J. Anim. Sci. 2004. 82:1997-2003
© 2004 American Society of Animal Science
Ileal amino acid digestibility and performance of growing pigs fed wheat-based diets supplemented with xylanase
M. Barrera*,
M. Cervantes*,1,
W. C. Sauer
,
A. B. Araiza*,
N. Torrentera* and
M. Cervantes*
* Instituto de Ciencias Agrícolas, Universidad Autónoma de Baja California, Mexicali, México; and
and
Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta T6G 2P5, Canada
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Abstract
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Two experiments were conducted to determine the effect of supplementation of xylanase to a wheat-based diet on the apparent ileal digestibility (AID) of AA and the performance of growing pigs fed diets limiting in AA. In Exp. 1, eight pigs (average initial BW = 20.5 ± 1.2 kg) fitted with a simple T-cannula at the distal ileum, were fed four diets according to a repeated 4 x 4 Latin square design. Diet 1 was a basal diet that contained 97.6% wheat. Diets 2, 3, and 4 were the basal diet supplemented with xylanase at rates of 5,500, 11,000, and 16,500 units of xylanase activity (XU), respectively (as-fed basis). There were linear and quadratic effects (0.062 < P < 0.001) of xylanase supplementation on the AID of CP and most of the AA. The largest increases in AID of CP and AA were obtained when xylanase was supplemented at a rate of 11,000 XU; no further increases were observed with xylanase supplementation at a rate of 16,500 XU. In Exp. 2, 30 pigs (average initial BW 21.4 ± 1.8 kg) were randomly allotted to six dietary treatments. Diets 1 to 4 were similar to those used in Exp. 1. Diet 5 was the same as Diet 1, but supplemented with 0.53% lysine, 0.12% threonine, and 0.05% methionine. Diet 6 (positive control diet) was a wheat-soybean meal diet that contained 18.2% CP (as-fed basis). The total contents of lysine, threonine, and methionine were similar for Diets 5 and 6. There was a linear effect of xylanase supplementation on ADG (P = 0.093) and feed:gain ratio (P = 0.089), and a quadratic effect on ADG (P = 0.067) and feed:gain ratio (P = 0.074). But, the greatest response was obtained with the supplementation of 11,000 XU. The supplementation of lysine, threonine, and methionine to Diet 1 increased (P = 0.001) ADG and ADFI and improved (P = 0.01) feed:gain ratio. There was no difference (P = 0.508) in the performance of pigs fed the AA-supplemented or control diet. In conclusion, the supplementation of xylanase to a diet in which wheat provided the sole source of protein and energy improved the AID of AA, ADG, and feed:gain ratio; however, this improvement was very small compared with that obtained with the supplementation of synthetic amino acids.
Key Words: Amino Acids • Ileal Digestibility • Pigs • Wheat • Xylanase
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Introduction
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The potential of the nutritive value of wheat in diets for pigs and poultry is not fully realized because of certain physical and chemical characteristics (Campbell and Bedford, 1992
). The nonstarch polysaccharides (NSP) in wheat include cellulose, pectins, ß-glucans, and arabino-xylans, of which the latter in particular affects its nutritive value (Souffrant, 2001). Arabino-xylans, which are present in the endosperm cell walls of wheat, are linear polymers of variable length consisting of D-xylose joined with ß (1–4) linkages, and single residues of arabinose attached along the primary xylan chain (Bedford, 1995). Arabinose renders this polymer soluble, which, in solution, interacts with long polymers, resulting in the formation of a viscous digesta in nonruminants. Choct and Annison (1992), in studies with broiler chicks, showed that an increase in the viscosity of the intestinal contents is closely associated with a decrease in nutrient and energy digestibility. Arabino-xylans represent about 70% of the total NSP in wheat; the content of arabino-xylans in wheat varies (from 2.6 to 6.5%), depending in part on the cultivar (Zijstra et al., 1999).
The digestive system of nonruminants does not produce the enzyme xylanase, which is required to hydrolyze arabino-xylans. At present, xylanase is commercially available. The response of xylanase supplementation to wheat-based diets for poultry is usually positive and includes improvements in nutrient and energy digestibilities, ADFI, and ADG (Bedford and Campbell, 1992; Bedford, 1995; 2000). However, only limited and conflicting research (Van Lunen and Schulze, 1996; and Mavromichalis et al., 2000) is available on the effects of xylanase supplementation on nutrient digestibility and performance in wheat-based diets for swine.
The objective of this study was to determine the effect of xylanase supplementation on the apparent ileal digestibility (AID) of AA and the performance of growing pigs fed diets in which wheat provided the only source of protein and energy. In addition, growth performance was compared with that of pigs fed the same diets but supplemented with the limiting AA.
