Efficacy of Phase-Feeding in Supporting Growth Performance of Broiler Chicks During the Starter and Finisher Phases 1

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1 Efficacy of Phase-Feeding in Supporting Growth Performance of Broiler Chicks During the Starter and Finisher Phases 1 W. A. Warren and J. L. Emmert 2 Department of Poultry Science, University of Arkansas, Fayetteville, Arkansas ABSTRACT A feeding regimen has been developed IICP, or PF diets. In Experiment 2, NRC or IICP requirements that uses regression equations to predict amino acid requirements over time. Phase-feeding (PF) of broilers was were fed from 40 to 61 d, whereas PF was tested using a series of three diets (40 to 47, 47 to 54, and 54 to tested to evaluate its efficacy compared with feeding 61 d). No differences (P > 0.05) in weight gain or feed broilers NRC or Illinois ideal chick protein (IICP) recommendations. In Experiment 1, NRC or IICP requirements intake were observed, but the feed efficiency of birds fed the IICP diet was decreased (P < 0.05). The IICP and PF diets resulted in decreased (P < 0.05) digestible lysine for lysine, sulfur amino acids, and threonine were fed and threonine intake; gain per unit digestible lysine and from 0 to 21 d, whereas PF was tested using a series of threonine intake was increased (P < 0.05) by PF. No differences (P < 0.05) in breast meat, wing, or leg yield were three diets (0 to 7, 7 to 14, and 14 to 21 d). No differences (P > 0.05) in weight gain, feed intake, feed efficiency, noted among treatments. Economic analysis indicated digestible amino acid intake, or gain per unit digestible amino acid intake were noted among chicks fed NRC, that PF may facilitate reduced dietary costs without sacrificing growth performance or carcass yield. (Key words: phase-feeding, broiler, growth performance, amino acids, feeding programs) 2000 Poultry Science 79: INTRODUCTION The poultry industry encompasses production systems that include grow-out periods of as little as 4 wk of age, and it is becoming increasingly common for companies to attempt to maximize economic, uniform production of breast meat by raising cockerels separately, in some cases for up to 10 wk of age. The NRC (1994) provides a single set of recommendations that encompasses both pullets and cockerels, and requirements are segregated into three fixed periods: starter, 0 to 3 wk of age; grower, 3 to 6 wk of age; and finisher, 6 to 8 wk of age. This regimen does not correspond with the growout periods used in typical production systems. In addition, many companies are now rearing cockerels separately beyond 8 wk of age to obtain large quantities of breast meat. A flexible set of amino acid requirements that can adapt to various production systems is needed. Amino acid requirements (% of diet or % of calories) decrease steadily throughout the grow-out period, and studies have shown that broilers may be switched to a less nutri- Received for publication August 30, Accepted for publication January 28, Support by the Arkansas Agricultural Experiment Station, Fayetteville, AR To whom correspondence should be addressed: jemmert@comp. uark.edu. ent-dense grower diet earlier than 3 wk of age without sacrificing growth performance or carcass yield (Watkins et al., 1993; Saleh et al., 1995, 1996a,b). Phase-feeding (PF) has been used in swine to decrease nitrogen excretion without sacrificing growth performance. In an attempt to provide a flexible feeding program that is adaptable and applicable to a wide range of commercial conditions, Emmert and Baker (1997) used the Illinois ideal chick protein (IICP; Baker and Han, 1994; Baker, 1997) to develop regression equations that predict amino acid requirements for use in a PF regimen for broilers. Because the equations express digestible amino acid requirement as a function of age, requirement predictions for any specified period of time may be derived, including periods beyond 8 wk of age for which few requirement estimates are available. Moreover, PF may support the elimination of some excess dietary supplemental amino acids, thereby potentially decreasing dietary costs. Our objective was to evaluate the efficacy of PF in supporting growth of broilers during the starter and finisher periods. MATERIALS AND METHODS All procedures were approved by the University of Arkansas Institutional Animal Care and Use Committee. Abbreviation Key: IICP = Illinois ideal crude protein, PF = phasefeeding, SAA = sulfur amino acids, SBM = soybean meal 764

