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

Introduction

Concerns about the possible contribution of P runoff from the application of poultry litter to eutrophication of surface waters has focused attention on means of reducing phosphorus excretion (Edwards and Daniel, 1992; Sharpley, 1999). Because of the demands for adequate skeletal development of the rapidly growing broiler and the variability of phosphorus content in animal byproduct feeds used to provide a major share of this mineral, it is necessary to provide an adequate margin of safety in broiler diets.

A considerable amount of the phosphorus in poultry diets is in the form of phytate phosphorus. This organically bound form of the mineral is poorly available to chickens because monogastrics are lacking or limited in phytase, the enzyme that is necessary for breakdown of the molecule and subsequent release of phosphorus for absorption (O’Dell et al., 1972; Raboy, 1990). Nelson et al. (1968, 1971) demonstrated that addition of phytase to broiler diets was an effective means of improving the availability of phytate-bound phosphorus. Recent commercial development of phytase enzymes offers promise in reducing phosphorus excretion by increasing the ability of the chick to utilize a portion of the phytatebound phosphorus (reviewed by Ravindran et al., 1995; Sebastian et al., 1998).

Another approach to reducing phosphorus in excreta is to develop feedstuffs with modified levels of phytatebound phosphorus. A corn mutation with low phytate phosphorus and high available phosphorus content has been developed by the USDA (Raboy and Gerbasi, 1996; Raboy, 1997) and has been bred into a hybrid by a major seed company4 using the low phytic acid 1-1 (lpal-1) allele of the corn LPA1 gene. This hybrid, designated as high available phosphate corn (HAPC), contains approximately 0.27% total phosphorus, of which 0.17% is estimated to be available to the chicken. The nonphytate phosphorus in HAPC has been found to be equivalent in biological value to a commercial dicalcium phosphate (Ertl et al., 1998; Waldroup et al., 1999). In contrast, normal yellow dent corn (YDC) contains similar levels of total phosphorus but only about 0.03% available phosphorus. Previous studies with HAPC have been limited primarily to birds grown to 3 wk of age (Ertl et al., 1998; Waldroup et al., 1999). Huff et al. (1998) grew broilers to 49 d using diets with YDC or HAPC; however, phosphorus levels used in the diets were greater than NRC recommendations. The present study was conducted to evaluate the use of YDC and HAPC in reduced-phosphorus diets for broilers grown to market weights with phytase supplementation to enhance utilization of phytatebound phosphorus.

Materials and methods

Floor Pen Studies

Using nutrient values provided by the seed company (Table 1), diets were formulated for growing broilers to 8 wk with either YDC or HAPC (Table 2). The YDC used in the study was the isogenic normal phytate counterpart to the HAPC. Both corns were grown in the same location during the same year. The diets contained aminimum of 110% of NRC (1994) suggested amino acid requirements and were formulated to contain NRC (1994) recommended levels of nonphytate phosphorus (NPP; 0.45% NPP for 0 to 21 d, 0.35% for 21 to 42 d, and 0.30% for 42 to 56 d) with either YDC or HAPC. By varying the level of supplemental dicalcium phosphorus and limestone in the diet, additional test diets were produced that contained reduced levels of NPP [−0.0%, −0.075, and −0.15% of NRC (1994)] for both corn types. Each of these diets was mixed and divided into two subplots. One lot was treated with microbial phytase enzyme at 1,000 units/kg5; this level of phytase was considered to be sufficient for maximum economical release of phosphorus (Kornegay et al., 1996; Van der Klis and Versteegh, 1996; Yi et al., 1996).

The combination of two corn sources, with and without phytase supplementation, at three levels of NPP resulted in 12 experimental treatments. Each treatment was fed to four replicate pens of 50 male broilers. Diets were fed in pellet form (starter diets were crumbled) with the phytase enzyme sprayed on the feeds in amixer after the pelleting process. Samples of diets were ana lyzed for calcium, total phosphorus, and phytase activity.