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Materials and Methods
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Two experiments were conducted with growing pigs (Landrace x Hampshire x Duroc) at the Swine Experimental Unit of the University of Baja California in order to determine the effect of xylanase supplementation on AID and performance. In both experiments, the animals were individually housed in 1.2 x 0.6 m metabolic cages. The same batch of soft winter wheat was used in both experiments. The enzyme used was of fungal origin (Natugrain Blend, Aspergillus niger and Trichoderma longibrachiatium) and was supplied by DSM Food Specialties (Delft, The Netherlands). The enzyme’s activity was 55,000 xylanase U/g. One unit of xylanase activity (XU) is defined as the quantity of enzyme that will liberate 4.53 µmol of reducing sugars (measured as xylose equivalents) per minute from arabino-xylans obtained from wheat flour (0.5%, wt/vol, at 40°C and pH 3.5). The enzyme product also contained 1,500 ß-glucanase U/g.
Experiment 1: Digestibility Trial
Eight barrows, average initial BW 20.5 ± 1.2 kg, were fitted with a simple T-cannula at the distal ileum according to procedures adapted from Sauer et al. (1983). A detailed description of pre- and post-operative care was previously described by Li et al. (1994). The cannulas were prepared from Tygon tubing (Norton Performance Plastics, Wayne, NJ). The pigs were returned to the metabolic crates immediately after surgery. During the 12-d recovery period, the pigs received a 16% CP sorghum-SBM diet ad libitum. Water was freely available. Following recovery, the pigs were fed one of four wheat-based diets with or without supplementation of xylanase at rates of (as-fed basis) 5,500, 11,000, and 16,000 XU per kg (Table 1). Vitamins and minerals were supplemented to meet or exceed NRC (1998) standards for pigs ranging in BW from 20 to 50 kg. All diets contained 0.2% chromic oxide as the digestibility marker. The average BW of the pigs at the conclusion of the experiment was 37.4 kg.
The study was conducted according to a repeated Latin square design (Steel and Torrie, 1980). Each experimental period consisted of 12 d: 8 d of diet adaptation followed by 4 d of collection of digesta. The pigs were fed equal amounts twice daily, at 0700 and 1900. The pigs were trained to consume their ration in 15 min or less; the amount of feed offered was limited to the feed intake of the pig that consumed the least to ensure that all pigs had the same daily consumption. The intake by the pig that consumed the least was equivalent to 3.3 times the DE requirement for maintenance (NRC, 1998), based on the average BW of the pigs at the start of each experimental period. The feed was mixed 1:1 with water. The enzyme product was added and mixed with the feed at the same time the water was added. Ileal digesta were collected in plastic bags tied to the barrel of the cannula. The bags were removed and replaced as soon as they were filled with digesta; no bag remained attached to the cannula longer than 20 min. The collection of ileal digesta was initiated at 0600 on d 9. Ileal digesta were collected for 24 h, in 6-h periods followed by a 6-h period in which no digesta were collected. The first six 6-h collections took place from 0600 to 1200 on d 9. The last 6-h collection took place from 0000 to 0600 on d 12 of each experimental period. Ileal digesta were stored immediately after collection at –20°C.
At the conclusion of the experiment, ileal digesta were thawed, pooled within pig and period, and homogenized. A subsample of each homogenate was freeze-dried and ground through a 1-mm mesh screen. Samples of diets and ileal digesta were analyzed for DM and CP (AOAC, 1995). Amino acid analyses of diets (Table 2) and ileal digesta were performed at the University of Missouri Experiment Station Chemical Laboratories (Columbia) using ion-exchange chromatography after acid hydrolysis. Cystine and tryptophan were not determined. Amino acid concentrations were not corrected for incomplete recovery resulting from hydrolysis. Chromic oxide was analyzed according to Hill and Anderson (1958) using an UV spectrophotometer.
The data were analyzed according to a 4 x 4 replicated Latin square design using the GLM procedures of SAS (SAS Inst., Inc., Cary, NC). Regression analyses between the AID of CP and AA and the rate of xylanase supplementation to the diets were performed to determine linear and quadratic relationships. Treatment differences were considered significant at an alpha level of 0.05.
The pigs used in this experiment were cared for in accordance with the guidelines established by CCAC (1993).
Experiment 2. Performance Trial
Thirty pigs (Landrace x Hampshire x Duroc) with an average initial BW 21.4 ± 1.8 kg were used in a 28-d growth assay. The pigs were grouped by initial BW and randomly assigned to six diets based on sex, BW, age, and litter of origin. There were five pigs (three barrows and two gilts) for each treatment, with one pig per pen. Each 0.6- x 1.2-m pen was equipped with a single-hole feeder and a water nipple to allow ad libitum consumption of feed and water. The animals were weighed weekly; feed intake was also determined weekly. The average final BW of the pigs was 34.5 kg.
