Phosphorus digestibility response of broiler chickens to dietary calcium-to-phosphorus ratios

Phosphorus digestibility response of broiler chickens to dietary calcium-to-phosphorus ratios J. B. Liu,*† D. W. Chen,†1 and O. Adeola* *Department of Animal Sciences, Purdue University, West Lafayette, IN 47907; and †Institute of Animal Nutrition, Sichuan Agricultural University, Ya’an, Sichuan 625014, P. R. China ABSTRACT This study was conducted to evaluate the true digestibility of P in soybean meal (SBM) for broiler chickens fed diets with different dietary calcium-tophosphorus ratios (Ca:P) using the regression method. The experiment used a 4 × 3 factorial arrangement with 12 diets formulated to contain combinations of 4 levels of dietary Ca:P: 0.8, 1.2, 1.6, or 2.0 and 3 levels of SBM: 31.0, 44.0, or 57.0%. A total of 576 male Ross 708 broilers were allocated to 12 dietary treatments with 8 cages (6 birds per cage) per treatment from d 15 to 22 posthatching, and the BW between groups were similar. Chromic dioxide was used as an indigestible marker to calculate P digestibility and retention. The results showed that BW gain and feed efficiency were increased (linear, P < 0.01), and prececal DM digestibility and DM retention were decreased (linear, P < 0.01) with graded SBM in diets for each Ca:P. De-

creasing linear (P < 0.01) relationships were observed for apparent prececal P digestibility and total tract P retention with increased dietary SBM levels. The prececal and excreta P output increased (linear, P < 0.01; quadratic, P < 0.05) as increasing levels of SBM were added to the experimental diets. True prececal P digestibility in SBM was greater (P < 0.05) for birds fed a diet with Ca:P of 0.8 compared with those fed higher Ca:P, but there was no difference among the Ca:P ratios between 1.2 and 2.0. However, the total tract retention of P from SBM was not affected by Ca:P between 0.8 and 2.0. In conclusion, results of the present experiment demonstrated that prececal digestibility of P in SBM was not affected by Ca:P ratio between 1.2 and 2.0; and there was no difference in total tract retention of P from SBM among the Ca:P ratios between 0.8 and 2.0 in broiler chickens.

Key words: broiler chicken, calcium-to-phosphorus ratio, phosphorus, soybean meal, true digestibility 2013 Poultry Science 92:1572–1578 http://dx.doi.org/10.3382/ps.2012-02758

INTRODUCTION Utilization of plant P by chickens is poor because a significant proportion of the total P in the vegetable feed ingredients is bound to phytate (Eeckhout and Paepe, 1994). Therefore, highly digestible inorganic P is routinely incorporated into the diets for optimal growth performance. Such practice, however, incurs the excretion of P into the environment (Honeyman, 1993). To ameliorate the environmental influence of P, phytase and genetically selected feedstuffs that contain less phytate-bound P have been introduced into poultry diets (Raboy et al., 2001; Dilger et al., 2004; Nyannor et al., 2009). Phosphorus digestibility by poultry has been shown to be affected by the amount of Ca and P and their ratio in the diet (Günther and al-Masri, 1988; al-Masri, 1995; Driver et al., 2005; Selle et al., 2009) due to the antagonizing relationship between Ca and P in the in©2013 Poultry Science Association Inc. Received September 9, 2012. Accepted February 2, 2013. 1 Corresponding author: [email protected]

testine of broiler chickens. Several studies have shown that dietary P concentration is an important factor that affects apparent P digestibility by poultry (Qian et al., 1996; Rodehutscord and Dieckmann, 2005). High variability exists between and within laboratories when determining the digestibility of P in diets (Rutherfurd et al., 2002; Dilger et al., 2004; Rutherfurd et al., 2004; Dilger and Adeola, 2006; Selle et al., 2009). We surmise this might be attributable to varying dietary Ca-to-P ratio (Ca:P). Therefore, we tested the hypothesis that increasing the dietary Ca:P ratio affects P utilization in soybean meal (SBM) by broiler chickens. Investigation of the impact of dietary Ca:P ratio on P utilization in SBM by broiler chickens will help to enhance the utilization of P in plant ingredients and ameliorate the environmental impact of the required use of P in poultry diets.

