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* Departments of Animal and Poultry Science,
and
Microbiology, and
and
Molecular Biology and Genetics, University of Guelph, Guelph, Ontario, Canada N1G 2W1
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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recommended total and available P requirements in growing pigs, respectively. In conclusion, approximately 51% of the total P in conventional SBM is digested in growing pigs. The large intestine does not play an important role in the digestion of P associated with SBM in the growing pig. The fecal loss of the gastrointestinal endogenous P is an important route of P excretion in the growing pig.
Key Words: Digestibility • Losses • Phosphorus • Pigs • Soybean Oilmeal
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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; Jongbloed et al., 1991). Thus, accurate determination of bioavailability of P in feed ingredients and formulation of swine diets on the basis of bioavailable P supply are essential to ensure efficient P utilization (Jongbloed et al., 1991; Fan et al., 2001).
We have recently developed a methodology for simultaneous measurements of true P digestibility and the endogenous P outputs associated with feed ingredients for pigs (Fan et al., 2001). Our results suggest that true, rather than apparent, fecal P digestibility should be determined in feed ingredients and used in swine diet formulation (Fan et al., 2001). Furthermore, it has also been demonstrated that endogenous fecal output is an important route of body P excretion and the large intestine does not play a role in P digestion in weanling pigs (Fan et al., 2001).
However, it is likely that digestive utilization of P is subjected to developmental changes in endogenous secretions and microbial activity (Fan, 2003) and intestinal mucosal alkaline phosphatase activity (Fan et al., 2002). Therefore, it is logical to anticipate possible differences in true P digestibility and the endogenous P loss associated with soybean meal (SBM) between weanling and growing pigs. True P digestibility and the endogenous P loss associated with SBM have been reported for weanling pigs but not for growing pigs (Fan et al., 2001).
Therefore, the objectives of this study were to determine true P digestibility, the gastrointestinal endogenous P outputs associated with SBM and the role of the large intestine in P digestion in growing pigs.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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). The apparent digestible P in diets, expressed as g/kg DMI, are calculated from Eq. [1] according to previous studies (Fan et al., 1995b).
 | [1] |
where PAi represents the apparent ileal or fecal digestible P in the ith diet (g/kg DMI); PDi is the total P in the ith diet (g/kg DMI); and DA is the apparent ileal or fecal P digestibility values in the ith diet (%).
The outputs of P in ileal digesta and feces consist of both dietary and endogenous origins. If there are linear relationships between P outputs in ileal digesta or feces and the graded levels of P inputs from diets, when expressed as g/kg of DMI, their relationships can be expressed according to Eq. [2].
 | [2] |
where PAi and PDi are as defined in Eq. [1]
; PE is the endogenous P levels in the ileal digesta or feces (g/kg of DMI); and DT is the true ileal or fecal P digestibility values (%) in the P-containing assay ingredient.
Equation [2] represents a simple linear regression model in which PAi is the dependent variable, PDi is the independent variable, and PE and DT are the regression coefficients and are estimated by fitting these simple linear regression models. If there are linear relationships between the apparent ileal or fecal digestible intake and the total intake of dietary P with significant intercepts, the endogenous P level in ileal digesta or feces can be directly determined by extrapolating the dietary inputs of P to zero to obtain the intercepts of the linear regression equations (PE). However, if intercepts of these linear relationships are not significant, the endogenous P level in ileal digesta or feces can then be indirectly calculated from corresponding true P digestibility values as described below in the Calculation and Statistical Analysis section.
To determine true ileal and fecal P digestibility values in a P-containing ingredient, a series of assay diets are formulated to contain graded dietary levels of P but only from the assay ingredient. The dietary concentrations of antinutritive factors that likely affect P digestion and endogenous P levels should be controlled between the assay diets.
Animals and Surgery
Four Yorkshire barrows, with an average initial BW of 40 kg, were fitted with a simple T-cannula at the distal ileum according to procedures adapted from Li et al. (1993). After surgery, the animals were individually housed in metabolic cages in a temperature-controlled room (20 to 22°C). During a 7-d recovery period, the pigs were fed a 16% CP grower diet. A detailed description of pre- and postoperative care was previously presented by Li et al. (1993). The experimental protocol, surgical procedures, and procedures for care and treatment of the pigs were reviewed and approved by the University of Guelph Animal Care Committee in accordance with the guidelines established by CCAC (1993).