Diets 1 to 4 were the same as those used in Exp. 1 (Table 3). Diet 5 (AA control) was similar to diet 1 but was supplemented with 0.53% L-lysine, 0.12% L-threonine, and 0.05% DL-methionine, to levels similar to those in the positive control diet. Diet 6 was the positive control diet and was made up of wheat (79.3%) and SBM (18.7%).
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Table 3. Contents of crude protein and indispensable amino acids in the basal, AA control, and positive control dietsa
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Data were analyzed by ANOVA according to a randomized complete block design (Steel and Torrie, 1980) with six treatments in six blocks, with BW, sex, and age as blocking factors, using the GLM procedures of SAS. Regression analyses were performed for the 0-, 5,500-, 11,000-, and 16,500-XU/g treatments in the basal diet to determine linear and quadratic relationships. Additionally, two contrasts were constructed to test the effect of adding the crystalline AA to the basal diet (Contrast 1: Diet 5 vs. Diet 1), and to compare the AA-supplemented diet to the wheat-soybean meal control diet (Contrast 2: Diet 5 vs. Diet 6).
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Results and Discussion
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Experiment 1. Digestibility Trial
The AID of CP and AA of the experimental diets are presented in Table 4. For most of the indispensable AA, there were linear and quadratic responses (0.062 < P < 0.001) in the AID of CP and AA to the increase in the rate of supplementation of xylanase. Amino acid digestibility values were greatest in diets supplemented with 11,000 XU for all indispensable and dispensable AA except for proline, which had the highest value at 5,500 XU supplemental xylanase. There is no clear explanation why the AID of CP and AA numerically decreased when the rate of xylanase supplementation was increased from 11,000 to 16,500 XU. The improvements in AID values upon xylanase supplementation are larger than those reported by Rattay et al. (1998), who reported an increase in the AID of CP from 77.9 to 81.6% in studies in which growing pigs were fed a diet containing barley, wheat, wheat bran, and soybean meal. Yin et al. (2000) also reported only small improvements (two percentage units) in the AID of CP and some of the indispensable AA in pigs fed diets based on wheat or wheat by-products supplemented with xylanase. The differences in response to xylanase supplementation among the different studies may be attributed in part to differences in the content of NSP. Wheat was the only major dietary ingredient in this study, whereas in the other studies, the diets contained other ingredients, including SBM. According to NRC (1998), wheat contains 9.5% hemicellulose, whereas SBM contains only 4.3%. In addition, it is possible that the content of arabino-xylans in the source of wheat used in this study was higher than in the sources of wheat used in the other studies. Unfortunately, the content of arabino-xylans in wheat was not determined in this study.
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Table 4. Effect of xylanase supplementation on the apparent ileal digestibilities of crude protein and amino acids (%) in pigs fed the wheat-based diets (Exp. 1)
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Arabinose and xylose are the main components of hemicellulose (Henry, 1985) and the predominant NSP in wheat (Zijlstra et al., 1999). Arabinose renders this polymer soluble, which in solution interacts with long polymers resulting in the formation of a viscous digesta in nonruminants. Choct and Annison (1992), in studies with poultry, reported that the digestibilities of nutrients decrease as the viscosity of digesta increases. Zijlstra et al. (1999), in studies with pigs, found negative correlations between the digestible energy content and the dietary contents of arabinose (–0.71) and xylose (–0.78) in 15 wheat samples. Gdala et al. (1997), in studies with pigs fed barley- and wheat-based diets supplemented with xylanase, reported increases in the digestibilities of xylose, arabinose, and mannose. The improvement in the digestibility of arabino-xylans, and a resulting decrease in the viscosity of digesta, may explain the increase in the AID of CP and AA reported in this study.
The effect of xylanase supplementation on the contents (%) of apparent ileal digestible CP and AA in the wheat-based diets is presented in Table 5. As for AID, except for histidine, isoleucine, and methionine, there were linear and quadratic responses (0.057 < P < 0.002) in the apparent ileal digestible CP and indispensable AA content to the increase in the rate of supplementation of xylanase. The contents of apparent ileal digestible CP and AA in the diet increased as the rate of xylanase supplementation also increased from 0 to 11,000 XU; however, no further improvement was observed when the rate of supplementation was increased from 11,000 to 16,500 XU.
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Table 5. Effect of xylanase supplementation on the apparent ileal digestible crude protein and amino acid contents (%) in the wheat-based diets (Exp. 1)
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Although the increases in the AID of AA in wheat in response to xylanase supplementation are considerable (Table 4
), there is just a modest increase in the content of digestible AA (Table 5). For example, the supplementation of xylanase at 11,000 XU increased the content of apparent ileal digestible lysine from 0.17 to 0.21%. This amount is equivalent to an increase in the daily digestible lysine intake from 3.4 to 4.0 g (0.6 g/d) in pigs consuming 2 kg/d of a diet formulated with wheat as the only major ingredient. According to NRC (1998) standards, the daily lysine requirement of growing pigs, assuming a daily lean tissue gain of 325 g, is 14.2 g. Thus, increasing the intake of digestible lysine by 0.6 g/d as a result of the supplementation of xylanase hardly increases the daily lysine supply in relation to the daily lysine requirement. Therefore, the increases in the AID of AA resulting from xylanase supplementation to a wheat-based diet may not necessarily be reflected by an improvement in pig performance.