MATERIALS AND METHODS The animal procedures used in the present study were approved by the Purdue Animal Care and Use Committee.

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as 1 g of chromic dioxide mixed with 4 g of soybean meal. per kilogram of diet: retinyl acetate, 688 mg; cholecalciferol, 7.5 μg; dl-α-tocopheryl acetate, 20 mg; menadione, 0.52 mg; thiamine, 4 mg; niacin, 15 mg; riboflavin, 4 mg; pantothenic acid, 12 mg; vitamin B12, 15 μg; pyridoxine, 2 mg; d-biotin, 0.1 mg; folic acid, 0.5 mg; choline, 0.6 g; Fe (ferrous sulfate), 90 mg; Mn (manganese oxide), 5 mg; Cu (copper sulfate), 8 mg; I (potassium iodate), 0.20 mg; Se (sodium selenite), 0.21 mg; Zn (zinc sulfate), 90 mg. 2Provided

1Prepared

42.00 21.60 2.00 5.00 15.00 10.00 0.40 1.25 2.50 0.25 100.00 42.00 21.93 2.00 5.00 15.00 10.00 0.40 0.92 2.50 0.25 100.00 29.00 35.18 2.00 5.00 15.00 10.00 0.40 0.67 2.50 0.25 100.00 55.00 9.52 2.00 5.00 15.00 10.00 0.40 0.33 2.50 0.25 100.00 42.00 22.60 2.00 5.00 15.00 10.00 0.40 0.25 2.50 0.25 100.00 Soybean meal Cornstarch Soy oil Casein Sucrose Dextrose Salt Limestone Chromic dioxide premix1 Mineral-vitamin premix2 Total

29.00 35.68 2.00 5.00 15.00 10.00 0.40 0.17 2.50 0.25 100.00

0.8 Ingredient, %

Table 1. Diet formulation of experimental diets (as-fed basis)

                   

 

29.00 35.43 2.00 5.00 15.00 10.00 0.40 0.42 2.50 0.25 100.00

42.00 22.27 2.00 5.00 15.00 10.00 0.40 0.58 2.50 0.25 100.00

55.00 9.11 2.00 5.00 15.00 10.00 0.40 0.74 2.50 0.25 100.00

                   

  1.2

Dietary Ca:P ratio

1.6

55.00 8.68 2.00 5.00 15.00 10.00 0.40 1.17 2.50 0.25 100.00

                   

29.00 34.93 2.00 5.00 15.00 10.00 0.40 0.92 2.50 0.25 100.00

2.0

55.00 8.27 2.00 5.00 15.00 10.00 0.40 1.58 2.50 0.25 100.00

PHOSPHORUS DIGESTIBILITY RESPONSE OF BROILERS

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Birds and Management One-day-old male broiler chicks (Ross 708) were used in the current study. The room temperatures were maintained at 35°C from d 1 to 8 posthatching and 32°C from d 8 to 15 posthatching. A standard broiler starter diet containing 54.22% corn, 36% SBM, 5% soybean oil, 2% monocalcium phosphate, 1.3% limestone, 0.4% NaCl, 0.3% vitamin-mineral premix, 0.38 dl-methionine, 0.29% lysine∙HCl, and 0.11% threonine that contained per kilogram: 226 g of CP, 3,208 kcal of MEn, 10.0 g of Ca, and 5.0 g of nonphytate P, was fed from d 1 to 15 posthatching. At d 15 posthatching, chicks were individually weighed, and 576 chicks were selected and assigned to 12 diets of a randomized complete block design such that the average BW among treatment groups was similar, which gave 8 replicate cages per treatment with 6 chicks per cage. Chicks had free access to feed and water from d 15 to 22 posthatching, and the room temperatures were maintained at 27°C. The BW of individual chicks and feed consumption per cage were recorded at d 22 posthatching.