2 PHASE-FEEDING 765 TABLE 1. Composition of diets for 0 to 21-d-old chicks (Experiment 1) NRC 1 IICP 2 PF 3 PF 3 PF 3 Ingredient (d 0 to 21) (d 0 to 21) (d 0 to 7) (d 7 to 14) (d 14 to 21) Corn Soybean meal Poultry fat Vitamin mix Mineral mix Dicalcium phosphate Limestone NaCl Choline Cl (60%) L-Lysine HCl DL-Methionine NRC diets contained lysine, sulfur amino acid, and threonine levels recommended by NRC (1994), with the exception of lysine, which was supplemented to the level recommended by the Illinois ideal chick protein (IICP; Baker and Han, 1994; Baker, 1997). 2 IICP diets contained lysine, sulfur amino acids, and threonine levels recommended by the IICP profile (Baker and Han, 1994; Baker, 1997). 3 Levels of amino acids in phase-feeding (PF) diets were predicted by linear regression equations (Table 3). 4 Han and Baker (1993). (%) Two experiments were conducted utilizing male broiler chicks of a commercial strain (Cobb Cobb 3 ) that were purchased from a local hatchery. Chicks were housed in floor pens containing a litter mixture of new pine shavings and rice hulls. A 24-h constant light schedule was maintained, and water and experimental diets were freely available. In both experiments, dietary treatments (Tables 1 and 2) consisted of 1) a single diet, formulated to contain NRC (1994) recommendations for lysine, sulfur amino acids (SAA), and threonine, and fed for the entire 3-wk experiment; 2) a single diet, formulated to contain IICP (Baker and Han, 1994; Baker, 1997) recommendations for lysine, SAA, and threonine, fed for the entire 3-wk experiment; and 3) a series of three diets, formulated to contain lysine, SAA, and threonine requirements predicted by linear regression equations (Emmert and Baker, 1997); dietary amino acid concentration was lowered after each week of the experiment. Regression equations from Emmert and Baker (1997) were modified to reflect male requirements and were used to predict PF requirements for the first, second and third weeks of both trials as follows: digestible lysine, y = x; digestible methionine and cystine, y = ( x)/2; and digestible threonine, y = x, where y = digestible amino acid level, and x = midpoint (day) of the desired age range (example: x = 3.5 d for the first week of age). These equations are based on a combination of the best available digestible lysine, SAA, and threonine requirements for the starter, grower, and finisher periods. Specifically, digestible amino acid requirements for the starter period were lysine, 1.07% (Han and Baker, 1993); SAA, 0.77% (NRC, 1994); and threonine, 0.70% (NRC, 1994); for the grower period were lysine, 0.865% (Han and Baker, 1994); SAA, 0.62% 3 Cobb-Vantress Inc., Siloam Springs, AR (Baker et al., 1996); and threonine, 0.593% (Webel et al., 1996); and for the finisher period were lysine, 0.745% (NRC, 1994); SAA, 0.54% (Baker and Han, 1994; Baker, 1997); and threonine, 0.51% (Webel et al., 1996). In both experiments, corn and soybean meal (SBM) were added in sufficient quantities to meet the target digestible threonine concentration, and crystalline lysine and methionine were supplemented to meet their requirements. Because NRC (1994) recommendations are based on total dietary amino acid needs, after we formulated the NRC diet to meet total amino acid recommendations, the digestible lysine, SAA, and threonine contents were calculated by applying digestibility coefficients for corn and SBM. Corn was analyzed (as fed) to contain 8.6% CP, 0.28% total lysine, 0.22% total methionine, 0.22% total cystine, and 0.30% total threonine, and the digestibility of lysine, methionine, cystine and threonine in corn was assumed to be 78, 91, 86, and 84%, respectively (Parsons, 1991). Soybean meal was analyzed (as fed) to contain 46.7% CP, 2.91% total lysine, 0.66% total