Experiment 2. Performance Trial
The results of the performance trial are presented in Table 6. There was a linear effect of xylanase supplementation on ADG (P = 0.093) and feed:gain ratio (P = 0.089); also, there was a quadratic effect on ADG (P = 0.067) and feed:gain ratio (P = 0.074). The greatest responses in ADG and feed:gain ratio were obtained with the supplementation of xylanase at a rate of 11,000 XU. This would support the results found in Exp. 1, wherein the greatest AID of AA was achieved when 11,000 XU of xylanase were added to the diet (Table 4).
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Table 6. Effect of xylanase and amino acid supplementation to wheat-based diets on the performance of growing pigs (Exp. 2)a
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The basal diet used in this study was deficient in lysine and threonine, which are the first- and second-limiting AA, respectively, in wheat for growing pigs (NRC, 1998). This diet was chosen to elicit a response in performance of the pigs fed the xylanase-supplemented diets, assuming that xylanase would increase the AID of AA. It was shown in Exp. 1 that the supplementation of xylanase at 11,000 XU improved the AID of lysine and threonine (Table 4). However, there was no improvement in ADG (Table 6). The lack of response can be attributed to the very small improvements in the contents of digestible lysine and threonine (Table 5). Therefore, the results from this study show that an improvement in nutrient digestibility upon xylanase supplementation is not necessarily accompanied by an improvement in pig performance. Also, it is possible that the increase in the AID of AA resulted from a reduction in the secretion and ileal recovery of endogenous AA rather than from an increase in true AA digestibility values (Nyachoti et al., 1997), which perhaps may explain the improvement in feed:gain ratio.
Van Lunen and Schulze (1996), in studies with pigs fed wheat-wheat middlings-SBM diets, reported that xylanase supplementation improved ADG and feed:gain ratio by 9.2 and 5.3%, respectively. On the other hand, Thacker et al. (1991) did not report a response in performance to xylanase supplementation in pigs fed rye- or barley-based diets. Inborr et al. (1993) did not find an improvement in performance with xylanase supplementation in weaning pigs fed wheat- or barley-SBM diets. Furthermore, Mavromichalis et al. (2000) reported no response in performance to xylanase supplementation in nursery pigs fed wheat-based diets. However, they reported an improvement in ADG in one of two studies with finishing pigs fed wheat-SBM diets. The differences in performance in response to xylanase supplementation in the aforementioned studies (including this study) can perhaps be attributed to differences in the dietary contents of arabino-xylans, as was discussed previously.
The supplementation of lysine, threonine, and methionine to the basal diet (Table 2), compared with the basal and xylanase-supplemented diets, increased ADG, ADFI, and improved feed:gain ratio (P = 0.001; Table 6). This improvement was expected because of the marked deficiency of lysine and threonine in the basal diet. It should also be mentioned that synthetic lysine and threonine are completely absorbed in the small intestine (Chung and Baker, 1992). The ADG (P = 0.755), ADFI (P = 0.508), and feed:gain ratio (0.872) of pigs fed the AA-supplemented diet were similar to those of pigs fed the wheat-soybean meal diet. In contrast to studies with pigs fed corn-based (Russell et al., 1983) or sorghum-based (Cervantes et al., 1997) diets, our study suggests that adding synthetic lysine, threonine, and methionine to a wheat-based diet can completely replace SBM in the diet. As such, the replacement of SBM by synthetic AA in a wheat-based diet will result in a considerable reduction in nitrogen output in manure (Kerr and Easter, 1995), which may further be reduced by xylanase supplementation.
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Implications
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Supplementation of xylanase at a rate of 11,000 xylanase units (as-fed basis) to a diet in which wheat provided the sole source of protein and energy increased the apparent ileal digestibilities of all indispensable amino acids in growing pigs. Supplementation of xylanase at 11,000 xylanase units tended to improve feed:gain ratio, but did not affect weight gain or feed intake. The supplementation of synthetic lysine, threonine, and methionine may replace soybean meal in wheat-based diets for growing pigs. Additional research with a larger number in pigs is warranted to verify these findings.
1 Correspondence: P.O. Box 4913, Calexico, CA 92231 (phone: 686-523-0217; e-mail: miguel_Cervantes{at}uabc.mx).
Received for publication July 5, 2003.
Accepted for publication March 1, 2004.
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Abstract
Introduction