Dietary Treatments A randomized complete block design with a 4 × 3 factorial arrangement of treatments was used to investigate the response of broiler chickens to dietary Ca:P ratios (0.8, 1.2, 1.6, or 2.0) combined with 3 graded levels of SBM (31.0, 44.0, or 57.0%) in the diets. Diets (Table 1) were semipurified, consisting primarily of cornstarch, sucrose, dextrose, casein, and SBM. Casein (for baseline protein) and SBM were the only dietary sources of P, and limestone was included into the diet to adjust the dietary Ca:P ratio. Chromic dioxide was incorporated into the diets at 5 g/kg on an as-fed basis as an indigestible marker. The calculated and analyzed nutrient profiles of experimental diets are presented in Table 2.

Sample Collection and Chemical Analyses Excreta samples were collected from pans beneath each cage between d 18 and 21 posthatching and dried in a forced-air oven at 55°C for 5 d. At d 22 posthatching, chicks were euthanized via carbon dioxide asphyxiation, and the prececal digesta samples from the distal two-thirds of the entire ileum (defined as extending from Meckel’s diverticulum to the ileocecal junction) were collected. Contents of this segment were flushed into a plastic container using distilled water, pooled per cage of 6 chicks, and subsequently lyophilized. Left tibias from all chicks were collected at d 22 for determination of tibia ash percentage on a fat-free DM basis. The percentage tibia ash was determined after ether extraction, followed by ashing at 600°C for 16 h. Diets, dried prececal digesta, and excreta samples were ground using a grinding mill (Retsch ZM 100,

1574 313 4,091 7.6 4.2 254 4,035 6.3 3.3 191 4,003 4.7 2.6         319 4,099 6.4 4.1 188 4,031 2.7 2.4

189 4,017 3.8 2.6         315 4,125 5.0 4.1 262 4,076 4.1 3.4

250 4,056 4.9 3.4

321 3,007 8.0 4.0 258 3,226 6.3 3.2 321 3,023 6.4 4.0 258 3,240 5.1 3.2 195 3,456 3.8 2.4         321 3,041 4.8 4.0 195 3,466 2.8 2.4

258 3,253 3.8 3.2

420 290 550

        318 4,118 3.4 4.2 258 4,063 2.8 3.3 194 4,037 2.1 2.6

321 3,057 3.2 4.0 258 3,267 2.5 3.2 195 3,476 1.9 2.4

Calculated nutrient composition   Protein, g/kg   ME, kcal/kg   Ca, g/kg   P, g/kg Analyzed nutrient composition   Protein, g/kg   Gross energy, kcal/kg   Ca, g/kg   P, g/kg

420 Item

290

550

       

 

290

420

1.6 1.2 0.8

Dietary Ca:P ratio (dietary soybean meal, g/kg)

Table 2. Analyzed and calculated nutrient composition of experimental diets (as-fed basis)

550

       

195 3,446 4.7 2.4

420 290

2.0

550

Liu et al.

Retsch GmbH and Co., K.G., Haan, Germany). Dry matter content was determined by drying samples at 105°C for 24 h. Diets, digesta, and excreta samples were prepared by a nitric-perchloric acid wet ash before the determination of Ca, Cr, and P contents (AOAC International, 2006). Chromium concentration was measured at 440 nm using a spectrophotometer (Spectronic 21D, Milton Roy Co., Rochester, NY; AOAC International, 2006). Acid molybdate and Fiske-Subbarow reducer solutions were used to measure the concentration of P through the formation of a phosphomolybdenum complex by spectrophotometric reading of absorption at a wavelength of 620 nm (Spectronic 21D, Milton Roy Co., Rochester, NY; AOAC International, 2006). Concentration of Ca in the supernatant was measured using flame atomic absorption spectrometry (Varian FS240 AA Varian Inc., Palo Alto, CA). Gross energy was measured by adiabatic bomb calorimetry (model 1261, Parr Instrument Co., Moline, IL) using benzoic acid as an internal standard, and EDTA was used as an internal standard to determine the nitrogen content by the combustion method (model FP2000, Leco Corp., St. Joseph, MI; AOAC International, 2006).