Diets and Experimental Design
Following recovery, the pigs were fed one of four experimental diets (Table 1) according to a 4 x 4 Latin square design. They were fed twice daily, equal amounts for each meal, at 0800 and 2000. The dietary allowance was 2,200 g/d during period 1 and increased by 200 g/d for each of the following periods. Water was freely available from low-pressure drinking nipples. At the end of the trial, the final BW of the barrows was approximately 58 kg.
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). The SBM sample was ground through a 4.8-mm mesh screen prior to diet incorporation. Corn oil was added to increase the digestible energy content similar to the level recommended by NRC (1998) and to reduce the dustiness of the diets; dextrose was included to improve the palatability of the diets. Vitamins and minerals were supplemented according to NRC (1998) standards. Chromic oxide was included in the diets as the digestibility marker for the determination of the digestibility values of the nutrients that were measured.
Sample Collection
Each experimental period comprised 8 d. After a 4-d adaptation, representative fecal samples were collected on d 5 and 6. Ileal digesta samples were collected for a total of 24 h from 0800 to 1000 on d 7, and every other 2 h thereafter until 0800 on d 8, and from 1000 to 1200 on d 8 and every other 2 h thereafter until 0800 on d 9. Ileal digesta samples were collected in soft plastic tubing (length, 10 cm; i.d., 1.5 cm), which was attached to the barrel of the cannula with Velcro tape. The tubing contained 10 mL of a solution of formic acid (2.86 M) to minimize further bacteria activity. The tubing was removed and replaced as soon as it was filled with digesta. Digesta samples were immediately frozen at -20°C.
Analytical Methods
At the end of the trial, the fecal and digesta samples were freeze-dried, pooled within pigs in the same period, ground to be homogenous through a 1-mm mesh screen, and mixed before analysis. The samples of the diets and soybean meal were similarly ground. All analyses were performed in duplicate. Analyses for DM were carried out according to AOAC (1993) methods. Chromic oxide was determined according to the procedure of Saha and Gilbreath (1991) by using an atomic absorption spectrometer (SpectrAA-10/20, Varian, Mulgrave, Australia). Approximately 1.0 g of diet and 0.5 g of digesta and fecal samples were weighed into 60-mL Pyrex beakers and ashed overnight at 550°C. Chromic oxide was then oxidized to dichromate by digestion in 6 mL of phosphoric acid (16.7 M)—manganese sulphate (13.5 mM) solution mixed with 8 mL of potassium bromate (0.27 M) solution on a hot plate with a surface temperature at about 350°C. Potassium dichromate was used as a standard. The absorbance for dichromate was read at 375 nm with a slit width of 0.5 nm on the atomic absorption spectrometer.
Analyses of total inorganic phosphate P in samples were carried out by spectrophotometeric analysis at 355 nm according to the procedure of Heinoen and Lahti (1981). Potassium monobasic phosphate was used as a standard inorganic phosphate compound for establishing standard curves. To partition total P in diet, digesta, and fecal samples into water-soluble inorganic phosphate and other forms of P, approximately 1.0 g of sample was weighed into 50-mL centrifuge tubes, mixed well, and centrifuged at 2,000 x g for 20 min to precipitate large particles. The supernatant was transferred into a 250-mL volumetric flask, brought up to the volume with distilled, deionized water and assayed for the content of inorganic phosphate P by using the same procedure as those described above. As the color reaction reagents did not react with any water-soluble organic phosphates in the supernatant samples, the difference between the total P and the water-soluble inorganic P was defined to be the water-insoluble P (Fan et al., 2000).
Calculations and Statistical Analyses
The apparent ileal and fecal digestibility values of DM and P in the experimental diets were calculated according to Eq. [3].
 | [3] |
where DAi represents apparent ileal and fecal P digestibility values in the assay diets (%); ID is digestibility marker concentration in the ith assay diet (%); PI is P concentration in ileal digesta or feces (%); II is digestibility marker concentration in ileal digesta or feces (%); and PD is P concentration in the ith assay diet (%).