Day-old male chicks of a commercial broiler strain (Cobb 5006) were obtained from a local hatchery and were randomly distributed into pens in a commercialtype house with new wood shaving litter over concrete floors. Fifty birds were placed in each of 48 pens (5.2 m2). Each pen was equipped with two tube feeders and an automatic water font. The birds were placed on their respective test diets at 1 d of age and were fed to 56 d with the test feeds and tap water provided ad libitum.

Table 1. Nutrient profile of normal yellow dent corn (YDC) and high available phosphorus (HAPC) corn (as-fed basis)¹

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 - Image 1

¹Values provided by Pioneer Hi-Bred International, Inc., Johnston, IA 50131.

Cage Studies

Portions of the test diets were fed to cohort birds maintained in wire-floored battery pens in a temperaturecontrolled room for bone ash and fecal phosphorus determination. Each diet was fed to eight pens of chicks with eight birds per pen.

Measurements

Birds in the floor pens were group weighed by pen at 21, 42, and 56 d of age, and pen feed consumption was determined. Mortality was checked twice daily; any bird that died was weighed, and the weight used to adjust feed conversion. Feed conversion ratio (FCR; grams of feed per gram of gain) was calculated by dividing total feed consumed by weight of live plus dead birds. Two birds from each battery pen (16 per treatment) were killed by CO2 inhalation at 21, 42, and 56 d for tibia ash determination as outlined by AOAC (1990). At the same ages, fecal samples were collected on aluminum foil, pooled from the eight replicate pens, and freeze-dried, and phosphorus content was determined.

Statistical Analysis

Data were subjected to analysis of variance as a factorial arrangement of treatments with corn source, phosphorus level, and phytase supplementation as main effects; all possible interactions among and between the main effects were evaluated. Mortality data were transformed to √n+1 prior to analysis; data are presented as natural numbers. Analysis used the general linear model of SAS (SAS Institute, 1991). Significant differences among or between means were separated by repeated t-tests using the lsmeans option of SAS. Statements of probability were based on P ≤ 0.05.

Results and discussion

The calculated and analyzed levels of calcium and total phosphorus are shown in Table 3. There was good agreement between calculated and analyzed levels of these two minerals. Results of phytase assays indicated that all phytase supplemented diets contained activity commensurate with expected levels; mean phytase values were 1,020, 935, and 998 units/kg for starter, grower, and finisher diets, respectively. A summary of the statistical analyses for the performance parameters is shown in Table 4.

Body Weight

There were no significant differences in body weight between birds fed the two types of corn at any age (Table 5). Overall, addition of phytase to the diets significantly improved body weight at 21 d; however, there was no significant improvement in body weight at 42 or 56 d from phytase supplementation. As expected, phosphorus level significantly influenced body weight. At every age, birds fed diets with −0.075% NRC levels did not differ significantly in body weight from those fed the NRC level; birds fed diets with −0.15% NRC were significantly lighter than birds fed NRC or −0.075% NRC diets.

Several interactions between main effects were observed. There was a significant interaction between type of corn and phytase supplementation at 56 d; body weight of chicks was significantly improved by phytase supplementation when they were fed YDC diets but not when fed HAPC diets. Because HAPC contains approximately half of the phytate-bound phosphorus that could be released by phytase supplementation as compared to YDC, this interaction was expected and is in agreement with previous research from our laboratory (Waldroup et al., 1999). There was a significant interaction between phytase supplementation and level of NPP in 56-d body weights; phytase supplementation significantly improved body weights when added to diets containing −0.15% NRC but not when added to diets containing NRC or −0.075% NRC. These two levels of NPP appeared to be adequate to maintain body weight and, thus, would not be expected to respond to phytase supplementation.