Calculations and Statistical Analyses Apparent P utilization (prececal digestibility and overall retention) was determined by the index method according to the following equation: APU (%) = 100 − [(CrI/CrO) × (PO/PI) × 100], where APU is the apparent P utilization (prececal digestibility or overall retention) expressed as a percentage; CrI is the chromium concentration of dietary intake; CrO is the chromium concentration of output (in prececal digesta or excreta); PI is the P concentration of dietary intake; and PO is the P concentration of output (in prececal digesta or excreta). All values used were expressed as milligrams per kilogram of DM. The P output in prececal digesta or excreta, expressed on a DM intake (DMI) basis, were calculated using the following equation: PO-DMI (mg/kg) = PO-DMO × (CrI/CrO), where PO-DMI and PO-DMO represent P output concentrations on a DMI and DM output (DMO) basis, respectively. In the present study, P output was regressed against dietary P intakes for each of the 4 dietary Ca:P ratios as described by Dilger and Adeola (2006) using the following statistical model: PO-DMI (mg/kg) = (TPI × PI) + EPL, where TPI represents the true P indigestibility, and EPL is the endogenous P loss for each dietary Ca:P ratio on a DMI basis. In this equation, TPI and EPL

0.903 0.651

TPU (%) = 100 − (TPI × 100), where TPU is the true P utilization (in prececal digesta or excreta) in percentage. All data were analyzed using the GLM Procedure (SAS Institute Inc., Cary, NC). Cage served as the experimental unit for all statistical analysis, and α levels of 0.05 (significant) and 0.01 (highly significant) were used. The model for this analysis included block (7 df), dietary Ca:P ratios (3 df), dietary SBM inclusion levels (2 df), and the interaction between dietary Ca:P ratios and SBM inclusion levels (6 df). Linear and quadratic contrasts were used for the effects of SBM levels on growth performance, apparent P utilization, and P output within each dietary Ca:P ratio. Regression coefficients were compared between dietary Ca:P ratio and type of samples (prececal digesta or excreta) by the CI derived from SE of respective regression coefficients.

0.951 0.206 0.312 0.510 0.273 replicate cages per treatment (6 chicks per cage). of fat-free tibia on DM basis. 2Percentage

46.7 45.9

46.9

46.6

47.9

47.8

46.8

47.5

47.2

47.6

47.4

47.2

0.42

RESULTS

1Eight

78.3 82.1

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are the slope and intercept of a simple linear regression of PO-DMI on PI, respectively. True P indigestibility is the inefficiency with which dietary P is extracted by the chick. Therefore, true P utilization is calculated as

0.295

0.557 <0.001 0.201

78.3 80.2

73.2

83.7

77.0

73.5

83.6

79.5

73.2

82.5

78.1

73.0

0.42

<0.001

<0.001

0.002

<0.001

0.024

0.551 <0.001 0.025

688 500 607 487

71.2

82.2

76.2

69.5

81.1

76.9

70.4

79.6

73.5

68.9

0.94

<0.001

<0.001

0.740

<0.001

0.327

0.036 0.069 <0.001 0.389 0.010 0.407

544 528

697 491

598 473

685 519

715 503

586 458

727 503

722 497

619 463

694 502

707 494

9.4 10.5

0.002 0.840

<0.001 0.582

0.044 0.019

<0.001 0.188

<0.001 0.053

0.058 0.316 0.453

374 321

BW gain,  g/chick Feed intake,  g/chick G:F, g/kg DM intake,  g/chick Prececal DM  digestibility,  % DM retention,  % Tibia ash,2 %

