Exp. 1. There was a large reduction (P < 0.05) in plasma urea as dietary protein was reduced from 16 to 12%, with little or no further reduction when the dietary protein was 11%. Supplementation with the combination of histidine + isoleucine + valine reduced plasma urea concentration on d 14 but not on d 35.

Experiment 3
In this experiment, there was also excellent performance (Table 7), and in this case there were only small reductions in growth traits when dietary protein concentration was reduced from 16 to 12%. A further reduction to 11% resulted in lower ADG, G/F, and FFLG, resulting in linear and/or quadratic effects of protein level (P 0.07). Supplementation with valine increased ADG, but the difference was significant (P < 0.05) only when isoleucine was included with valine. Inclusion of histidine or isoleucine resulted in better G/F than when valine was supplemented alone, but again the difference was significant (P < 0.05) only when the combination of isoleucine and valine was included. Neither backfat thickness nor LMA was affected by dietary treatment in this experiment.

	Discussion

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The results of this research show clearly that neither isoleucine nor histidine alone is the fifth-limiting amino acid in a low-protein corn-soybean meal diet for growing gilts. Addition of isoleucine alone (Exp. 1) reduced ADG and FFLG by 24 and 16%, respectively. Addition of histidine alone (Exp. 2) reduced ADG and FFLG by 7 and 5%, respectively.

In contrast, addition of valine alone induced small positive responses in two experiments. In Exp. 1, valine addition increased ADG by 2% and decreased FFLG by 3%, and in Exp. 3, valine addition increased ADG and FFLG by 9 and 3%, respectively. The beneficial effect of valine addition was mediated via increased feed intake, and there was essentially no change in G/F in either experiment. These data suggest valine was limiting in the 11% CP diet that was supplemented with lysine, tryptophan, threonine, and methionine. Additional support for this conclusion is the fact that in all three experiments pigs fed diets containing valine in combination with (an) other amino acid(s) had greater ADG and FFLG than those of pigs fed the 11% CP diet. The average improvement was 14% for ADG and 11% for FFLG.

Russell et al. (1987) also reported that addition of valine alone to an 11% CP diet (supplemented with lysine, tryptophan, and threonine) improved ADFI and ADG, but not G/F of growing gilts. These researchers also found that addition of isoleucine alone was not beneficial. They did not test histidine addition. In experiments with nursery pigs fed low-protein corn-soybean meal diets that contained 8% whey, Mavromichalis et al. (1998) found positive responses to valine but not to isoleucine or histidine.

Thus, both our research and that of Russell et al. (1987) indicate that an 11% CP corn-soybean meal diet contains an insufficient amount of valine to support maximal performance of growing pigs. Our experimental design was similar to that of Russell et al. (1987) in that we both used pigs with an initial weight of 20 kg and included diets with 16 and 11% CP. However, our low-protein diets included supplemental methionine, whereas methionine addition was imposed as an experimental treatment by Russell et al. (1987). Also, our experiments were for 35 d, whereas theirs averaged 28 d. However, the greatest difference between the two data sets was in the growth performance achieved. Pigs fed the positive control, 16% CP diet had an ADG of 713 g in the research reported by Russell et al. (1987), whereas our pigs fed a similar diet had an ADG of 834 g. Although some of this difference is probably due to the longer feeding period we employed, most of it is likely due to the genetic improvement made since 1987.

The sequence of limiting amino acids after valine is unclear. Although differences were not statistically significant, addition of isoleucine to the 11% CP diet supplemented with valine seemed to increase growth performance (Exp. 1 and 3). However, histidine addition to the valine-supplemented diet also produced nonsignificant positive responses (Exp. 2 and 3). In a direct comparison of valine + isoleucine vs. valine + histidine (Exp. 3), the diet with valine + isoleucine gave greater performance, but supplementation with all three amino acids (Exp. 2) was no better than supplementation with valine + histidine. These trends are not consistent with classical concepts of protein quality, whereby individual amino acids become limiting in a sequential pattern or two amino acids become equally colimiting.

In previous research (Lewis et al., 1982;Grosbach et al., 1985;Gibb et al., 1992), changes in plasma urea and plasma amino acid concentrations have aided in the identification of limiting amino acids. In the current experiments, plasma urea concentrations fell as dietary protein decreased from 16 to 12% CP. This effect has been observed in numerous previous experiments (Chen et al., 1995; Figueroa et al., 2002;Gómez et al., 2002). There were, however, no effects on plasma urea concentration of histidine, isoleucine, or valine supplementation, either alone or in various combinations.