Table 2. Composition and calculated nutrient content of broiler starter, grower, and finisher diets with normal yellow dent corn (YDC) or high available phosphate corn (HAPC)

¹Alpharma, Inc., Ft. Lee, NJ 07024. Provides 110 g/kg bacitracin activity as bacitracin methylene disalicylate.
²Elanco Animal Health, division of Eli Lilly & Co., Indianapolis, IN 46285. Provides 132 g monensin/kg as monensin sodium.
³Provides per kilogram of diet: 7,714 IU vitamin A, 2,204 IU cholecalciferol, 16.53 IU vitamin E, 0.013 mg vitamin B12, 6.6 mg riboflavin, 39 mg niacin, 10 mg pantothenic acid, 465 mg choline, 1.5 mg vitamin K, 0.9 mg folic acid, 1.54 mg thiamin, 2.76 mg pyridoxine, 0.066 mg d-biotin, 125 mg ethoxyquin, and 0.1 mg Se.
⁴Provides per kilogram of diet: Mn (from MnSO4_H2O), 100 mg; Zn (from ZnSO4_7H2O), 100 mg; Fe (from FeSO4_7H2O), 50 mg; Cu (from CuSO4_5H2O), 10 mg; and I (from Ca(IO3)2_H2O), 1 mg.
⁵Variable amounts of dicalcium phosphate, limestone, or washed builders sand as required to adjust level of nonphytate phosphorus.

Table 3. Calculated (C) and analyzed (A) levels of calcium and phosphorus in test diets containing normal yellow dent corn (YDC) or high available phosphorus corn (HAPC)

Table 4. Results of statistical analysis of performance parameters

Feed Conversion

Feed conversion by birds fed the experimental diets is shown in Table 6. These data may be influenced in part by the relatively high mortality experienced at the lowest level of nonphytate phosphorus. Even though data were adjusted to account for the weight of birds that died during the study, this adjustment is imperfect at best. This adjustment resulted in numerous interactions between and among the main effects. Overall, FCR by birds fed diets containing HAPC was equal (21 and 42 d) or superior (56 d) to that of chicks fed diets with YDC. Phytase supplementation resulted in superior overall FCR at 56 d. At 42 d, the FCR of birds fed the phytase-supplemented diets with reduced phosphorus levels was inexplicably greater than that of birds fed the unsupplemented diets.

At 56 d, there were several interactions among and between the main effects. Phytase supplementation significantly improved FCR of diets based on YDC but not when added to diets containing HAPC; note that the diets with HAPC had less phytate-bound phosphorus to be released by the phytase. Phytase supplementation also had a greater influence on FCR when added to diets with reduced phosphorus levels, having a greater influence on low-phosphorus diets containing YDCcompared with diets containing the HAPC.

Mortality

Mortality of birds fed the experimental diets is shown in Table 7. Mortality was higher than anticipated, especially at the later ages, and was exacerbated by heat stress between 42 and 56 d. Mortality was not significantly influenced by the type of corn used in the diets but was significantly influenced by dietary phosphorus level. At 42 and 56 d, birds fed the diets with −0.15% NRC had significantly higher mortality than those fed diets with NRC or −0.075% NRC. Birds fed diets supplemented with phytase had significantly lower mortality tan those fed unsupplemented diets, with a significant interaction between phosphorus level and phytase supplementation. Addition of phytase significantly reduced mortality at the lower levels of phosphorus, especially at 42 and 56 d. This reduction in mortality by phytase supplementation may be one of the greatest contributions of phytase in a feeding strategy to minimize fecal excretion of phosphorus by broilers.

Table 5. Effect of normal yellow dent corn (YDC) or high available phosphate corn (HAPC), phytase supplementation, and level of nonphytate phosphorus on body weight of male broilers

^a–c; x,yWithin comparisons, means with common superscript do not differ significantly (P ≤ 0.05).
¹Indicates reduction (% of diet) from NRC (1994) recommended level.
²With or without 1,000 units of phytase/kg.

Table 6. Effect of normal yellow dent corn (YDC) or high available phosphate corn (HAPC), phytase supplementation, and level of nonphytate phosphorus on feed conversion by male broilers

Tibia Ash

Tibia ash at different ages is shown in Table 8. Tibia ash was not influenced by corn source, indicating that the NPP found in the HAPC was utilized as well as the dicalcium phosphate supplement used in the diets with YDC, in agreement with studies by Ertl et al. (1998) and Waldroup et al. (1999). Phytase supplementation resulted in significant improvements in tibia ash at all ages. Tibia ash of birds fed diets with the reduced phosphorus levels was significantly lower than that of birds fed the diets with NRC recommended levels at all ages, even with phytase supplementation. There was a significant interaction between phosphorus level and phytase supplementation; phytase supplements improved tibia ash to a greater extent at the lowest level of phosphorus supplementation. At 21 d, but not at older ages, phytase was more effective in improving tibia ash when added to diets containing YDC than in diets with HAPC.

