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ANIMAL NUTRITION |
* Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta, Canada T6G 2P5;
and
Instituto de Ciencias Agricolas, Universidad Autónoma de Baja California, Mexicali, México 21100; and and
Field Crop Development Centre, Lacombe, Alberta, Canada T4L IW8
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Key Words: barley • low phytate • nutrient excretion • phosphorus • phytase • pig
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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). In Canada, barley production ranks second after that of wheat in terms of cereal grains (FAO, 2002).
Approximately two-thirds of the P in feedstuffs of plant origin, including barley, is present in the form of phytate (Cromwell, 1992; Ravindran et al., 1995). Phytate P is poorly utilized by swine because of insufficient activity of endogenous phytases (Nelson, 1967). Inorganic P (iP) sources are therefore routinely added to swine diets to meet P requirements, and most of the phytate P is excreted. Phosphorus in swine manure is increasingly becoming an environmental concern (Jongbloed and Lenis, 1998; Kornegay, 2001), and strategies to reduce P excretion warrant investigation.
One method to reduce P excretion by pigs, besides the use of supplemental phytase (e.g., Mroz et al., 1994), is the use of low-phytate feedstuffs (Raboy, 2002). In recent years, several low-phytate crops have been developed, including low-phytate hulled barley (LPHB), which is phenotypically identical to normal-phytate hulled barley (HB), and low-phytate hull-less barley (LPHLB). Normal-phytate hull-less barley (HLB) has an 8 to 10% greater energy digestibility than HB (Huang et al., 2003), and thus provides further nutritional benefits. The effectiveness of low-phytate feedstuffs to improve P utilization should be determined and compared with phytase supplementation.
The objective of Exp. 1 was to determine and compare the digestibility, balance, and excretion of P in pigs fed diets that contained normal-phytate or low-phytate cultivars of either hulled or hull-less barley origin. The objective of Exp. 2 was to determine the effect of iP and phytase supplementation of an 18% CP barley-soybean meal (SBM) diet deficient in available P (aP) and the effect of replacing normal-phytate barley with low-phytate barley in the barley-SBM diet on the digestibility, balance, and excretion of P.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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. Two experiments were conducted at the Swine Research and Technology Center of the University of Alberta (Edmonton, Alberta, Canada).
Exp. 1.
Nine crossbred barrows (Large White x Landrace), with an average initial BW of 25.0 ± 0.77 kg, were housed individually in stainless steel metabolic crates (height = 82 cm; length = 124 cm; width = 76 cm) in a temperature-controlled room (22 ± 1°C). During a 7-d adaptation to the crates, the barrows were fed an 18% CP grower diet ad libitum. Water was freely available from a low-pressure drinking nipple. Based on their feed intake pattern, 8 barrows were selected and fed 4 diets according to a repeated 4 x 4 Latin square design. Each of the 4 experimental periods lasted 13 d. The barrows were fed twice daily at 0800 and 1500, with equal amounts at each meal. The diets were fed at a rate of 2.5 times the maintenance requirement for ME (i.e., 106 kcal of ME/kg of BW0.75; NRC, 1998) based on the average BW of the pigs, which was recorded at the beginning of each experimental period.
Four barley-based diets were formulated with 4 barley cultivars (Field Crop Development Center, Lacombe, Alberta, Canada): 95.6% HB (cultivar Metcalfe), 95.8% LPHB (mutant line 99043), 95.8% HLB (cultivar Falcon), and 95.8% LPHLB (mutant line 99054). The phytate P content was 69 and 88% lower in the low-phytate HB and HLB than in the normal-phytate HB and HLB, respectively, and the normal-phytate barley cultivars were, thereby, limiting in calculated aP (Table 1). As such, the HB and HLB diets were supplemented with iP, so that the calculated aP content met or exceeded the NRC (1998) recommendation. The diets were supplemented with Lys, Met, Thr, and Trp (Table 2) to meet their apparent ileal digestible supplies (NRC, 1998). The HB and HLB diets were supplemented with monocalcium phosphate to meet the NRC (1998) recommendation for aP, which is 0.23% for grower pigs over the range from 20 to 50 kg of BW. The LPHB and LPHLB diets were not supplemented with monocalcium phosphate because these diets contained sufficient aP (0.26 and 0.33%, respectively). The diets were formulated to contain 0.60% Ca. Canola oil was included to meet the recommendation for ME, and vitamins and minerals were supplemented to meet or exceed the recommendations (NRC, 1998). Chromic oxide (0.3%) was included as an indigestible marker. The barley was ground through a 2-mm mesh screen before incorporation into the diets. The diets were fed in the form of mash. Water was mixed with the feed at a ratio of 2.5 to 1 (wt/wt) and was freely available between meals.