Based on the apparent ileal and fecal P digestibility values and the levels of endogenous P extrapolated with regression analysis, the true P digestibility values in the assay diets, and also in the test ingredient, SBM, can be determined according to Eq. [4].
 | [4] |
Alternatively, the endogenous P outputs corresponding to individual diets can also be calculated according to Eq. [5] if corresponding true ileal and fecal P digestibility values are determined.
 | [5] |
where DTi, DAi, PE, and PDi are as defined in Eq. [2].
The digestibility values were first subjected to ANOVA for a 4 x 4 Latin square design including diets, animals, and periods. The diets were the major test factor, and animals and periods were the controlled factors. The intervals between the treatment levels of P were designed to be equal by increasing equal amounts of soybean meal (136 g/kg) in the diets at the expense of cornstarch. The treatment effect (diets) was partitioned and tested according to the equally spaced orthogonal polynomials based on the principle described by Steel and Torrie (1980). The ANOVA and the orthogonal polynomial analyses were carried out using the GLM procedures of (SAS Inst., Inc., Cary, NC). Related linear and curvilinear regression analyses were conducted by using the Fig. P program (Fig. P, 1993, Biosoft, Cambridge, U.K.). The comparison of true P digestibility and the endogenous P losses between the distal ileal and fecal levels and between the weanling and the growing-finishing pigs was conducted according to the pooled t-test (Byrkit, 1987).
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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for SBM. A recent study by Leech (2002) revealed that there was a large variability in total P and phytate P contents in 108 Ontario-grown soybean samples, and there was a strong linear relationship between total P and phytate P contents in soybean samples. Thus, phytate P content in the SBM sample used in this study was estimated by using the linear regression equation reported by Leech (2002). The estimated phytate P content is 0.36% on as-fed basis, accounts for 56.2% of the total P in the SBM sample used in this study, and is close to the value of 52 ± 3.7% of the total P as phytate P in soybean meal samples reported by Eeckhout and Paepe (1994).
Graded levels of dietary protein, calcium, and P intake as a result of graded levels of SBM inclusion did not affect normal digestive functions, and this was reflected by changes in apparent DM digestibility values (Table 2). There were linear decreases (P < 0.05) in the apparent ileal and fecal DM digestibility values, which was a direct result of replacing cornstarch with SBM, indicating that SBM has a lower apparent ileal DM digestibility than cornstarch (Table 2). There were increases (P < 0.05) in the apparent ileal and fecal P digestibility values in SBM, the highest values for the dietary P content of 0.293%, when the dietary P content was increased from 0.098 to 0.391%. The increases were 79.4 and 44.3 percentage units for the apparent ileal and fecal digestibility values, respectively (Table 2). There were no animal and period effects (P > 0.05).
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). The presence of this significant linear relationship suggests that the ileal and fecal P flow at different dietary levels can be linearly partitioned into the components of P of endogenous and exogenous sources. Thus, the true P digestibility values and gastrointestinal endogenous P outputs in the ileal digesta and feces were determined according to Eq. [2]. There were no differences (P > 0.05) between the ileal and fecal levels in true P digestibility values and the endogenous P output associated with SBM in growing pigs (Figures 2A and B).
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and 4). In general, there were linear increases (P < 0.05) in the total ileal and fecal P outputs, expressed as g/kg of DMI, as dietary P contents increased from 0.098 to 0.391% (Tables 3 and 4). However, the levels of endogenous P in ileal digesta and feces were not different (P > 0.05) between the diets when results were expressed as g/kg of DMI. On the other hand, ileal and fecal outputs of P of dietary origin showed a linear (P < 0.05) pattern of increases when these were expressed as g/kg of DM for digesta, feces, or diets from dietary P content of 0.098 to 0.391%. Furthermore, it is noteworthy that when expressed as g/kg of DM digesta or feces, total ileal P outputs were considerably lower than total fecal P outputs due to fermentation loss of organic nutrients in the large intestine, resulting in increased P concentrations in feces (Tables 3 and 4).
