J. Appl. Poult. Res.
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J APPL POULT RES 2006. 15:207-218
© 2006 Poultry Science Association
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Research Reports

Modified Phosphorus Program for Broilers Based on Commercial Feeding Intervals to Sustain Live Performance and Reduce Total and Water-Soluble Phosphorus in Litter1

C. A. Fritts2 and P. W. Waldroup3

Department of Poultry Science, University of Arkansas, Fayetteville 72701

Correspondence: 3 Corresponding author: waldroup{at}uark.edu


   SUMMARY
TOP
 SUMMARY
DESCRIPTION OF PROBLEM
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES AND NOTES
 
Two experiments were conducted to adapt the results of previous studies from our laboratory on calcium and nonphytate phosphorus (NPP) requirements to feeding intervals more similar to the commercial broiler industry. Feeding periods of 0 to 14, 14 to 35, 35 to 42, and 42 to 56 d were used. In the first experiment, a positive control diet similar to industry levels (0.45% NPP and 1.0% Ca, 0.40% NPP and 0.9% Ca, 0.35% NPP and 0.8% Ca, and 0.30% NPP and 0.8% Ca for the various feeding periods, respectively) was compared with various combinations of Ca and NPP levels, with phytase supplementation used in some diets with low NPP. It was observed that birds fed starter diets with 0.40% NPP followed by 0.30% during the grower phase grew as well with equivalent feed conversion as those fed the positive control diet. Feeding less than 0.30% during the grower period resulted in excessive mortality over the duration of the study. In the second experiment, consisting of 2 consecutive trials in which birds were housed beginning on new softwood shavings, adjustments were made in the NPP and Ca levels used in the first experiment; diets were fed with or without phytase supplementation. At the conclusion of the second trial, samples of litter were evaluated for total and soluble P contents. All of the modified diets supported BW gain, feed conversion, livability, and bone parameters that did not differ significantly from that of birds fed the positive control diets. Total and soluble P contents of the litter were significantly reduced by the modified diets fed with or without phytase. Use of the modified diets resulted in significant savings in dietary costs associated with reduced levels of P supplementation.

Key Words: broiler • phosphorus • litter • eutrophication • soluble phosphorus • phytase


   DESCRIPTION OF PROBLEM
TOP
 SUMMARY
 DESCRIPTION OF PROBLEM
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES AND NOTES
 
The presence of P in poultry litter has resulted in severe limitations to land application in many countries around the world. There are areas of northwestern Arkansas and eastern Oklahoma lying in the Tulsa, OK, watershed where farmers are prohibited from applying poultry litter on land until a P index has been established [1]. Many nutritionists are adding the enzyme phytase to broiler diets in conjunction with reduced dietary P levels in an effort to reduce P excretion. It has been suggested that phytase may actually increase water-soluble P in poultry litter [2]. However, other reports do not support this theory and have reported lower values for water-soluble P when phytase is added to broiler diets [3, 4].

Phosphorus is a critical and expensive nutrient provided to poultry for supporting growth and development of a strong skeleton to withstand the rigors of growth, transport, and processing and must be supplied in adequate but not excessive amounts. In previous research, our laboratory has investigated minimum nonphytate P (NPP) needs in various age periods to support tibia ash, based on NRC [5] suggested feeding periods of 0 to 21, 21 to 42, and 42 to 56 d of age [6, 7, 8, 9], and has applied these successfully in an overall feeding program [10, 11]. However, the age periods evaluated in these studies do not match current US commercial industry practices.

Two experiments were conducted in the present report to adapt our previous findings to industry feeding intervals to support live performance and skeletal development while reducing total and soluble P in broiler litter. A preliminary study (Experiment 1) was conducted to determine the NPP needs of male broilers based upon commercial feeding periods. Based on the results of Experiment 1, a second series of trials (Experiment 2) was conducted to apply these results to longer-term trials with broilers in litter floor pens to evaluate the effects of the dietary treatments on live performance and bone development and also to evaluate the effects on total and soluble P content of broiler litter.