Fecal Phosphorus

The phosphorus content of freeze-dried feces at different ages is shown in Table 9. These data are expressed in actual quantity and as relative amounts. The relative amount represents the percentage fecal phosphorus in relation to the YDC diet with the NRC-recommended phosphorus level with no phytase supplementation. Fecal phosphorus was markedly reduced by reducing the dietary phosphorus level and by substitution of HAPC for YDC. Addition of phytase had little effect on actual fecal phosphorus output when dietary phosphorus was reduced by 0.075% but had a greater impact on reduction of fecal phosphorus when dietary phosphorus was reduced by 0.15%. It must be noted, however, that addition of phytase to these diets reduced mortality and allowed the birds to grow at these lower phosphorus levels; thus, phytase supplementation is an important contributor to the overall strategy for reducing fecal phosphorus content.

Results of this study suggest that fecal phosphorus content can be markedly reduced without sacrificing live performance and skeletal integrity by the use of normal YDCor HAPCin conjunction with reduced dietary phosphorus content and phytase supplementation. Reducing dietary phosphorus levels in YDC diets to 0.15% below NRC recommendations along with phytase supplementation resulted in a 37.5% reduction in fecal phosphorus levels. A similar phosphorus level and phytase supplementation with HAPC diets reduced fecal phosphorus by 58.2%, as compared with broilers fed the YDC diet with NRC-recommended phosphorus levels. Although tibia ash was lower with these diets as compared with those fed the NRC-recommended levels, no apparent leg problems or excessive mortality was noted. Phytase supplementation appeared to be especially beneficial in reducing mortality associated with the lower dietary phosphorus levels, in agreement with the report by Waldroup et al. (1999).

The extent to which dietary phosphorus levels may be reduced over the growing period of the broiler needs further study, utilizing diets designed for specific age periods. The greatest part of the skeletal development of the broiler takes place during the first 3 wk of life (Skinner and Waldroup, 1995); after this time phosphorus levels may be greatly reduced with no adverse effects on performance (Waldroup et al., 1974; Sauveur, 1978; Yoshida and Hoshii, 1982; Nys et al., 1983; Tortuero and Tardon, 1983; Skinner et al., 1992). The extent to which tibia ash must be maximized to prevent breaking problems during processing must also be determined (Yoshida and Hoshii, 1982).

Table 7. Effect of normal yellow dent corn (YDC) or high available phosphate corn (HAPC), phytase supplementation, and level of nonphytate phosphorus on mortality by male broilers

Table 8. Effect of normal yellow dent corn (YDC) or high available phosphate corn (HAPC), phytase supplementation, and level of nonphytate phosphorus on tibia ash content of male broilers maintained in battery brooders

^a–e; x,yWithin comparisons, means with common superscript do not differ significantly (P ≤ 0.05).
¹Indicates reduction (% of diet) from NRC (1994) recommended level.
²With or without 1,000 units of phytase/kg.

Table 9. Effect of source of corn, phytase supplementation, and level of nonphytate phosphorus on phosphorus content of freeze-dried feces from male broilers maintained in battery brooders

¹Values in parentheses represent percentage fecal phosphorus in relation to normal corn diets at NRCrecommended level of nonphytate phosphorus.
²Yellow dent corn (YDC) diets contain NRC recommended level of nonphytate phosphorus.
³Addition of 1,000 units of phytase/kg. HAPC = high available phosphorus corn.

Acknowledgments

This study was supported by donation of the corn and a grant from Pioneer Hi-Bred International, Johnston, IA 50131, and a grant from the US Poultry and Egg Association, Tucker, GA 30084. Donation of phytase and phytase assays by BASF, Mt. Olive, NJ 07828, is greatly appreciated.

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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

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