methionine, 0.71% total cystine, and 1.84% total threonine, and the digestibility of lysine, methionine, cystine, and threonine in SBM was assumed to be 90, 92, 83, and 89%, respectively (Parsons, 1991). The energy contents of corn, SBM, and poultry fat were assumed to be 3,350, 2,440, and 8,800 kcal ME n /kg, respectively (NRC, 1994). Experiment 1 Experiment 1 was conducted to assess the efficacy of PF in supporting growth of chicks during the starter period (0 to 21 d). It should be noted that the NRC diet (Treatment 1) was based on NRC (1994) recommendations for SAA and threonine, but the IICP starter period lysine recommendation was used for the NRC diet (Treatment 1) in this experiment because of previous research suggesting that the NRC (1994) lysine recom-

3 766 WARREN AND EMMERT TABLE 2. Composition of diets for 40 to 61-d-old chicks (Experiment 2) NRC 1 IICP 2 PF 3 PF 3 PF 3 Ingredient (d 40 to 61) (d 40 to 61) (d 40 to 47) (d 47 to 54) (d 54 to 61) Corn Soybean meal Poultry fat Vitamin mix Mineral mix Dicalcium phosphate Limestone NaCl Choline Cl (60%) L-Lysine HCl DL-Methionine Sacox salinomycin BMD-50 Bacitracin NRC diets contained lysine, sulfur amino acid, and threonine levels recommended by NRC (1994). 2 IICP diets contained lysine, sulfur amino acid, and threonine levels recommended by the IICP profile (Baker and Han, 1994; Baker, 1997). 3 Levels of amino acids in phase-feeding (PF) diets were predicted by linear regression equations (Table 3). 4 Han and Baker (1993). 5 Sacox 60, Hoechst-Roussel Agri-Vet Co., Somerville, NJ Provides 66 mg/kg salinomycin activity. 6 BMD-50, AlPharma, Inc., Ft. Lee, NJ Provides 55 mg/kg bacitracin methylene disalicylate activity. (%) mendation is too low (Han and Baker, 1991; Han and Baker, 1993). Ten pens of 40 male chicks were assigned to each of the three experimental feeding regimens. Experimental diets (Tables 1 and 3) based on NRC (Treatment 1) or IICP (Treatment 2) requirements were fed from 0 to 21 d, whereas PF (Treatment 3) was tested using a series of three diets (0 to 7, 7 to 14, and 14 to 21 d). Because PF lysine, SAA, and threonine requirements are based on digestible amino acid requirements as a function of age, the levels of dietary lysine, SAA, and threonine in the PF diets decreased when diets were switched on Days 7 and 14 (Table 3). Additionally, CP and ME n varied according to the dietary content of corn and SBM (Table 3). With PF, CP decreased from 23.0 to 21.1% over the course of the experiment, whereas ME n increased from 3,123 to 3,173 kcal ME n /kg. Chicks and feed were weighed at 7, 14, and 21 d of age for determination of weight gain, feed intake, and feed efficiency. TABLE 3. Calculated digestible amino acid levels in Experiments 1 and 2 Digestible content, % of diet 1 CP, ME n, 3 Experiment 1 2 Day Lysine Methionine Cystine Threonine % kcal/kg NRC 4 0 to ,173 IICP 0 to ,143 PF 0 to ,123 7 to , to ,173 Experiment 2 6 NRC 4 40 to ,247 IICP 40 to ,288 PF 40 to , to , to ,313 1 Digestible amino acid, CP, and dietary ME content calculated from the analytical values for total lysine, sulfur amino acids, and threonine in corn and soybean meal and published digestibility coefficients (Parsons, 1991; see Materials and Methods). 2 Experiment 1 was conducted from 0 to 21 d posthatching. 3 Metabolizable energy values for corn, soybean meal, and poultry fat were assumed to be 3,350, 2,440, and 8,800 kcal ME n /kg, repectively. 4 Although the NRC (1994) provides total dietary amino acid recommendations, digestible amino acid levels for Experiments 1 and 2 were calculated after formulation of diets to meet total NRC (1994) recommendations for dietary lysine, sulfur amino acids, and threonine. PF = phase-feeding. 5 The lysine requirement for the NRC treatment in Experiment 1 was based on Illinois ideal chick protein (IICP) (Baker and Han, 1994; Baker, 1997) recommendations. 6 Experiment 2 was conducted from 40 to 61 d posthatching. 7 Corn and soybean meal were added to the basal diet in sufficient quantities to meet the NRC (1994) recommendation for threonine, resulting in a level of dietary lysine that exceeded NRC (1994) recommendations.