   MATERIALS AND METHODS
TOP
 SUMMARY
 DESCRIPTION OF PROBLEM
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES AND NOTES
 
Diet Formulation and Dietary Treatments
Feeding periods of 0 to 14, 14 to 35, 35 to 42, and 42 to 56 d were used, based on typical industry feeding periods [12]. The basal diets were formulated to be adequate in all nutrients with variation only in levels of Ca and NPP. Diets in Experiment 1 (Table 1Go) made use of extensive amino acid supplementation to minimize dietary protein levels, whereas diets in Experiment 2 (Table 2Go) were higher in crude protein content without extensive amino acid supplementation. Diets were fortified with commercial vitamin and trace mineral supplements. To prepare the test diets a common basal diet was mixed, and aliquots were used to prepare the experimental diets. Alterations among diets were made by variations in dicalcium phosphate, ground limestone, and washed sand to attain the desired Ca and NPP levels.


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  		Table 1. Composition (g/kg) and nutrient content of basal diets (Experiment 1)
 

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  		Table 2. Composition (g/kg) and nutrient content of basal diets (Experiment 2)
 
The dietary treatments (% NPP and % Ca) used in Experiment 1 are shown in Table 3Go. The level of NPP and Ca in Treatment 1 is similar to commercial levels [12] and also similar to NRC [5] recommendations with a slight adjustment for feeding period. Treatments 2 and 3 are similar to our previous recommendations for modified early feeding [10, 11] adjusted to the different time intervals. No phytase was added to Treatment 2, and Treatment 3 was supplemented with 1,000 U of phytase/ kg [13]. Treatments 4, 5, and 6 had even more stringent reductions in P in the later stages of growth in conjunction with 1,000 U of phytase supplementation/kg.


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  		Table 3. Dietary treatments fed to male broilers from 0 to 56 d (Experiment 1)
 
Dietary treatments used in Experiment 2 are shown in Table 4Go. Levels of NPP and Ca used in Experiment 2 are based upon results from Experiment 1. Three modified (MOD) series were compared with an industry standard (IND). All MOD diets contained 0.40% NPP for 0 to 14 d followed by 0.30% from 14 to 35 d and varied thereafter in NPP levels. All diets were fed with or without an additional 1,200 U of phytase /kg [13]. Diets in both trials were fed in mash form.


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  		Table 4. Dietary treatments fed to male broilers from 0 to 56 d (Experiment 2)
 
Birds and Housing
The house used in these studies was an insulated steel truss house of commercial design with a 3-ft curtain sidewall containing 48 pens with 56 ft2 / pen. Temperature was maintained by thermostatically controlled gas brooders, adjustable sidewall curtains, and exhaust fans. Each pen was equipped with 2 tube feeders and 1 automatic water fount. Supplemental waterers and feeders were used for the first 7 d post-hatch. Care and management of the chicks followed recommended guidelines [14]. In the first experiment, chicks were placed on used wood shavings litter over concrete floors. The second experiment consisted of 2 consecutive flocks of broilers using the same experimental design with diet assignments maintained in both flocks. New softwood shavings litter was used for the first flock. At the conclusion of the first flock, caked litter underneath feeders and waterers was removed, and the litter was stirred and used for the second flock.

Male chicks of a commercial broiler strain [15] were obtained from a local hatchery where they had been vaccinated in ovo for Marek’s disease and had received vaccinations for New-castle disease and infectious bronchitis post-hatch via a coarse spray. In the first experiment, 30 chicks were placed in each of the 48 pens with 8 pens assigned to each of the 6 treatments. In the second experiment, 70 chicks were placed in each of the 48 pens with 6 pens assigned to each of the 8 treatments in 2 consecutive feeding trials on the same litter. The same pens were assigned to the various dietary treatments in both flocks of birds.