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Four diets based on barley and SBM were formulated to contain 18% CP by using a HB or LPHB (same batches as Exp. 1): HB-SBM, HB-SBM + iP, HB-SBM + phytase supplemented at a rate of 500 phytase units/kg of diet, and LPHB-SBM (Table 3). Monocalcium phosphate (17% Ca and 21.1% P) was used as the iP source, and Aspergillus niger phytase (Natuphos, DSM Food Specialties, Delft, the Netherlands) was used as microbial phytase. The HB-SBM and HB-SBM + phytase diets were formulated to contain less aP than the NRC (1998) recommendation. The HB-SBM + iP and LPHB-SBM diets were formulated to meet the NRC (1998) recommendation for aP, which is 0.23% for pigs from 20 to 50 kg of BW. One phytase unit is defined as the quantity of enzyme that liberates one micromole of orthophosphate per minute from 5.1 mM Na-phytate at pH 5.5 and 37°C (Engelen et al., 1994). The diets were formulated to contain 0.6% Ca. Chromic oxide (0.3%) was included in the diets.
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Prior to chemical analyses, feces and urine were pooled for each pig in each experimental period. Feces were dried to constant weight in a forced-air oven at 55°C. The dried samples of feces and samples of ingredients and diets were ground through a 0.5-mm mesh screen (Thomas-Wiley Laboratory Mill, Arthur H. Thomas Co., Philadelphia, PA). Urine samples were filtered through Whatman No. 2 filter paper and dried in a forced-air oven at 55°C to constant weight.
Analyses for DM and OM were carried out according to the AOAC (2000). Crude protein (N x 6.25) was determined by combustion (FP-428 N Determinator, Leco Corporation, St. Joseph, MI). The P contents were determined spectrophotometrically by the molybdovanadate procedure (AOAC, 2000). Barley phytate P content was analyzed according to the procedures described by Haug and Lantzsch (1983). Chromic oxide was determined with a spectrophotometric procedure according to the methods of Fenton and Fenton (1979). Diet and barley samples were analyzed in triplicate; feces and urine were analyzed in duplicate.
Calculations and Statistical Analyses
The apparent total tract digestibilities (ATTD) of DM, OM, and P were calculated by using the following equation:
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where AF is the concentration of a component in feces (%), ID is the chromic oxide concentration in the assay diet (%), AD is the concentration of a component in the assay diet (%), and IF is the chromic oxide concentration in feces (%). The total P excretion in feces was determined by taking into account the intake and ATTD of P (determined via chromic oxide). For normal-phytate barley, digestibility and retention of P and retained P (g/d) contributed solely by the barley samples were calculated by assuming that the bioavailability of mono-calcium phosphate is 100% (NRC, 1998).
To compare differences in the variables among main effects or diets, data were analyzed by ANOVA for a repeated, 4 x 4 Latin square design using PROC GLM (SAS Institute Inc., Cary, NC). In Exp. 1 and 2, the effects of diet (n = 4) and experimental period (n = 4) were included in the model. The diet effects were described and considered significant at P < 0.05. Means were reported as least squares means.
In Exp. 1, 3 orthogonal contrasts were constructed to test the main effects and interaction: 1) the effect of hulled vs. hull-less barley, 2) the effect of normal- vs. low-phytate, and 3) the interaction barley x phytate. In Exp. 2, when a significant F-value (P < 0.05) was indicated for diet by ANOVA, means were separated using the Student-Newman-Keuls’ multiple range test.