4 PHASE-FEEDING 767 Experiment 2 Experiment 2 was conducted to assess the efficacy of PF in supporting growth of chicks during the finisher period (40 to 61 d). It should be noted that the finisher period NRC (1994) recommendation for threonine is high relative to lysine when compared with IICP recommendations. Therefore, the decision to meet the NRC (1994) dietary threonine recommendation from corn and SBM led to an excess of dietary lysine in the NRC diet (Treatment 1). In addition, although NRC (1994) and IICP recommendations for the finisher period encompass 42 to 56 d of age, no attempt was made to adjust NRC (1994) or IICP lysine, SAA, and threonine levels for the extended finisher period used in this experiment. Experimental diets (Tables 2 and 3) based on NRC (Treatment 1) or IICP (Treatment 2) requirements were fed from 40 to 61 d, whereas PF (Treatment 3) was tested using a series of three diets (40 to 47, 47 to 54, and 54 to 61 d). As in Experiment 1, dietary lysine, SAA, and threonine levels decreased when diets were switched on Days 47 and 54 (Table 3). Again, CP and ME n varied according to the dietary content of corn and SBM (Table 3). With PF, CP decreased from 17.2 to 15.3% over the course of the experiment, whereas ME n increased from 3,265 to 3,313 kcal ME n /kg. Because Experiment 2 was initiated on Day 40, chicks were fed a common starter diet from 0 to 21 d and a common grower diet from 21 to 40 d that were formulated to meet or exceed NRC (1994) requirements for all essential nutrients. Ten pens of 20 male chicks were assigned to each of the three experimental feeding regimens. Chicks and feed were weighed at 47, 54, and 61 d of age for determination of weight gain, feed intake, and feed efficiency. Feeders were removed from experimental pens on Day 60, 10 h prior to experiment termination. After weights were taken on Day 61, five birds per pen were randomly selected for processing at the University of Arkansas processing plant. Part weights were recorded for wings, legs (drum and thigh), breast, abdominal fat, and rack, and parts yields were calculated as a percentage of eviscerated weight. Statistical Analysis Both experiments were analyzed as completely randomized designs, and the general linear models procedure of SAS (SAS Institute, 1996) was used to conduct ANOVA on all data. Differences among treatment means were established using the least significant difference multiple-comparison procedure (Carmer and Walker, 1985). RESULTS AND DISCUSSION Experiment 1 was conducted to assess the efficacy of PF in supporting growth performance during the starter period. No differences (P > 0.05) in weight gain, feed intake, or feed efficiency were observed among birds fed NRC, IICP, or PF diets (Table 4). Although PF resulted in slight numerical improvements, no differences (P > 0.05) among treatments were noted with regard to digestible lysine, SAA, or threonine intake, weight gain per unit digestible lysine, SAA, or threonine intake. During the starter period, only a small amount of feed is consumed relative to the total feed consumed by a broiler over a 6 to 9 wk grow-out period. In addition, IICP requirement recommendations for lysine, SAA, and threonine are higher than NRC (1994) recommendations when expressed on a digestible basis (Table 3). Because PF amino acid requirements are largely based on the IICP (Emmert and Baker, 1997), the authors were not expecting an increase in growth performance or efficiency of amino acid utilization. The ability of PF to support equivalent growth performance to chicks fed the diet based on NRC (1994) requirements may not be surprising. This finding is in light of work showing that broiler chickens may be switched to a less nutrient-dense grower diet earlier than the recommended 3 wk of age without sacrificing 6-wk growth performance or carcass yield (Watkins et al., 1993; Saleh et al., 1995, 1996a,b). Although the results of Experiment 1 indicate that amino acid levels may be gradually lowered during the starter period without negatively impacting