Measurements
In Experiment 1, birds were group weighed by pen and pen feed consumption determined at 14, 35, 42, and 56 d. Mortality was checked twice daily; any bird that died was weighed, and the weight used to adjust feed conversion ratio (FCR; g of feed / g of gain). Two birds per pen were randomly selected at 14, 35, 42, and 56 d and killed by CO2 inhalation followed by cervical dislocation. The right tibia was used to determine bone ash [16]. At 56 d, the left tibia was scored for incidence and severity of tibial dyschondroplasia (TD) using the scoring system of Edwards and Veltmann [17]. At the conclusion of the study, all remaining birds were processed by automated evisceration, and the number of broken bones (wings, legs, clavicles, and rib cage) was determined. Feed, but not water, was removed from birds 8 h before processing. Birds were placed in coops and transported approximately 2 km to a pilot processing plant where they were processed as described [18].

In Experiment 2, birds were grown to 56 d with BW and feed consumption determined at 14, 35, 42, and 56 d of age. Mortality was checked twice daily; any bird that died was weighed, and the weight used to adjust FCR. At 56 d, 2 birds per pen were killed by CO2 inhalation followed by cervical dislocation, and the tibia was removed for bone ash determination as previously described. Ten randomly selected birds per pen were processed by automated evisceration as previously described, and the number of broken bones (wings, legs, clavicles, and rib cage) was determined after processing.

At the conclusion of the second trial, birds and equipment were removed from the pens. A uniform area of the pen away from feeders and waterers served as the collection area. The area was marked with an inverted 5-gal bucket. Approximately 1,000 g of litter was collected from the marked area and placed in plastic bags. Subsamples of the litter from each pen from the same dietary treatment were combined as one composite sample and mixed for approximately 15 min in a small mixer. Eight subsamples of the composite litter from each treatment were analyzed for total and soluble P content by a commercial laboratory specializing in mineral analysis [25]. Soluble P content was determined as described by Self-Davis and Moore [26]. All test diets were analyzed for Ca and total P by a commercial laboratory [25] and for phytase activity by a commercial company [13]

Statistical Analysis
Data were subjected to statistical analysis [27] with statements of probability based on P ≤ 0.05.


   RESULTS AND DISCUSSION
TOP
 SUMMARY
 DESCRIPTION OF PROBLEM
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY AND CONCLUSIONS
 REFERENCES AND NOTES
 
Calcium and total P assays indicated that the diets contained at or near the expected levels of these minerals for Experiment 1 (Table 5Go) and Experiment 2 (Table 6Go). Phytase analyses indicated that the test diets contained levels higher than anticipated, with a mean phytase level for supplemented diets of 1,105 ± 32 phytase U/kg in Experiment 1 and 1,318 ± 38 phytase U/kg in Experiment 2.


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  		Table 5. Calculated (C) and analyzed (A) levels of calcium and total phosphorus in test diets (Experiment 1)
 

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  		Table 6. Calculated (C) and analyzed (A) levels of calcium and total phosphorus in test diets (Experiment 2)
 
Experiment 1
The effects of the various treatments on live performance of male broiler chicks are shown in Table 7Go. No significant differences in BW or FCR at 14, 35, 42, or 56 d were noted among birds fed the different dietary treatments. However, significant differences in mortality were observed among the birds fed the various dietary treatments. No significant differences were noted at 14 d of age. At 35 d of age and continuing through the end of the study, birds fed less than 0.30% during the period of 14 to 35 d typically had higher mortality than those fed 0.30% or more NPP up to 35 d of age. In most instances these differences were statistically significant and in all cases were numerically greater. This finding indicated that birds could be fed a starter diet from 0 to 14 d with a NPP level of 0.40%, but during the period from 14 to 35 d birds cannot be fed NPP levels less than 0.30% without having detrimental effects, even when the diets are supplemented with phytase.