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RESULTS AND DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Because of the lower phytate P intake, the ATTD and retention of P were greater (P = 0.004) in pigs fed the low-phytate barley diets compared with pigs fed the normal-phytate barley diets (Table 4). Similarly, the low-phytate barley had greater ATTD (P < 0.001) and retention of P (P < 0.001) than the normal-phytate barley. The ATTD of P averaged 65% for low-phytate barley and 49% for normal-phytate barley. Thacker et al. (2003, 2004) also reported a greater ATTD of P in finishing pigs fed low-phytate hulled and hull-less barley diets compared with normal-phytate hulled and hull-less barley diets. The normal-phytate barley diets used by Thacker et al. (2004) were deficient in aP for finishing pigs, and the measured ATTD of P was 11% for normal-phytate barley and 37% for low-phytate barley. The greater ATTD coefficients in the current study may reflect true P digestibility in normal-phytate barley because endogenous P output will drastically reduce measured ATTD of P in diets that are limiting in aP (Ajakaiye et al., 2003). Differences in P digestibility may also reflect differences in endogenous phytase activity between hulled and hull-less barley or among the samples of hulled or hull-less barley (Tran and Skiba, 2005). In barley, effects of phytase supplementation and low-phytate cultivars on P digestibility were not additive (Thacker et al., 2004), indicating that combining both treatments does not result in a cumulative increase in P digestibility. The ATTD of DM did not differ between pigs fed the normal- and low-phytate diets in the current study. The digestibility of OM tended to be 1% greater (P = 0.052) in normal-phytate barley, indicating that the phytate content of barley might play a minor role in determining energy digestibility.
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; Liao et al., 2006), indicating that pigs did not meet their P requirement (Ekpe et al., 2002). For this reason, the values for ATTD and retention of P for each dietary treatment are nearly identical numerically. Although the retention of P (%) was greater, the amount of P retained (g/d) was decreased (P = 0.002) in pigs fed the low-phytate compared with the normal-phytate barley diets (Table 4
), indicating that the low-phytate barley diets might not have been nutritionally complete to stimulate further reductions in P excretion in feces. The reduced amount of P retained was also reported by Veum et al. (2002) in a study with growing pigs fed a diet in which normal-phytate hulled barley was replaced by low-phytate hulled barley in a barley-SBM diet. This observation requires further study because the objective of nutrient management is to reduce P excretion while maintaining the amount of P retained. When considering the barley only following correction for the P contributed by mono-calcium phosphate, the amount of P retained (g/d) was greater (P < 0.001) for low-phytate than for normal-phytate barley. The total excretion of P was reduced by 35% (P < 0.001) when normal-phytate barley was replaced by the low-phytate barley in the diets. These reductions are in agreement with previous studies with low-phytate barley (Li et al., 2001a,b; Veum et al., 2002), corn (Veum et al., 2001; Bohlke et al., 2005), and SBM (Sands et al., 2003) in swine and poultry diets. Thus, the current study provides further evidence that the low-phytate genotype in barley is just as important as in other feedstuffs to improve P utilization in pigs.
In the current study, the diets were formulated to meet the recommendation of aP for grower pigs with a BW from 20 to 50 kg (0.23%; NRC, 1998). However, because of the restricted access to feed (2.5 times maintenance requirement for ME), the daily intakes of P and aP ranged from 43 to 64% and from 70 to 100% of the NRC (1998) recommended intake, respectively. In other P digestibility and balance studies, similar approaches have been used to study low-phytate feed-stuffs. For example, Bohlke et al. (2005) restricted the feed allowance to 3 times the maintenance requirement for ME to study P digestibility of low-phytate corn. Veum et al. (2002) gave free access to feed, but restricted the aP content of the diet to study low-phytate barley. Therefore, both studies limited P supply using 2 different approaches, but were able to study effects of low-phytate feedstuffs on P digestibility and retention. Assuming that P and aP intake does not affect P retention from the amount fed in this study up to the daily recommended intake by NRC (1998), the relative reduction in P excretion will be constant when normal-phytate barley is replaced with low-phytate barley in feed formulation.
The ATTD of DM and OM were greater (P < 0.001) in pigs fed hull-less than hulled barley diets (Table 4), similar to other studies (Huang et al., 2003). Although the ATTD and retention of P did not differ, the amount of retained P was greater (P = 0.018) in pigs fed the hull-less compared with the hulled barley diets because intake of aP was greater for the hull-less barley diets (3.20 vs. 2.97 g/d); indicating that hull-less barley contained more total P and aP than hulled barley. The excretion of P in pigs fed the hull-less barley diets was greater than in pigs fed the hulled barley diets because the intake of phytate P was greater in hull-less barley (1.51 vs. 1.18 g/d).
The effects of iP and phytase supplementation of the HB-SBM diet and replacing HB with LPHB were investigated in Exp. 2. Because of the use of the same batches of HB and LPHB in both Exp. 1 and 2, the content of phytate P was considerably lower in the LPHB diet than in the HB diet and was reflected in a reduced phytate P in the LPHB-SBM diet. The average BW of the pigs was 20.7, 24.1, 32.4, and 38.9 kg at the beginning of experimental periods 1, 2, 3, and 4, respectively, and was 46.3 kg at the conclusion of the experiment. The daily intakes of P and aP ranged from 34 to 43% and from 31 to 49% of the NRC (1998) recommended intake, respectively.