growth performance, the impact of PF during the finisher period is of even more interest. In Experiment 2, no difference (P > 0.05) in weight gain or feed intake was observed, but feed efficiency of birds fed the IICP diet was decreased (P < 0.05) relative to chicks consuming the NRC diet (Table 5). Both the IICP and PF diets were effective at decreasing (P < 0.05) digestible lysine and threonine intake, and PF also resulted in an increased (P < 0.05) weight gain per unit digestible lysine and threonine intake relative to chicks fed the NRC diet. In contrast, no differences (P > 0.05) in SAA intake or weight gain per unit digestible SAA intake were observed between chicks fed NRC and PF treatments, and chicks fed IICP diets exhibited the lowest (P < 0.05) weight gain per unit digestible SAA intake. The IICP diet resulted in a slight increase (P < 0.05) in abdominal fat relative to birds consuming the NRC diet, but no differences (P < 0.05) in breast meat, wing, or leg yield were noted among treatments. The chosen age range can play a large role in the results of studies evaluating PF. Because a single set of NRC and IICP recommendations is provided for the finisher period, and no requirement estimates are made beyond 8 wk of age, it would be expected that PF would be increasingly economically advantageous as the length of growout increases beyond the traditional finisher period. The authors are planning future studies to evaluate whether the magnitude of the improvements associated with PF are magnified when birds are raised beyond the age used in Experiment 2. It is possible that eventually PF would result in reductions in digestible SAA intake as well. Nevertheless, PF was effective in promoting equivalent growth performance and carcass composition and improved the efficiency with which lysine and thre-

5 768 WARREN AND EMMERT TABLE 4. Growth performance of chicks fed NRC, Illinois idel chick protein (IICP), or phase-feeding (PF)-based diets from 0 to 21 d of age (Experiment 1) 1 Treatment Parameter NRC 2 IICP 3 PF 4 SEM Weight gain, g Feed intake, g Gain:feed, g:kg Digestible lysine intake, g Digestible SAA 5 intake, g Digestible threonine intake, g Gain:digestible lysine intake, g:g Gain:digestible SAA intake, g:g Gain:digestible threonine intake, g:g Values are means of 10 pens of 40 male chicks fed the experimental diets from 0 to 21 d posthatching; average initial weight was 46 g. 2 NRC diets contained 20.9% CP, 1.12% digestible Lys, 0.41% digestible Met, 0.38% digestible Cys, and 0.70% digestible Thr (digestible amino acid levels were calculated after formulation of diets to meet total NRC (1994) recommendations). Values are based on analysis of total amino acid content of corn and soybean meal combined with digestibility coefficients from Parsons (1991). 3 IICP diets contained 22.2% CP, 1.12% digestible Lys, 0.41% digestible Met, 0.41% digestible Cys, and 0.75% digestible Thr. Values are based on analysis of total amino acid content of corn and soybean meal combined with digestibility coefficients from Parsons (1991). 4 PF diets contained 23.0% CP, 1.19% digestible Lys, 0.43% digestible Met, 0.43% digestible Cys, and 0.78% digestible Thr from 0 to 7 d posthatching; 21.9% CP, 1.12% digestible Lys, 0.41% digestible Met, 0.41% digestible Cys, and 0.74% digestible Thr from 7 to 14 d posthatching; and 21.1% CP, 1.05% digestible Lys, 0.38% digestible Met, 0.38% digestible Cys, and 0.71% digestible Thr from 14 to 21 d posthatching. Values are based on analysis of total amino acid content of corn and soybean meal combined with digestibility coefficients from Parsons (1991). 