Reducing dietary protein from 16 to 11% CP resulted in lower plasma concentrations of most essential amino acids (except lysine, tryptophan, threonine, and methionine, which were supplemented) and higher plasma concentrations of most nonessential amino acids. This presumably reflects the decrease in soybean meal and the increase in corn and the differences in their amino acid profiles. Supplementation with histidine, isoleucine, and/or valine elicited large increases in the plasma concentration(s) of the amino acid(s) that was (were) supplemented, but this was the only consistent effect. Thus, changes in plasma urea and amino acid concentrations did not assist in the identification of limiting amino acids in these experiments. Nevertheless, the plasma amino acid data provide important supplementary information. First, the increases in lysine, tryptophan, threonine, and methionine as dietary protein level was reduced, although predictable, provide support that these amino acids were unlikely to be limiting in the reduced-protein diets. Second, the increases in histidine, isoleucine, and valine when these amino acids were supplemented show that plasma levels do respond to supplementation and, more importantly, show the magnitude of the response.

One explanation for the positive response to both histidine and isoleucine supplementation is that these amino acids simply provided a source of nonspecific nitrogen that was used to correct a deficiency of nonessential amino acids. However, three lines of evidence make this unlikely. First, no benefit has been obtained from nonspecific nitrogen/nonessential amino acid supplementation in previous experiments with a similar design (Russell et al., 1987; Brudevold and Southern, 1994;Ward and Southern, 1995) or in an experiment with low-protein corn-soybean meal-whey-based diets for nursery pigs (Mavromichalis et al., 1998). Second, the essential:nonessential amino acid ratio of the diets ranged from 37:63 to 43:57, indicating that the sum of the nonessential amino acids exceeded the sum of the essential amino acids and therefore nonspecific nitrogen was unlikely to be limiting (Lewis, 2001). Third, plasma concentrations of most nonessential amino acids were higher in pigs fed the 11% CP diet than in pigs fed the 16% CP diet, suggesting that there was no deficiency of nonessential amino acids.

In summary, this research suggests that valine becomes limiting when the protein content of the diet of growing gilts is reduced by more than four percentage units (i.e., from 16 to 11% CP). Histidine and/or isoleucine also seem to be limiting, but the sequence of their limitation or whether they are colimiting is unclear.

	Implications

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Reducing the crude protein content of corn-soybean meal diets for growing gilts by four or five percentage units leads to reductions in growth traits. A portion of the reduction in growth can be restored by judicious supplementation with crystalline amino acids. A reduction of five percentage units requires supplementation with valine in addition to lysine, tryptophan, threonine, and methionine. Supplements of histidine and/or isoleucine also seem to be required, but the precise sequence of limitation is unclear.

	Footnotes

 
¹ A contribution of the Univ. of Nebraska Agric. Res. Div., Lincoln 68583-0704. Journal series No. 13640.

² Appreciation is expressed to Nutri-Quest, Chesterfield, MO for the donation of some of the crystalline amino acids used in this research.

³ Present address: Colegio de Postgraduados en Ciencias Agry’colas, Montecillo, Texcoco, Edo. de México, 56230, México.

Received for publication February 26, 2002. Accepted for publication January 23, 2003.

	Literature Cited

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Brudevold, A. B., and L. L. Southern. 1994. Low-protein, crystalline amino acid-supplemented, sorghum-soybean meal diets for the 10- to 20-kilogram pig. J. Anim. Sci. 72:638–647.[Abstract]

Chen, H.-Y., P. S. Miller, A. J. Lewis, C. K. Wolverton, and W. W. Stroup. 1995. Changes in plasma urea concentration can be used to determine protein requirements of two populations of pigs with different protein accretion rates. J. Anim. Sci. 73:2631–2639.[Abstract]

Figueroa, J. L., A. J. Lewis, P. S. Miller, R. L. Fischer, R. S. Gómez, and R. M. Diedrichsen. 2002. Nitrogen metabolism and growth performance of gilts fed standard corn-soybean meal diets or low-crude protein, amino acid-supplemented diets. J. Anim. Sci. 80:2911–2919.[Abstract/Free Full Text]

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Lewis, A. J., M. B. Barnes, D. A. Grosbach, and E. R. Peo, Jr. 1982. Sequence in which the amino acids of corn (Zea mays) become limiting for growing rats. J. Nutr. 112:782–788.

Mavromichalis, I., D. M. Webel, J. L. Emmert, R. L. Moser, and D. H. Baker. 1998. Limiting order of amino acids in a low-protein corn-soybean meal-whey-based diet for nursery pigs. J. Anim. Sci. 76:2833–2837.[Abstract/Free Full Text]

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Russell, L. E., B. J. Kerr, and R. A. Easter. 1987. Limiting amino acids in an 11% crude protein corn-soybean meal diet for growing pigs. J. Anim. Sci. 65:1266–1272.

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