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  		Table 7. Effects of dietary nonphytate phosphorus (NPP) and calcium, with or without phytase, on live performance of male broilers (Experiment 1, mean of 8 pens of 30 male chicks / treatment)
 
The effects of the various treatments on bone parameters are shown in Table 8Go. Tibia ash was not significantly affected by the dietary treatments at 14 or 35 d of age. Samples for 42 and 56 d were lost due to a freezer malfunction. At 56 d there were no significant differences among birds fed the various P programs for incidence (% of birds having TD scores greater than zero) and severity (% of birds having a TD score of 3) of TD. No significant differences were observed among birds fed the various levels of NPP on the number of wings, legs, rib cage, or clavicles broken during automated processing at 56 d.


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  		Table 8. Effects of dietary nonphytate phosphorus (NPP) and calcium, with or without phytase, on bone parameters of male broilers (Experiment 1, mean of 8 pens of 30 males / treatment)
 
The results of Experiment 1 indicate that the various levels of NPP evaluated were adequate for BW, feed conversion, and bone development. However, the effects on mortality demonstrated that some of the levels were too low during 14 to 35 d of feeding and prompted the change in dietary treatments used in Experiment 2.

Experiment 2
Two consecutive feeding trials were conducted in this experiment. Because there were no trial x treatment interactions, data from the 2 studies were combined. The effects of the various treatments on live performance and bone parameters are shown in Table 9Go. There were no significant effects on BW, feed conversion, or mortality when birds were fed the various modified P programs with or without phytase supplementation, indicating that feeding lower levels of NPP for the various feeding periods was sufficient to support these live performance parameters for birds fed based on commercial feeding times when compared with the industry diet.


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  		Table 9. Effect of the various phosphorus feeding programs based on commercial feeding intervals on performance of male broilers (Experiment 2, mean of 2 trials each with 6 pens of 70 males / treatment)
 
Birds fed the modified P diets with phytase had significantly higher percentages of tibia ash at 56 d than birds fed the industry diets with or without phytase and the modified P diets without phytase. This finding indicates the levels fed from 0 to 56 d could be successfully accomplished to support skeletal development; however, it is necessary that phytase be included. Considerable variation occurred among dietary treatments in number of bones broken during automated processing. No consistent pattern related to presence or absence of phytase or NPP program was observed.

The results of the present studies are in agreement with previous studies from our laboratory [29, 30, 31] and elsewhere [32], indicating that the NPP levels can be markedly reduced in the latter stages of broiler production provided that adequate levels are fed during the starter and early grower phases. Failure to provide sufficient NPP in starter and early grower diets can lead to extensive bone breaking during processing [33, 34]. Overall levels used in the present studies confirm previous recommendations from our laboratory [10, 11] adjusted to commercial feeding periods. These recommendations agree with reports of Angel et al. [35, 36] who found the NPP requirement to be 0.28 to 0.32% for 18 to 35 d and 0.11% for 42 to 49 d. Angel et al. [37] reported that 0.45, 0.36, 0.18, and 0.14% NPP for starter (1 to 18 d), grower (18 to 32 d), finisher (32 to 42 d), and withdrawal (42 to 49 d) periods resulted in no increase in number of birds with broken wings or legs at processing vs. birds fed industry-average NPP levels of 0.43, 0.36, 0.32, and 0.28% for the respective periods. The present studies indicate even lower levels of NPP can be successfully utilized.

Implementation of the 3 different modified P feeding programs resulted in significant reductions in total and soluble P in the litter compared with that from birds fed the industry P program (Table 10Go). Within a P treatment, no significant differences were observed for total or soluble P with or without addition of phytase. This result is in agreement with those of Moore et al. [3], Applegate et al. [4], and Saylor [38] who reported no significant differences in total or soluble P content of litter from birds fed diets with or without phytase supplementation. Soluble P was increased when phytase was added to the IND diet, which was already sufficient in total P, but not when added to the MOD diets with reduced P levels. Implementation of any of the 3 MOD programs resulted in significant reductions in both total and soluble P in the litter compared with the IND program. Although the addition of phytase resulted in no major reductions in total or soluble P in the litter, the use of phytase in conjunction with reduced NPP levels allowed for better tibia ash and better overall livability, as has been demonstrated in previous trials from our laboratory [6, 7, 8, 9, 10, 11].