As expected, the supplementation of iP to the HB-SBM diet, which was deficient in aP, increased the amount of P retained (P < 0.05; Table 5). However, the excretion of total P was also increased (P < 0.05). Similarly, phytase supplementation to the HB-SBM diet increased the ATTD, retention (%), and the amount of P retained, and simultaneously decreased the excretion of total P (P < 0.05). Similar results were previously reported in many studies with growing pigs when phytase was supplemented to diets deficient in aP (e.g., Lei et al., 1993; Mroz et al., 1994; Kemme et al., 1999).
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). Similarly, Harper et al. (1997) reported that growing-finishing pigs excreted 22% less P when phytase was supplemented to a diet deficient in P compared with a diet adequate in P. The supplementation of iP and phytase to the HB-SBM diets increased the daily amount of P retained to approximately the same extent (i.e., from 1.49 to 2.03 and from 1.49 to 1.97 g, respectively). These results are in agreement with Parr (1996) and Sauer et al. (2003) who reported that the supplementation of 500 phytase unit/kg of diet (as in this study) is equivalent to the supplementation of 1.0 g of P from monocalcium phosphate. Supplementation of the HB-SBM diet with 0.45% mono-calcium phosphate supplied approximately 1.0 g of P.
Another approach to improve P utilization and simultaneously reduce excretion, besides phytase supplementation, is the use of low-phytate feedstuffs. Replacing HB with LPHB in the HB-SBM diet increased (P < 0.05) the ATTD and retention (%) of P and amount of P retained, and decreased (P < 0.05) the excretion of total P (Table 5). The amount of P retained for pigs fed the LPHB-SBM and the HB-SBM diets supplemented with iP or phytase did not differ, although the amount of P retained by pigs fed the LPHB-SBM diet was 0.20 to 0.25 g/d lower than in pigs fed the other 2 diets.
Pigs fed the HB-SBM + phytase and the LPHB-SBM diet excreted 20.2 and 16.7% less total P, respectively (P < 0.05; Table 5), compared with pigs fed the HB-SBM diet. These results, in which normal-phytate barley was replaced with low-phytate barley, are in agreement with previous studies (Li et al., 2001a,b; Veum et al., 2002). Pigs fed the HB-SBM + phytase and LPHB-SBM diets excreted 32.4 and 29.3% less total P, respectively, compared with the HB-SBM + iP diet.
In the 2 experiments, the Ca:aP ratios in the diets ranged from 1.85:1 to 2.57:1 in Exp. 1 and 2.40:1 to 3.75:1 in Exp. 2, and the ratios remained below the recommended upper limit of 3:1 (NRC, 1998). In Exp. 1, an increased Ca:aP ratio was inversely related to P digestibility (R2 = 0.47), and in Exp. 2, this relationship also existed (R2 = 0.61) for the 3 diets that were not supplemented with phytase. A greater Ca:aP decreased P absorption, especially in diets that are marginal in P (NRC, 1998). Nonetheless, in Exp. 1, the Ca:aP ratio did not differ between the HB and LPHB diets, whereas P digestibility was 6% greater in the LPHB diet. The Ca:aP ratio was lower in the LPHLB than the HLB diet, but P digestibility was 8% greater for the LPHLB diet. These results indicate that although the dietary Ca:aP ratio might be important, P digestibility is changed more drastically by low-phytate cultivars of barley.
In conclusion, the results of Exp. 1 showed that the ATTD and retention of P were greater in pigs fed the low-phytate diets compared with the normal-phytate barley diets and less total P was excreted. However, the amount of P retained was decreased in pigs fed the low-phytate barley cultivars. The results of Exp. 2 showed that the replacement of HB with LPHB in the HB-SBM diet was as effective as phytase supplementation of the HB-SBM diet in increasing the amount of P retained and decreasing the excretion of total P. Low-phytate barley is as effective as supplemental phytase in improving P digestibility and utilization and decreasing P excretion in grower pigs.
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Footnotes |
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2 The authors thank K. Sauer and J. Hairong for technical assistance.
3 Present address: Degussa Canada Inc., PO Box 1000, Hwy 643 East, Gibbons, Alberta, Canada T0A 1N0.
4 Corresponding author: ruurd.zijlstra{at}ualberta.ca
Received for publication December 13, 2006. Accepted for publication June 20, 2007.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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