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Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1
2To whom correspondence should be addressed. E-mail: mfan{at}uoguelph.ca.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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35%. Current diet formulation on the bases of total, apparent P digestibility and availability values in corn inevitably leads to P overfeeding and excessive P excretion in pigs.
KEY WORDS: • phosphorus • endogenous output • true digestibility • corn • pigs
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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,2
). In addition, there is a large variation in P availability between feed ingredients due to differences in phytate-P content and intrinsic phytase activity (2
). As a result, formulation of swine diets on the basis of total P content is usually conducted with a big safety margin to compensate for these differences. For an example, as recommended by the National Research Council (NRC) (3
), total P requirements for growing and the early finishing phases are 0.50 and 0.45%, respectively, whereas available P requirements are 0.23 and 0.19%, respectively, on as-fed basis for these types of pigs. Therefore, accurate determination of bioavailability of P in feed ingredients and the formulation of swine diets on the basis of bioavailable P supply are essential to ensure efficient P utilization (2
,4
).
Digestibility studies and the slope-ratio assay are the two major evaluation systems for assessing the bioavailability of P in feed ingredients for pigs (2
,4
,5
). Digestibility studies estimate P availability by measuring its digestive utilization, whereas the slope-ratio assay provides a combined estimation of digestive and postabsorptive utilization of P at the tissue level (2
,6
). The slope-ratio assay is limited to, at least, the following aspects in assessing P bioavailability. First, assay results are very variable and are affected by assay criteria selected. For example, P availability in soybean meal was considerably lower (17.0%) using bone variable than the estimate (61%) based on empty body composition or P retention (7
,8
). Second, it is not clear whether the assay results for individual ingredients are additive in diet formulation. Third, it is relatively expensive for routine determination (7
,8
).
At present, apparent P digestibility and availability values in corn for pigs measured by the slope-ratio assay are reported and used in diet formulation (2
,5
). Corn is a major feed ingredient in commercial swine diets around the world. Three major issues face the use of apparent P digestibility values reported in corn for the formulation of swine diets. First, reported apparent P digestibility values of corn are highly variable among studies, ranging from 12 to 48% (2
,5
,6
). Second, apparent P digestibility values considerably underestimate the true digestive utilization of P (4
). Third, apparent P digestibility values measured in single feed ingredients are not always additive when used for diet formulation (9
). Therefore, it is less accurate to use apparent P digestibility values in swine diet formulation.
The gastrointestinal endogenous fecal P output is an important route of whole body P excretion in pigs (4
). However, it is not clear if feed ingredients per se can affect output of the endogenous fecal P. We have recently developed a novel methodology that allows simultaneous measurement of true P digestibility and the endogenous P output associated with assay ingredients (4
). However, this methodology was validated with soybean meal as a model ingredient. It is unknown whether this methodology, referred to as the regression analysis technique, is also suitable for cereal grains such as corn that is relatively low in P content compared with oil seed meals (3
).
In addition, the role of the large intestine in the digestion and absorption of plant sources of P remains a controversial topic. As measured by apparent P digestibility, differences between the ileal and fecal digestibility values were not consistent in previous studies (9
). Recent studies with weanling pigs suggested that the large intestine did not play a role in absorption of the dietary source of P (4
). Further studies should be conducted on this particular topic.
Thus, the objectives of this study were to determine true P digestibility and the endogenous P output associated with corn for growing pigs. A related objective was to examine whether the large intestine plays a role in digestive utilization of both the exogenous and the endogenous P in pigs.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Determination of the gastrointestinal endogenous nutrient outputs by the regression analysis technique relies on establishing linear relationships between apparent digestible and total intake of assay nutrients in diets (10
). The apparent digestible P contents in diets, expressed as g/kg dry matter intake (DMI), are calculated from Equation 1
according to the previous studies (10
).
 | (1) |
where PAi represents the apparent ileal or fecal digestible P content in the ith diet (g/kg DMI), PDi is the total P content in the ith diet (g/kg DMI) and DA is the apparent ileal or fecal P absorption values in the ith diet (%).
The outputs of P in ileal digesta and feces consist of both dietary and the 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 DMI, their relationships can be expressed according to Equation 2
.
 | (2) |
where PAi represents the apparent ileal or fecal digestible P content in the ith diet (g/kg DMI) determined from Equation 1 , PE is the endogenous P levels in the ileal digesta or feces (g/kg DMI), DT is the true ileal or fecal P absorption values (%) in the P-containing assay ingredient and PDi is the total P content in the ith diet (g/kg DMI).