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, differences in the dietary levels of P had dramatic effects (P < 0.05) on the apparent ileal and fecal P digestibility values. However, these differences had no effects (P > 0.05) on the true P digestibility values (Figures 3A and B).
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). Between 90 and 93% of the total dietary P were not water-soluble inorganic phosphates and had to be released after enzymatic hydrolysis before absorption. In the distal ileal digesta, only about 3 to 4% of total P remained as water-soluble inorganic phosphates. However, the majority of P in ileal digesta (96 to 98%) did not exist as water-soluble inorganic phosphates and was not readily available for absorption (Table 5). In feces, between 12 and 16% of total P was converted into water-soluble inorganic phosphate. However, this proportion of P escaped absorption and was defecated in feces (Table 5).
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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There are three major phases in swine nutrition, including weanling (5 to 20 kg), growing (20 to 50 kg), and finishing (50 to 120 kg) phases (NRC, 1998). Unless there are no differences in true P digestibility values in feed ingredients between different feeding stages, true P digestibility values should be determined for each of the specific feeding phase and used for diet formulation. Therefore, the major objective of this study was to determine true P digestibility in SBM for growing pigs. Interestingly, we observed no period effects (P > 0.05) on true P digestibility in this study, suggesting that there are also no differences in true P digestibility values within the growing phase (data not shown). True ileal and fecal P digestibility values and the endogenous P outputs associated with SBM for weanling pigs were previously reported (Fan et al., 2001). The same source of soybean meal was used in this study as the weanling pig study by Fan et al. (2001). There were no differences (P > 0.05) in the ileal endogenous P outputs and fecal endogenous P outputs between the weanling and the growing pigs, respectively (Figure 2A). There were also no differences (P > 0.05) in the true ileal and true fecal P digestibility values in SBM between the weanling and the growing pigs, respectively (Figure 2B). However, these observations are only specific to P in SBM. Due to differences in texture and physicochemical properties, potential differences in digestive utilization of P between growth stages of pigs may exist for other feed ingredients. On the other hand, our comparison on true P digestibility values as affected by growth stages is different from previous reports on ileal crude protein and amino acid digestibility values in SBM (Fan et al., 1994; 1995a). For example, ileal CP digestibility in SBM was 67.0 ± 1.98 and 81.3 ± 1.05% in weanling and growing pigs, respectively. Digestibility values of the limiting AA, lysine, threonine and methionine, were 76.2 ± 2.37, 65.6 ± 2.17, and 77.4 ± 4.21% in SBM, respectively, for weanling pigs, whereas these were 86.3 ± 0.75, 76.6 ± 1.22, and 89.4 ± 0.49% in SBM, respectively, for growing pigs. Therefore, the effects of growth stages on nutrient digestibility values are dependent on types of nutrients examined.
The gastrointestinal endogenous fecal P loss is an inevitable route of body P excretion (Fan et al., 2001). However, there were no differences in the endogenous ileal and fecal P outputs between the weanling and the growing pigs when these were expressed as g/kg of DMI. Total and available P requirements are 0.67 and 0.56% for weanling (10 to 20 kg) vs. 0.36 and 0.26% for growing (20 to 50 kg) pigs, respectively, on a DM basis (NRC, 1998). Compared as a percentage of the recommended total and available P intake levels, the endogenous fecal P losses accounted for 4.6 and 8.6% in the weanling pig and 8.1 and 17.6% in the growing pig, respectively. Therefore, the endogenous fecal P loss is more important in growing pigs than in weanling pigs due to the relatively low P requirement level in growing pigs.
There were no statistical differences in the endogenous P output, when expressed as g/kg of DMI, between the ileal and the fecal levels (Figure 2A). The numerical difference of 0.13 g/kg of DMI suggested that about 22.4% of the endogenous P entering the large intestine was reabsorbed (Fan et al., 2001). Thus, it appears that the large intestine does play a role in partially recycling the endogenous P.
It was observed that there were no differences in true P digestibility values in SBM between the ileal and fecal levels in growing pigs, consistent with our previous findings in weanling pigs (Fan et al., 2001). Taken together, we conclude that the large intestine does not play a role in digestive utilization of dietary P from SBM in pigs. At present, there is a lack of cellular and molecular evidence to explain our whole-animal level observations. It is likely that transporters responsible for transcellular transport of inorganic phosphates are not expressed in the large intestinal mucosa. Further research needs to be conducted to verify this speculation.