539

513

565

551

496

546

542

502

547

539

11.5

0.852

0.419

0.020

0.145

0.058

0.009 0.006 <0.001 <0.001 0.005 <0.001 0.043 0.031 10.4 381 380 310 391 397 290 394 388 307

550 420 290 550 420 290 550 420 290 550 420 290 Measurement

376

Linear Quadratic Linear Quadratic

Linear Quadratic

1.6 1.2

0.8 Pooled SEM Linear Quadratic 2.0 1.6 1.2 0.8

Dietary Ca:P ratio (dietary soybean meal, g/kg)

Table 3. Growth performance, DM intake, and DM utilization from chicks fed graded levels of P from 4 ratios of Ca:P1

Dietary Ca:P ratio (P-value)

2.0

PHOSPHORUS DIGESTIBILITY RESPONSE OF BROILERS

The calculated and analyzed nutrient compositions of the experimental diets are presented in Table 2. The analyzed values for P and Ca were close to calculated values. Growth performance of chicks, as well as DMI and DM utilization, are presented in Table 3. There were both linear and quadratic increases (P < 0.05) in BW gain and feed efficiency with increasing SBM level within each dietary Ca:P ratio. At a Ca:P ratio of 1.2, feed intake quadratically increased (P < 0.05), and there were tendencies (P < 0.06) for quadratic increases in feed intake at Ca:P ratios of 1.6 and 2.0. Additionally, linear decreases (P < 0.01) of prececal DM digestibility and excreta DM retention were observed in chicks for each dietary Ca:P ratio. With increasing dietary levels of SBM, no linear or quadratic response was found in tibia ash content of chicks fed diets with different Ca:P ratios. Intake and output of P, and apparent P utilization in prececal digesta and excreta are presented in Table 4. Prececal P output increased (linear, P < 0.01; quadratic, P < 0.05) with graded increasing inclusion of SBM within each dietary Ca:P ratio. There was a linear increase (P < 0.01) in excreta P output for different dietary Ca:P, and the quadratic effect in excreta P output was only significant for chicks fed diets with Ca:P ratios of 0.8 and 1.6. Apparent prececal P digestibility and P retention exhibited a linear decrease (P < 0.01) with increasing levels of SBM for each dietary Ca:P ratio. The true P digestibility and retention were determined by regression of P output in the prececal digesta and excreta against dietary P intake within each of the

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<0.001

0.967

4 Ca:P ratios. Regression equations for the diets with different Ca:P ratios are presented in Table 5. There was no significant difference in the true P retention between each group. However, the prececal true P digestibility was higher (P < 0.05) at the Ca:P ratio of 0.8 than at 1.2, 1.6, or 2.0.

DISCUSSION

replicate cages per treatment (6 chicks per cage). = DM intake. 2DMI

1Eight

79.5 81.9

70.9 

90.7

82.9

77.0 

85.5

84.3

73.7 

84.9

79.3

73.6

1.34 <0.001

0.059

<0.001

0.576

<0.001

0.005

0.275 0.004 89.1 87.5

81.4 

81.0

80.6

69.4 

80.1

77.2

67.2 

72.2

70.9

64.4

1.97

0.006

0.057

<0.001

0.029

<0.001

0.143

0.141 <0.001 616 1,118   429

205

537

863  

346

489

989  

366

618 1,025

42.7

<0.001

0.004

<0.001

0.948

<0.001

<0.001

0.028 <0.001 0.049 <0.001 0.018 <0.001 0.015 <0.001 58.3 868 1,382 673 709 1,231   473 610 1,146   417 713   328

— 2,362 3,004 3,836   2,193 3,138 3,745   2,372 3,114 3,756   2,417 2,981 3,876

P intake, mg/kg   of DMI2 Prececal P output,   mg/kg of DMI Excreta P output,   mg/kg of DMI Apparent prececal   P digestibility, % Apparent P   retention, %

420 290 550 420 290 550 420 290 550 420 290 Measurement

296

Linear Quadratic Linear Quadratic Linear Quadratic

2.0 1.6 1.2

0.8 Pooled 550 SEM Linear Quadratic 2.0 1.6 1.2 0.8

Dietary Ca:P ratio (dietary soybean meal, g/kg)

Table 4. Dietary P intake, total P output, and apparent P utilization from chicks fed graded levels of P from 4 ratios of Ca:P1

Dietary Ca:P ratio (P-value)

Liu et al.