Equation 2 represents a simple linear regression model in which PAi is the dependent variable and PDi is the independent variable. PE and DTare the regression coefficients and are estimated by fitting this simple linear regression model. If there are linear relationships between the apparent ileal and fecal digestible intake and the total intake of dietary P with significant intercepts, the endogenous P level in ileal digesta or feces can then be directly determined by extrapolating the dietary inputs of P to zero by obtaining the intercepts of the linear regression equations (PE).
Animals, diets and experimental design.
Four Yorkshire barrows, with an average initial body weight of 25 kg, were obtained from the University of Guelph Arkell Swine Research Station swine herd and surgically fitted with a simple T-cannula at the distal ileum according to the procedures adapted from previous studies (11
). After surgery, the animals were individually housed in stainless steel metabolic crates in a temperature-controlled room (20–22°C). During a 7-d recovery period, the barrows were fed a regular grower diet. A detailed description of pre- and postoperative care was previously presented (11
).
After recovery, the barrows were fed one of the four experimental diets (Table 1)
according to a 4 x 4 Latin square design. The pigs were fed twice daily, equal amounts each meal, at 0800 and 2000 h. The dietary allowances were 1300, 1500, 1800 and 2150 g/d during periods 1, 2, 3 and 4, respectively. Water was freely available from a low-pressure drinking nipple. The barrows were electrically stunned before they were killed at the conclusion of the experiment and dissected to determine whether cannulation had caused intestinal abnormalities. The average body weight of the barrows at the conclusion of the experiment was 45 kg.
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) and to reduce the dustiness of the diets; dextrose was included to improve the palatability of the diets. Solkafloc was included to balance neutral-detergent fiber content between diets. Vitamins and other minerals were supplemented according to the NRC (3
). Chromic oxide was included in the diets as the digestibility marker for the determination of the apparent nutrient digestibility values in the diets. Each experimental period comprised 8 d. After a 4-d adaptation, all possible fecal samples were collected on d 5 and 6. Ileal digesta samples were collected for a total of 24 h: from 0800 to 1000 h on d 7 and every other 2 h thereafter until 0800 h on d 8 and from 1000 to 1200 h on d 8 and every other 2 h thereafter until 0800 h on d 9. Ileal digesta 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 mol/L) to minimize further bacterial activity. The tubing was removed and replaced as soon as it was partially filled with digesta. Digesta were immediately frozen at -20°C.
The experimental proposal, surgical procedures and procedures for care and treatment of the barrows were reviewed and approved by the University of Guelph Animal Care Committee. The animals used in this experiment were cared for in accordance with the guidelines established by Canadian Council on Animal Care (12
).
Chemical analyses.
After the conclusion of the experiment, the digesta and fecal samples were freeze-dried, pooled within the same barrow and period for the same diet, ground through a 1-mm mesh screen, and mixed before analysis. The samples of the diets and corn were ground similarly. Analyses were performed in duplicate.
Analyses for DM were carried out according to procedures of the Association of Official Analytical Chemists (13
). Analyses for neutral-detergent fiber were carried out according to an established procedure (14
).
Chromic oxide was determined (15
) by using an atomic absorption spectrometer (SpectrAA-10/20; Varian, Mulgrave, Australia). Approximately 1.0 g of diet and 0.4–0.6 g of digesta and fecal samples were weighed into 100-mL Pyrex beakers and ashed overnight at 550°C. Chromic oxide, as part of ash, was then oxidized to dichromate by digestion in 6 mL of phosphoric acid (16.7 mol/L)-manganese sulfate (13.5 mmol/L) solution mixed with 8 mL of potassium bromate (0.27 mol/L) solution on a hot plate. Potassium dichromate was used as 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 P in samples were carried out by spectrophotometric analysis at 355 nm (16
). Potassium monobasic phosphate was used as standard. To partition total P contents in diet, digesta and fecal samples into water-soluble inorganic phosphate-P and water-insoluble P, 1.0 g of sample was weighed into 50-mL centrifuge tubes, mixed and centrifuged at 2000 x g for 20 min to precipitate large particles. The supernatant was transferred into a 250-mL volumetric flask and assayed for the content of inorganic phosphate-P. Because 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 contents was defined to be the water-insoluble P (4
).