5 Sulfur amino acids. TABLE 5. Growth performance of chicks fed NRC, Illinois ideal chick protein (IICP) or phase-feeding (PF)-based diets from 40 to 61 d of age (Experiment 2) 1 Treatment Parameter NRC 2 IICP 3 PF 4 SEM Weight gain, g 1,576 1,483 1, Feed intake, g 3,733 3,852 3, Gain:feed, g:kg 422 a 386 b 402 ab 7 Digestible Lys intake, g 31.0 a 29.3 b 28.0 b 0.6 Digestible SAA 5 intake, g Digestible Thr intake, g 22.4 a 20.4 b 20.2 b 0.4 Gain:digestible Lys intake, g:g 50.9 b 50.7 b 54.2 a 1.0 Gain:digestible SAA intake, g:g 72.8 a 66.5 b 71.6 a 1.3 Gain:digestible Thr intake, g:g 70.4 b 72.8 ab 75.2 a 1.4 Eviscerated carcass, % of live weight Breast, % of eviscerated carcass Wing, % of eviscerated carcass Leg, % of eviscerated carcass Abdominal fat, % of eviscerated carcass 2.7 b 3.1 a 2.9 ab 0.1 a b Means within a row lacking a common superscript differ (P < 0.05). 1 Values are means of 10 pens of 20 male chicks fed the experimental diets from 40 to 61 d posthatching; average initial weight was 1.82 kg. 2 NRC diets contained 17.9% CP, 0.83% digestible Lys, 0.30% digestible Met, 0.28% digestible Cys, and 0.60% digestible Thr (digestible amino acid levels were calculated after formulation of diets to meet total NRC (1994) recommendations). Values are based on analysis of total amino acid content of corn and SBM combined with digestibility coefficients from Parsons (1991). 3 IICP diets contained 16.1% CP, 0.76% digestible Lys, 0.29% digestible Met, 0.29% digestible Cys, and 0.53% digestible Thr. Values are based on analysis of total amino acid content of corn and SBM combined with digestibility coefficients from Parsons (1991). 4 PF diets contained 17.2% CP, 0.81% digestible Lys, 0.30% digestible Met, 0.30% digestible Cys, and 0.57% digestible Thr from 40 to 47 d posthatching; 16.1% CP, 0.74% digestible Lys, 0.28% digestible Met, 0.28% digestible Cys, and 0.53% digestible Thr from 47 to 54 d posthatching; and 15.3% CP, 0.67% digestible Lys, 0.26% digestible Met, 0.26% digestible Cys, and 0.50% digestible Thr from 54 to 61 d posthatching. Values are based on analysis of total amino acid content of corn and SBM combined with digestibility coefficients from Parsons (1991). 5 Sulfur amino acids. 6 Live weights for birds fed NRC, IICP, and PF diets were 3.46, 3.49, and 3.48 kg, respectively.

6 PHASE-FEEDING 769 TABLE 6. Impact of phase-feeding (PF) on dietary costs (Experiment 2) Finisher period, days NRC IICP 1 PF PF PF (40 to 61) (40 to 61) (40 to 47) (47 to 54) (54 to 61) Dietary cost ($/kg) Feed cost ($/bird) Difference from NRC 4 ($/bird) Illinois ideal chick protein. 2 Dietary costs based on prices of $0.1036/kg for corn, $0.1631/kg for soybean meal, $1.1023/kg for L-lysine HCl, and $2.4251/kg for DL- methionine. 3 Feed cost per bird determined by multiplying the dietary cost by the total feed consumed per bird. 4 Calculated by subtracting the feed cost of the IICP and PF programs from the NRC feed cost. onine were used for weight gain during the finisher phase. A potential concern associated with formulating diets to match the requirements predicted by PF equations is the substantial decrease in dietary CP that occurs (Table 3), bringing into question whether dietary indispensable nitrogen levels are sufficient to support dispensable amino acid synthesis. Our results suggest that despite substantial CP reductions associated with the latter phases of PF, indispensable nitrogen levels were sufficient to support growth performance and meat yield. No differences in growth performance were detected with the exception of the feed efficiency of birds fed the IICP diet in Experiment 2, and it is possible that the decreased feed efficiency (relative to the NRC diet) was the result of a combination of low CP and increased ME n. Previous research also suggests that growth performance may be maintained when dietary protein levels are slightly to moderately decreased, provided diets are supplemented with essential amino acids such as lysine and methionine (Daghir, 1983; Han et al., 1992; Morris et al., 1992; Deschepper and De Groote, 1995). However, other researchers (Fancher and Jensen, 1989a,b) have been unable to maintain growth performance and protein accretion when feeding CP levels at which other researchers have noted no impact on performance, which may reflect differences in experimental protocols such as assay length and age of chick. Fancher and Jensen (1989a,b) included supplemental amino acids in their calculation of dietary CP