As expected, only 3 to 4% of the total P in the ileal digesta remained as water-soluble inorganic phosphate (Table 5), suggesting that the end of the small intestine absorbed the majority of the hydrolyzed phosphate. In growing pig feces, between 12 and 16% of the total P was in the form of water-soluble inorganic phosphate (Table 5). This was much lower than the values of 40% observed in weanling pigs fed SBM-based diets (Fan et al., 2001). This discrepancy was likely due to the fact that relatively high levels of microbial activity occurred in growing pigs, which in turn resulted in an increased utilization of inorganic phosphates.
Many studies were conducted to measure apparent P digestibility and availability values in SBM for pigs (Jongbloed et al., 1991; Weremko et al., 1997). As summarized in Table 6, there was a large variability in the apparent P digestibility values between studies ranging from 24 to 41%. Similar to our recent study (Fan et al., 2001) with weaning pigs, this study observed much larger variability in the apparent ileal (-26.7 to 52.7%) and fecal P (3.7 to 48.1%) digestibility values in SBM than those reported in the literature (Table 6). Furthermore, there was also a large variability in the P availability values between studies ranging from 18 to 38% as determined by the slope-ratio assay. Intrinsic factors such as differences in phytate P content and intrinsic phytase activity in different SBM samples between studies might have, in part, contributed to this variability. However, the majority of this variability was likely due to the wide range in dietary P contents needed for our experiment (Fan et al., 2001). Our results confirmed the previous observations that differences in the P contents between studies were the largest single factor responsible for the large variability in the apparent P digestibility values reported in the literature (Fan et al., 2001). This is due to the fact that the relative contribution of the endogenous P loss, as a percentage of the total dietary P content, decreased as dietary P content increased from low to high levels (Figure 3). On the other hand, the large differences in P availability in SBM among studies also likely resulted from using different response criteria, as well as possible variation in experimental conditions associated with the slope-ratio assays, as was fully discussed by Ketaren et al. (1993a,b). We also conclude that true P digestibility values are better than apparent P digestibility or P availability values for formulating swine diets. Furthermore, a comparison of true fecal P digestibility of 51.8 ± 8% and the phytate P contribution of 56.2% to the total P in the SBM sample used in this study suggests that nearly all nonphytate P associated with SBM can be digested by growing pigs.
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Implications |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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Footnotes |
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2 Correspondence: Room 250, #70 Animal Science/Nutrition Building (phone: 519-824-4120, ext. 53656; fax: 519-836-9873; E-mail: mfan{at}uoguelph.ca).
Received for publication October 22, 2002. Accepted for publication June 11, 2003.
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Literature Cited |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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) and fecal (•) digestible dietary P intake (y: g/kg of DMI, mean ± SE, n = 4) and total dietary levels of P intake (x: g/kg of DMI) in growing pigs fed soybean meal-based diets varying from low to high in P content. A) Ileal level: linear relationship analysis, y = 0.590, x = 0.587, n = 16, r2 = 0.74, P < 0.05 for both the intercept and the slope of the equation; quadratic relationship analysis, Y = -17.981x2 +110.567x - 118.245, n = 16, R2 = 0.47, P > 0.05 for the quadratic parameter estimate, P < 0.05 for the linear and intercept parameter estimates; the data were also fitted according to various curvilinear models, such as exponential and sigmoidal models; however, associated parameter estimates were not significant, P > 0.05. (B) Fecal level: linear relationship analysis, y = 0.513x - 0.454, n = 16, r2 = 0.75, P < 0.05 for both the intercept and the slope of the equation; quadratic relationship analysis, Y = -7.615x2 + 49.716x - 40.364, n = 16, R2 = 0.58, P > 0.05 for the quadratic and the intercept parameter estimates, P < 0.05 for the linear parameter estimate; the data were also fitted according to various curvilinear models such as exponential and sigmoidal models; however, associated parameter estimates were not significant, P > 0.05.
), as reported by Fan et al. (2001)
) from this study determined by the regression analysis technique.