Calculations and statistical analyses.
The apparent ileal and fecal digestibility values of DM and P in the experimental diets were calculated according to marker technique (4
).
Based on the apparent ileal and fecal P digestibility values and the levels of endogenous P extrapolated with the regression analysis, the true P digestibility values in the assay diets, also in the test ingredient, corn, can be determined according to Equation 3
.
 | (3) |
Alternatively, the endogenous P outputs corresponding to individual diets can also be calculated according to Equation 4
, if corresponding true ileal and fecal P digestibility values are determined.
 | (4) |
where DTi is true ileal and fecal P digestibility values in the assay diets (%), DAi is apparent ileal and fecal P digestibility values in the assay diets (%), PE is the levels of endogenous P in ileal digesta and feces (g/kg DMI), and PDi is P content in the assay diets (g/kg DM diet).
The digestibility values were first subjected to three-way ANOVA for a 4 x 4 Latin square design. The intervals between the treatment levels of P were designed to be equal by increasing an equal amount of corn (180 g/kg) in the diets at the expense of cornstarch. The treatment effect was, therefore, partitioned and tested according to equally spaced orthogonal polynomial analyses (17
). The ANOVA and the orthogonal polynomial analyses were carried out using the General Linear Procedures of SAS (18
). Related linear and curvilinear regression analyses were conducted by using the Fig. P program (Fig. P; Biosoft, Cambridge, UK). The comparison of true P digestibility values and the endogenous P outputs between the ileal and the fecal levels was conducted according to the pooled t test (19
).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Corn ground according to commercial practice was used as a test ingredient and contained 3.6 g/kg total P on as-fed basis. This P content is higher than the value of 2.8 g/kg summarized by the NRC (3
). Corn grain usually contains 68 ± 5.9% of total P as phytate P and 15 ± 18 U/kg intrinsic phytase activity (20
).
Graded levels of dietary protein, calcium and P intake, as a result of graded levels of corn inclusion, did not affect normal digestive functions as was reflected by changes in apparent DM digestibility values (Table 2)
. There were quadratic increases in the apparent ileal and linear increases the apparent fecal P digestibility values (P < 0.05) in corn when the dietary P content was increased from 0.7 to 2.8 g/kg DMI. The increase was 84.3 and 80.5 percentage units for the ileal and fecal P digestibility values, respectively (Table 2)
. In addition, there were no animal or period effects (P > 0.05) on apparent DM and P digestibility values (data not presented).
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, A and B). The presence of this significant linear relationship suggests that the ileal and fecal total outputs of P at different dietary levels can be linearly partitioned into the components of P of the endogenous and the exogenous sources. Thus, the gastrointestinal endogenous P outputs in ileal digesta and feces were determined according to Equation 2
. As shown in Figure 2
A, there were no differences (P > 0.05) in the endogenous P output between the ileal and the fecal determinations. True ileal and fecal P digestibility values in corn were determined according to Equation 2
. Furthermore, there were no differences (P > 0.05) between the true ileal and fecal P digestibility values associated with corn (Fig. 2B
).
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and 4). The patterns of P flows were similar in ileal digesta and feces. In general, there were linear and quadratic increases (P < 0.05) in the total ileal and fecal P outputs, expressed as g/kg DM for digesta, feces or diets, as dietary P levels increased from diets 1 to diet 4. There were no differences (P > 0.05) in the levels of the endogenous P outputs in ileal digesta and feces between the diets when these were expressed as g/kg DM digesta or feces. Furthermore, the levels of the endogenous P in ileal digesta and fecal were not different (P > 0.05) between the diets when these results were expressed as g/kg DMI. In contrast, ileal and fecal outputs of P of dietary origin showed linear and quadratic patterns of increases (P < 0.05) when these were expressed as g/kg DM digesta or feces or DMI from diets 1 to diet 4 (Tables 3
and 4
). Furthermore, it is noteworthy that when expressed as g/kg DMI, the endogenous P flow was similar (P > 0.05) at the ileal and fecal levels. There were also no differences (P > 0.05) in the levels of P originating from dietary sources between the fecal and the ileal levels when these were expressed as g/kg DMI (Tables 3
and 4)
.