The regression technique is a widely used approach to estimate the endogenous loss of nutrients and, therefore, to calculate the true digestibility of nutrients (Fan and Sauer, 1997; Fan et al., 2001; Rodehutscord et al., 2004). Several previous studies have reported the influence of dietary factors on the endogenous loss and true digestibility of amino acids and P in swine and poultry using the regression method (Fan and Sauer, 1997; Fan et al., 2001; Rodehutscord et al., 2004; Petersen and Stein, 2006). The BW gain and feed efficiency of broiler chicks exhibited increasing linear and quadratic responses to the increasing inclusion of SBM at the different dietary Ca:P ratios, and the feed intake was increased quadratically with graded inclusion of SBM in the present study. These growth performance responses are consistent with other results from our laboratory (Dilger and Adeola, 2006). Additionally, the prececal DM digestibility and excreta DM retention were decreased from 82 to 69% and 83 to 73% with increased SBM content in the diets, respectively. The reductions in DM digestibility and retention could be explained by the replacement of highly digestible cornstarch with SBM. The dietary requirements for Ca and nonphytate P in growing chicks are 1.0 and 0.45%, respectively (NRC, 1994). The concentrations of Ca and P in our experimental diets were below these suggested requirements because this is a prerequisite for valid use of the regression method (Fan et al., 2001; Dilger and Adeola, 2006). The casein and SBM were the main sources of dietary P, and the Ca in the diets originated from casein, SBM, and limestone. The concentration of P in diets used to determine the digestibility of P in feed ingredients was not important because apparent P digestibility was not affected by the inclusion level of P as reported by Stein et al. (2008). However, the apparent prececal P digestibility and excreta P retention were decreased linearly at each dietary Ca:P ratios by the supplementation of graded SBM, which might be caused by the source of P used in the experimental diets (Petersen and Stein, 2006; Stein et al., 2008). The ingredient used in the present study to increase concentrations of P in diets was SBM, the P digestibility of which was lower than that of casein. Therefore, the addition of graded SBM to diets in this study led to a shift toward a higher proportion of the less digestible P, which was presented as decreased apparent prececal P digestibility and total tract P retention. Additionally, the apparent prececal digestibility and total tract retention of P ranged from 64 to 90% and 71 to 91%, respectively. These values

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PHOSPHORUS DIGESTIBILITY RESPONSE OF BROILERS

Table 5. Linear relationships between prececal digesta or excreta output [mg of P/kg of DM intake (DMI)] and dietary intake (mg of P/kg of DMI) in 4 dietary ratios of Ca:P fed to chicks Dietary Ca:P ratio Prececal digesta  0.8  1.2  1.6  2.0 Excreta  0.8  1.2  1.6  2.0

Regression equation1

SE of the linear term2

SE of the intercept2

r2

True P utilization,3 %

Y Y Y Y

= = = =

0.292X 0.447X 0.542X 0.494X

− − − −

448 627 864 553

0.0323 0.0655 0.0858 0.0701

100.9 202.6 268.6 220.9

0.79 0.68 0.64 0.69

70.8a 55.3b 45.8b 50.6b

Y Y Y Y

= = = =

0.474X 0.417X 0.458X 0.451X

− − − −

733 728 803 726

0.0578 0.0486 0.0756 0.0407

180.6 150.4 236.9 128.1

0.75 0.77 0.63 0.85

52.6 58.3 54.2 54.9

a,bMeans

within prececal digesta or excreta with no common superscript differ significantly (P < 0.05). of prececal or excreta output (mg of P/kg of DMI) against dietary intake (mg of P/kg of DMI) as determined from feeding chicks different ratio of Ca:P. The linear term represents true P indigestibility, and the regression intercept provides an estimate of endogenous P loss (mg/kg of DMI). 2Standard error of regression components (n = 24 observations). 3Calculated as (1 − true P indigestibility) × 100%, as described in the Materials and Methods section. 1Regression