calculations, whereas other researchers reported the dietary CP level from only intactprotein sources, without regard to the nitrogen furnished by supplemental amino acids. The lower CP levels associated with PF may also be a concern because of the potential impact on carcass composition. Carcass fat has been shown to be elevated in birds consuming low-protein diets for a period of weeks (Fancher and Jensen, 1989a,b; Deschepper and De Groote, 1995). Low-protein diets contain fewer excess amino acids that require energy expenditure for catabolism, thereby likely increasing the net energy of the diet and the dietary energy available for fat synthesis. The increased abdominal fat percentage associated with the IICP and PF treatments in Experiment 2 seem to support this conclusion, but the observed increase was slight, and the relative economic importance may be minor. Of greater economic importance is breast muscle accretion under PF conditions. Although previous research has suggested that the level of dietary lysine and methionine needed to maximize breast yield may exceed the amount needed to maximize weight gain and feed efficiency (Sibbald and Wolynetz, 1986; Hickling et al., 1990; Moran and Bilgili, 1990; Bilgili et al., 1992; Han and Baker, 1993), we observed no negative impact of PF on breast, wing, or leg yield in Experiment 2. Economic analysis of amino acid-containing ingredients (Table 6) indicates that the cost of feeding broilers during the finisher period may be reduced by PF. Although seemingly small on an individual basis, the reduction in dietary cost associated with PF in Experiment 2 would be substantial when applied to the billions of birds processed annually in the US. Other potential benefits of PF have yet to be explored. We did not analyze litter, but it is possible that PF may reduce nitrogen excretion due to elimination of excess nitrogen from the diet. Phase-feeding has been evaluated in swine as a means of reducing nitrogen excretion (Boisen et al., 1991). Pigs fed under PF conditions maintained a rate of gain and feed efficiency that was similar to pigs fed diets containing higher crude protein levels. Conversely, pigs given the PF regimen excreted significantly less nitrogen, indicating an increased efficiency of dietary nitrogen utilization. Clearly, PF would not be economically feasible if six or more diets are fed during the grow-out period, due to the increased cost associated with diet preparation, transport, and storage. Rather, it may be possible to accomplish PF by initially delivering a nutrient-dense starter-type diet and a less-dense finisher-type diet, which could be blended at a desired rate to achieve gradual decreases in dietary amino acid levels. This procedure is similar to the approach used in parts of Europe, where diets are diluted by one or few relatively inert dietary ingredients as broiler chickens advance in age and weight. However, that system is functional because the basal diet is over-fortified, whereas PF would closely meet dietary amino acid requirements over the entire grow-out period.

7 770 WARREN AND EMMERT Further investigation is needed to verify the efficacy of PF over the entire grow-out period, with particular emphasis on the impact of PF during the starter period on growth performance and carcass composition of broiler chickens marketed between 6 and 10 wk of age. In addition, the impact of factors such as dietary energy level and bird density should be investigated. However, early indications suggest the PF may offer nutritionists a flexible alternative that facilitates application to commercial poultry nutrition programs. Substantial savings in the cost of production may be possible should PF be proven feasible under commercial conditions. REFERENCES Baker, D. H., Ideal amino acid profiles for swine and poultry and their application in feed formulation. Pages 1 24 in: Biokyowa Technical Review No. 9. Biokyowa Press, St. Louis, MO. Baker, D. H., S. R. Fernandez, D. M. Webel, and C. M. 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