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. Thus, differences in the dietary levels of P had dramatic effects (P < 0.05) on the apparent ileal and fecal P digestibility values (Fig. 3
, A and B). However, these had no effects (P > 0.05) on the true P digestibility values (Fig. 3
, A and B). In contrast, relative contributions of endogenous P outputs, as a percentage of total dietary P content, were not constant and decreased exponentially (P < 0.05) in response to the increases in the dietary P contents (Fig. 4
). Thus, the endogenous P contributions had relatively larger effects on their respective apparent P digestibility values at lower than at higher levels of dietary P.
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. Approximately 93% of total dietary P from corn was not water-soluble inorganic phosphate-P, likely in the forms of phytate P and P associated with DNA, RNA and proteins. These forms of P had to be hydrolyzed before absorption. In ileal digesta, 5% of total P was in the form of water-soluble inorganic phosphate P. The majority of the P (95%) in ileal digesta existed as the water-insoluble P that was not readily available for absorption as shown in Figure 5A
. In feces, between 16 and 19% of total P was converted into water-soluble inorganic phosphate P. However, this proportion of P escaped absorption and was excreted in feces as shown in Figure 5B
. Furthermore, the majority of P (80%) in feces was in the form of water-insoluble P.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). However, cereal grains such as corn are much lower in P than oilseed meals. The major concern in using the regression analysis technique for cereal grains is whether there are linear relationships between the ileal and fecal P outputs and the dietary inputs of total P, and between the apparent ileal and fecal digestible P and the total inputs of dietary P. The P contents, ranging from 1.1 to 4.3 g/kg DMI, were used in the previous studies with soybean meal (4
), whereas P contents from 0.67 to 2.83 g/kg DMI were used in this study with corn (Table 2)
. Thus, there were 30–40% lower P contents in the present corn study than those of the previous studies with soybean meal (4
). In spite of the differences in the contents of P gradients between the corn and the previous soybean meal studies, there were also linear relationships (P < 0.05) between the ileal and fecal outputs and dietary inputs of total P, and between the apparent ileal and fecal digestible and the total intake of dietary P, when expressed as g/kg DMI in corn of this study. These linear relationships suggest that true P digestibility values and the endogenous P outputs associated with corn and other cereal grains can be determined by the regression analysis technique.
In this study, the endogenous P outputs were determined to be 0.693 ± 0.128 in ileal digesta and 0.670 ± 0.166 g/kg DMI in feces, respectively. Thus, for pigs between 20 and 45 kg body weight with voluntary feed intake at 5% of their body weight, the endogenous P outputs were estimated to be 0.62 and 1.40 g/d in ileal digesta and feces. Ileal and fecal endogenous P outputs were reported to be 0.86 ± 0.09 and 0.31 ± 0.06 g/kg DMI, respectively, in weanling pigs fed soybean meal (4
). Compared with soybean meal for weanling pigs, the values associated with corn for growing pigs were similar for ileal endogenous P outputs, but were relatively higher for the fecal endogenous outputs. Intrinsic differences between the two feed ingredients as well as possible differences between developmental stages were likely responsible for this discrepancy.
Total and available P requirements are 0.50 and 0.23%, respectively, on as-fed basis, for pigs between 20 and 45 kg body weight (3
). In this study, the ileal endogenous P output associated with corn represents 12.3% of the animal’s (20–45 kg) daily requirement for total P and 26.6% of the animal’s daily requirement for available P, respectively. Fecal endogenous P output represents 12.0% of the animal’s daily requirement for total P and 25.8% of the animal’s (20–45 kg) daily requirement for available P, respectively. Therefore, fecal endogenous P output contributes to a large portion of daily P requirement and is an inevitable metabolic P loss.
There was no difference, expressed as g/kg DMI, between the ileal and the fecal endogenous P outputs (Fig. 2A
), suggesting that the large intestine does not play a role in secreting endogenous P. Furthermore, we also observed no recycling of the endogenous P recovered from the upper gastrointestinal tract in the large intestine from this study. However, recycling of the endogenous P recovered from the upper gastrointestinal tract in the large intestine was reported in weanling pigs fed soybean meal-based diets in previous studies (4
).