were higher than previous observations with SBM (Dilger and Adeola, 2006), which may be due to the inclusion of casein that had a higher P digestibility than SBM. The concentrations of P in diets of this study were below the recommended P requirement (NRC, 1994). Therefore, chicks might have adapted to these low P diets by increasing their P utilization capacity to maintain P homeostasis. This adaptive capacity has been reported when chicks were exposed to P- or Cadeficient diets by Yan et al. (2005). Taken together, this suggested that the digestibility of P was not affected by dietary P concentration when the dietary P concentration was at requirement for the bird, and when there was similarity in the ratio of digestible to total P for experimental diets. Both Ca and P are essential for bone mineralization (Wasserman, 1960). Calcium has been demonstrated to bind to the phytate molecule and decrease absorption of P in the gut. Growing White Pekin ducklings fed low P diets reduced their tibia ash concentration when dietary Ca content was increased (Xie at al., 2009), which shows that the absorption of P was decreased. Studies conducted to examine the interaction between dietary Ca and P have shown that chicks fed diets with a high concentration of Ca and low P decreased tibia ash and increased the incidence of rickets (Driver et al., 2005). However, these negative effects of high dietary Ca concentration were ameliorated by the inclusion of additional P in diets. In the present study, no difference in the percentage of tibia ash was found for each dietary Ca:P ratio. This might be due to the short experimental period (7 d), which may not be adequate to induce significant changes in bone mineralization. It has been shown that dietary P utilization was improved at low dietary concentration of Ca and narrow Ca:P ratios (Qian et al., 1997; Liu et al., 1998; Brady et al., 2002; Selle et al., 2009). Higher Ca levels and wider Ca:P ratios decreased P digestibility by the formation of Ca-phytate complexes, which reduced the absorption

of P in the intestine (Hurwitz and Bar, 1971; Lei et al., 1994). Several studies in broilers, pigs, and turkeys have been published to support the assertion that increasing dietary Ca:P ratio decreased the digestibility of P in the gut (Qian et al., 1996, 1997; Liu et al., 1998; Brady et al., 2002). In this study, the estimates of true prececal P digestibility from SBM ranged from 46 to 71%. These values were consistent with previous data (49 to 53%, Rutherfurd et al., 2002; Dilger et al., 2004; Rutherfurd et al., 2004) except the value of 71% for the diet at a Ca:P ratio of 0.8 in the present study. Using regression method, we found that the true prececal digestibility of P was decreased from 71% to 46% and 51% when dietary Ca:P ratio increased from 0.8 to 1.6 and 2.0, respectively. This observation directly demonstrated that greater P digestibility in SBM at low Ca:P ratio in diets and was consistent with a previous study (Plumstead et al., 2008). However, it was noteworthy that dietary Ca:P ratios between 0.8 and 2.0 did not affect P retention. This would suggest an increase in the urinary excretion of P as the dietary Ca concentration decreased. This notion was consistent with the observation that a Ca-deficient diet led to low retention of P for growing broiler chickens (Onyango et al., 2003; Driver et al., 2005). In conclusion, our results showed that a low dietary Ca:P ratio (0.8) increased true prececal digestibility of P but did not affect retention of P from SBM. Furthermore, there was no difference in total tract retention of P from SBM among the Ca:P ratio between 0.8 and 2 in broiler chickens during the experimental period.

ACKNOWLEDGMENTS The authors thank Hengxiao Zhai, Pengcheng Xue, Hang Lu, and Pat Jaynes for their assistance with bird management and technical assistance with analytical procedures.

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