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  		Table 10. Effect of the various phosphorus feeding programs based on commercial feeding intervals on total and soluble phosphorus content of the litter after 2 consecutive flocks of male broilers (Experiment 2, mean of 2 trials each with 6 pens of 70 males / treatment)
 
At the conclusion of the study, calculations were made on quantity of supplemental P consumed by birds on the various feeding programs (Table 11Go). The cost of supplemental P consumed was then calculated based upon current US price of $250 / ton for 18% supplemental P. Cost savings per million birds of $7,545.09 to $6,419.41 could be attained by use of 1 of the 3 modified feeding programs for birds fed to 42 d, and cost savings of $12,443.32 to $14,101.41 could be attained for birds fed to 56 d. Producers may adjust these values based on current prices of P.


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  		Table 11. Mean phosphorus consumption and cost of supplemental phosphorus for birds grown to 42 or 56 d based on $250/ton for 18% dicalcium phosphate1,2
 

   SUMMARY AND CONCLUSIONS
TOP
 SUMMARY
 DESCRIPTION OF PROBLEM
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
REFERENCES AND NOTES
 
  1. Feeding the Ca and NPP modified diets did not negatively affect live performance of broilers grown to 56 d, when the modified diets contained a dietary phytase enzyme providing 1,200 U of phytase/kg of diet.
  2. There was significant improvement in the percentage of tibia ash found in birds fed the modified P diets when phytase was supplemented.
  3. The use of 1 of the 3 modified P feeding programs could significantly reduce the excretion of total or water-soluble P by broilers, and the supplementation of phytase to the diet did not increase water-soluble P in the litter.
  4. Considerable savings in dietary cost could be obtained by utilizing any of the modified P feeding programs.
  5. Reduction of dietary P in the diet with the supplementation of phytase could sustain live performance and skeletal development while reducing total and soluble P in the litter when compared with levels of NPP currently being fed in the commercial poultry industry.


   FOOTNOTES
 
1 Published with approval of the Director, Arkansas Agricultural Experiment Station, Fayetteville, AR 72701. Mention of a trade name, proprietary product, or specific equipment does not constitute a guarantee or warranty by the University of Arkansas and does not imply its approval to the exclusion of other products that may be suitable. Back

2 Present address: Cobb-Vantress Inc., Siloam Springs, AR 72761. Back


   REFERENCES AND NOTES
TOP
 SUMMARY
 DESCRIPTION OF PROBLEM
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES AND NOTES
 