By aid of the regression analysis technique, true ileal and fecal P digestibility values in corn for growing pigs were determined and there was no difference between true ileal and fecal P digestibility values (54.0 ± 6.5 vs. 59.8 ± 8.5%; P > 0.05). This indicates that the large intestine does not play a major role in the absorption of exogenous P. This observation is consistent with the previous reports (4
,21
). Furthermore, the results from this study also suggest that 16–19% of total P in feces were water-soluble inorganic phosphates (Fig. 5B
). This form of P is readily available for absorption. Because only 5% of total P leaving the small intestine was water-soluble inorganic phosphates (Fig. 5A
), microbial fermentation in the large intestine might have played a role in converting the water-insoluble P including phytate P into free inorganic phosphates. This is consistent with reports that various microbes, such as E. coli, secrete phytase that specifically hydrolyzes phytate-associated phosphates (22
). Relatively speaking, less P leaving the small and the large intestine was in the form of water-soluble inorganic P in the growing pigs in this study than that in the weanling pigs from the previous report (4
). This discrepancy may be due to the following. First, total P loading levels were considerably higher in the previous studies than the levels used in this study. Second, the capacity of P absorption in the small intestine and the microbial growth activities in the large intestine of the growing pigs were likely to be relatively large. This is the first report of the determination of true P digestibility in corn fed to growing pigs. True P digestibility values determined in this study (54–60%) suggest that an average of 57% of total P in conventional corn could be digested and absorbed by growing pigs.
Many studies have been conducted to measure apparent P digestibility and availability values in corn for pigs (1
,2
,6
). As summarized in Table 6
, there is a large variability in apparent P digestibility values among studies ranging from 12 to 48% (averaged 22.6%). Furthermore, there is also a large variability in P availability among studies, ranging from 9 to 44%, as determined by the slope-ratio assay. Intrinsic factors such as differences in phytate-P content and intrinsic phytase activity in different samples of corn used in the studies might have, in part, contributed to this variability (4
). In this study, much larger variabilities in apparent ileal (-51.0–33.3%) and fecal P (-41.4–39.1%) digestibility were observed than those reported in the literature (Table 6)
. This is due to the fact that large differences in P content between diets were created in this study to demonstrate principles. These data suggest that differences in P contents among studies are the largest single factor responsible for the large variability in the apparent P digestibility values in corn reported in the literature. The relative contribution of the endogenous P outputs, as a percentage of total dietary P contents, decreased exponentially as dietary P content increased (Fig. 4
). Furthermore, the average apparent P digestibility and availability values are 22% in the literature, whereas true P digestibility is 57% in corn as determined from this study. Therefore, the current literature data underestimate the true digestive utilization of P in corn for pigs by 35%.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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3 Abbreviations used: DM, dry matter; DMI, dry matter intake; NRC, National Research Council; P, phosphorus.
Manuscript received 16 November 2001. Initial review completed 13 December 2001. Revision accepted 14 February 2002.
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Fan, M. Z., Archbold, T., Sauer, W. C., Lackeryam, D., Rideout, T., Gao, Y., de Lange, F. M. & Hacker, R. R. (2001) Novel methodology allows simultaneous measurement of true phosphorus digestibility and the gastrointestinal endogenous phosphorus outputs in studies with pigs. J. Nutr. 131:2388-2396.
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R. N. Dilger and O. Adeola Estimation of true phosphorus digestibility and endogenous phosphorus loss in growing pigs fed conventional and low-phytate soybean meals J Anim Sci, March 1, 2006; 84(3): 627 - 634. [Abstract] [Full Text] [PDF]
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T. W. Kim and X. G. Lei An improved method for a rapid determination of phytase activity in animal feed J Anim Sci, May 1, 2005; 83(5): 1062 - 1067. [Abstract] [Full Text] [PDF]
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C. H. Stahl, K. R. Roneker, W. G. Pond, and X. G. Lei Effects of combining three fungal phytases with a bacterial phytase on plasma phosphorus status of weanling pigs fed a corn-soy diet J Anim Sci, June 1, 2004; 82(6): 1725 - 1731. [Abstract] [Full Text] [PDF]
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C. W. Forsberg, J. P. Phillips, S. P. Golovan, M. Z. Fan, R. G. Meidinger, A. Ajakaiye, D. Hilborn, and R. R. Hacker The Enviropig physiology, performance, and contribution to nutrient management advances in a regulated environment: The leading edge of change in the pork industry J Anim Sci, February 1, 2003; 81(14_suppl_2): E68 - 77. [Abstract] [Full Text] [PDF]
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