  1. Cody, C. 2003. Farmers told to stop spreading litter. Arkansas Democrat Gazette, Little Rock, AR. Thursday, April 24, 2003.
  2. DeLaune, P. B., P. A. Moore, Jr., D. C. Carman, T. C. Daniel, and A. N. Sharpley. 2001. Development and validation of a phosphorus index for pastures fertilized with animal manure. Proc. Int. Symp. Addressing Anim. Prod. Environ. Issues. North Carolina State Univ., Raleigh, NC.
  3. Moore, P. A., Jr., M. L. Self-Davis, T. C. Daniel, W. E. Huff, D. R. Edwards, D. J. Nichols, W. F. Jaynes, G. R. Huff, J. M. Balog, N. C. Rath, P. W. Waldroup, and V. Raboy. 1998. Use of high oil corn and phytase enzyme additions to broiler diets to lower phosphorus levels in poultry litter. Pages 346–352 in Proc. 1998 Natl. Poult. Waste Manage. Symp. J. P. Blake and P. H. Patterson, ed. Auburn Univ. Printing Service, Auburn, AL.
  4. Applegate, T. J., B. C. Joern, D. L. Nussbaum-Wagler, and R. Angel. 2003. Water-soluble phosphorus in fresh broiler litter is dependent upon phosphorus concentration fed but not on fungal phytase supplementation. Poult. Sci. 82:1024–1029.[Abstract/Free Full Text]
  5. National Research Council. 1994. Nutrient Requirements of Poultry. 9th rev. ed. Natl. Acad. Press, Washington, DC.
  6. Waldroup, P. W., J. H. Kersey, E. A. Saleh, C. A. Fritts, H. L. Stilborn, R. C. Crum, Jr., and V. Raboy. 2000. Nonphytate phosphorus requirement and phosphorus excretion of broilers fed diets composed of normal or high available phosphorus corn with and without microbial phytase. Poult. Sci. 79:1451–1459.[Abstract/Free Full Text]
  7. Yan, F., J. H. Kersey, C. A. Fritts, P. W. Waldroup, H. L. Stilborn, R. C. Crum, Jr., D. W. Rice, and V. Raboy. 2000. Evaluation of normal yellow dent corn and high available phosphorus corn in combination with reduced dietary phosphorus and phytase supplementation for broilers grown to market weights in litter pens. Poult. Sci. 79:1282–1289.[Abstract/Free Full Text]
  8. Yan, F., J. H. Kersey, and P. W. Waldroup. 2001. Phosphorus requirements of broiler chicks three to six weeks of age as influenced by phytase supplementation. Poult. Sci. 80:455–459.[Abstract/Free Full Text]
  9. Yan, F., C. A. Fritts, J. H. Kersey, and P. W. Waldroup. 2003. Phosphorus requirements of broiler chicks six to nine weeks of age as influenced by phytase supplementation. Poult. Sci. 82:294–300.[Abstract/Free Full Text]
  10. Yan, F., C. A. Fritts, and P. W. Waldroup. 2003. Evaluation of modified dietary phosphorus levels with and without phytase supplementation on live performance and fecal phosphorus levels in broiler diets. 1. Full-term feeding recommendations. J. Appl. Poult. Res. 12:174–182.[Abstract/Free Full Text]
  11. Yan, F., C. A. Fritts, and P. W. Waldroup. 2004. Evaluation of modified dietary phosphorus levels with and without phytase supplementation on live performance and fecal phosphorus levels in broiler diets. 2. Modified early phosphorus levels. J. Appl. Poult. Res. 13:394–400.[Abstract/Free Full Text]
  12. Agri Stats Inc., Fort Wayne, IN.
  13. Natuphos 1200 dry powder, BASF Corp., Mt. Olive, NJ. One unit of phytase activity is defined as the quantity of enzyme required to produce 1 µmol of inorganic P/min from 5.1 mmol/L of sodium phytate at pH 5.5 and a water bath temperature of 37°C.
  14. FASS. 1999. Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching. 1st rev. ed. Fed. Anim. Sci. Soc., Savoy, IL.
  15. Cobb 500. Cobb-Vantress Inc., Siloam Springs, AR.
  16. Association of Official Analytical Chemists. 1990. Official Methods of Analysis. 15th ed. AOAC, Arlington, VA.
  17. Edwards, H. M., Jr., and J. R. Veltmann, Jr. 1983. The role of calcium and phosphorus in the etiology of tibial dyschondroplasia in young chicks. J. Nutr. 113:1568–1575.[Abstract/Free Full Text]
  18. The birds were wing-banded and hung on the slaughter line. They were then subjected to a 7-s stun at 10 to 12 mA and 12 V in a 4-ft stunning cabinet [19]. Approximately 10 s after being stunned, a deep manual throat cut was made to sever the carotid artery and jugular vein with a 120-s bleeding time. Carcasses were scalded at 128°F in an inline scalder [20], defeathered in an inline picker [21] for 70 s, and rinsed for 10 s in an inside-outside online rinse cabinet. The carcasses were removed from the slaughter line by passing through an automatic hock cutter [22], and necks were severed by pneumatic cutters. The carcasses were then rehung on the evisceration line and proceeded through an automated vent cutter and opening machine [23], and viscera were drawn from the carcass by automated evisceration [24]. The viscera were then manually removed from the carcass.
  19. Simmons Model SF 7000, Simmons Engineering Co., Dallas, GA 30132.
  20. Cantrell Model SS3300CF, Cantrell Machine Co., Gainesville, GA.
  21. FoodCraft Complex VF-1000, FoodCraft Equipment Co., Lancaster, PA.
  22. Cantrell Model PHC-4, Cantrell Machine Co.
  23. Model V-163, Stork Gamco Inc., Gainesville, GA.
  24. Model CDM-20, Stork Gamco Inc.
  25. Analyses were conducted by the Agricultural Diagnostic Laboratory (University of Arkansas, Fayetteville, AR) using inductively coupled plasma-atomic spectroscopy following HNO3 digestion.
  26. Self-Davis, M. L., and P. A. Moore, Jr. 2000. Determining water-soluble phosphorus in animal manure. Pages 74–75 in: Methods of Phosphorus Analysis for Soils, Sediments, Residuals, and Waters. Southern Coop. Series Bull. 396. G. M. Pierzynski, ed. Kansas State University, Manhattan, KS.
  27. Data were subjected to the analysis of variance using the GLM procedure of SAS software [28]. Pen means served as the experimental unit. Mortality data were transformed to {surd}n + 1 before analysis; data are presented as natural numbers. Significant differences among or between treatments were separated by repeated t-tests using the least square means option of SAS.
  28. SAS Institute. 1991. SAS User’s Guide: Statistics. Version 6.03 ed. SAS Institute Inc., Cary, NC.
  29. Waldroup, P. W., R. J. Mitchell, and K. R. Hazen. 1974. The phosphorus needs of finishing broilers in relation to dietary nutrient density level. Poult. Sci. 53:1655–1663.[ISI][Medline]
  30. Skinner, J. T., A. L. Izat, and P. W. Waldroup. 1992. Effects of removal of supplemental calcium and phosphorus from broiler finisher diets. J. Appl. Poult. Res. 1:42–47.[Abstract/Free Full Text]
  31. Skinner, J. T., M. H. Adams, S. E. Watkins, and P. W. Waldroup. 1992. Effect of calcium and nonphytate phosphorus levels fed during 42 to 56 days of age on performance and bone strength of male broilers. J. Appl. Poult. Res. 1:167–171.[Abstract/Free Full Text]
  32. Tortuero, F., and M. V. Diez Tardon. 1983. Possibilities in the use of low phosphorus concentrations for broiler diets during the finishing period. Adv. Aliment. Mejora Anim. 24:63–66.
  33. Chen, X., and E. T. Moran Jr. 1994. Response of broilers to omitting dicalcium phosphate from the withdrawal period: Live performance, carcass downgrading and further-processing yields. J. Appl. Poult. Res. 3:74–79.[Abstract/Free Full Text]
  34. Chen, X., and E. T. Moran Jr. 1995. The withdrawal feed of broilers: Carcass responses to dietary phosphorus. J. Appl. Poult. Res. 4:69–82.[Abstract/Free Full Text]
  35. Angel, R., T. J. Applegate, and M. Christman. 2000. Effects of dietary non-phytate phosphorus (nPP) on performance and bone measurements in broilers fed on a four-phase feeding system. Poult. Sci. 79(Suppl. 1):21–22. (Abstr.)
  36. Angel, R., T. J. Applegate, M. Christman, and A. D. Mitchell. 2000. Effect of dietary non-phytate phosphorus (nPP) level on broiler performance and bone measurements in the starter and grower phase. Poult. Sci. 79(Suppl. 1):22. (Abstr.)
  37. Angel, R., M. Christman, and T. Applegate. 2001. Phosphorus requirements of broilers and effect of phytase, citric acid, and 25-hydroxycholecalciferol on phosphorus availablitity of broilers and turkeys. Pages 72–87 in Proc. Maryland Nutr. Conf. Univ. Maryland, College Park.
  38. Saylor, W. W. 2001. Use of phytase and high available phosphorus corn in broiler diets: Impact on litter phosphorus levels and solubility. Pages 43–58 in Proc. Maryland Nutr. Conf. Univ. Maryland, College Park.




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