J APPL POULT RES 2008. 17:157-167. doi:10.3382/japr.2007-00071
© 2008 Poultry Science Association
Dietary Amino Acid Responses of Broiler Chickens1
W. A. Dozier, III*,2,
M. T. Kidd
and
A. Corzo
* USDA, Agriculture Research Service, Poultry Research Unit, PO Box 5367, Mississippi State 39762-5367; and Department of Poultry Science, Mississippi State University, Mississippi State 39762
Correspondence: 2 Corresponding author: bill.dozier{at}ars.usda.gov
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SUMMARY
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In commercial practice, formulating diets to adequate amino acid (AA) minimums is critical to optimize live production and meat yield of broiler chickens. The modern broiler has lower feed intake per unit BW gain and also has the potential to accrete more white meat than the commercial broiler of previous decades. Broilers consuming less feed per unit of gain have led to formulating higher AA density diets in commercial production for improved performance and meat yield. This manuscript reviews current literature in dietary AA density. In addition, it provides estimates of consumption and dietary percentages of critical AA needed to optimize growth and meat yield calculated from published research.
Key Words: amino acid • broiler • lysine • methionine • nutrient density
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DESCRIPTION OF PROBLEM
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Within the last 10 yr, demand for breast fillets and value-added products has contributed to increasing market weights of broiler chickens. Modern broilers reach their target weights in fewer days than the commercial broiler used in previous years [1, 2]. Fancher [3] reported that genetic selection by primary breeding companies has increased 42-d BW by 0.55 kg/yr during the past decade. With genetic improvements, broilers consume less feed per unit of BW gain [4, 5]. Hence, dietary amino acid (AA) needs of the modern broiler should be increased compared with AA minimums used with broilers in previous years.
Nutritional programs differ in AA density among broiler complexes in the United States [3]. Market weight, product mix, live cost, and genetic strain are factors that may govern AA supplementation. In the United States, 2 diverse nutritional strategies are practiced with regard to AA supplementation: 1) formulating diets to low AA density to minimize feed cost or 2) formulating diets to high AA density to optimize breast meat yield. The former philosophy can limit meat accretion of the modern broiler while not maximizing profits, especially when consideration is given to breast meat yield and breast meat prices [6–9].
This manuscript will review current literature of broiler responses to dietary AA density. Also, data are summarized to estimate critical dietary AA percentages and AA intake on a daily basis to optimize performance for a 49-d production period.
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IMPORTANCE OF DIETARY LYS FOR BREAST MEAT YIELD
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Amino acids are critical for muscle development [10] and Lys content in breast muscle is relatively higher than other AA [11]. Lysine represents approximately 7% of the protein in breast meat (pectoralis major and minor muscles) [11]. Dietary Lys inadequacy has been shown to reduce breast meat yield compared with other muscles [10]. Therefore, defining dietary AA needs for optimum growth and meat yield is of utmost importance.
Vazquez and Pesti [12] reviewed 16 data sets throughout the world and estimated the dietary Lys requirement as 1.21% for BW gain and 1.32% (total) for feed efficiency for the young chick from 0 to 21 d of age. These estimates based on growth were much higher than the 1.10% dietary Lys requirement recommended by NRC [13]. Dietary Lys needs vary with the response criterion and breast meat has a higher estimate than growth responses [14–22]. In a recent study, de Leon [21] determined the Lys requirement for 15- to 35-d-old Ross x Ross 708 broilers as 1.17% [1.04% digestible (dig) Lys] for optimum breast meat yield. From 6 to 8 wk, Corzo et al. [17] reported the dietary Lys requirement as 0.93% for breast meat yield with male Ross x Hubbard broilers, but significant quadratic responses were not observed for BW gain or feed conversion and so a requirement could not be determined. Female broilers did not respond to increasing dietary Lys for growth and meat yield parameters; thus a requirement was not estimated. Although feeding high Lys diets throughout production optimizes breast meat yield [15, 16], it may not always be economically justified. However, evidence in the literature suggests that feeding diets high (H) in Lys during the starter period impacts subsequent breast meat yield at marketing [14–16]. This strategy can potentially be economically advantageous because feed intake is the lowest during the starter period compared with subsequent periods, yet increases with white meat yield may be obtained. Kidd et al. [15] evaluated various Lys concentrations in the starter, grower, and finisher periods during a 49-d production cycle. Feeding broilers a diet formulated to 1.25% Lys from 1 to 18 d produced a 1.1% higher (P 
0.05) 49-d breast meat yield compared with broilers fed 1.04% dietary Lys from 1 to 18 d of age. Broilers fed the 1.04% dietary Lys during the starter period and provided 1.05 or 1.25% from 19 to 49 d of age did not respond with similar breast meat yield compared with broilers consuming 1.25% dietary Lys throughout 49-d production period.
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ROLE OF AMINO ACIDS IN MUSCLE DEVELOPMENT
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Postnatal protein accretion results from an increase in protein synthesis or a decrease in protein degradation. Diets containing low Lys can limit breast meat formation early in development by reducing protein accretion from protein synthesis and RNA content [10, 23, 24]. Dietary protein has been shown to regulate insulin-like growth factor 1 (IGF-I) concentrations [10, 25–27] in avian species. Doumit et al. [28] reported that IGF-I stimulates differentiation and proliferation of satellite cells. Other research [29] found a 62% increase in protein synthesis and a 38% decrease in protein degradation when muscle cells were incubated with IGF-I. The expression of transcription factors c-fos are activated by IGF-I [30], and c-fos transcription factor binds to another transcription factor, c-jun, to form a complex known as AP-1 protein. The AP-1 protein activates transcription of myogenic genes. The activation of transcription by IGF-I has also been reported by Ong et al. [31] who reported a 4-fold increase in mRNA of skeletal muscle cells when incubated with IGF-I. Muscle DNA accumulates as myofiber size increases during postnatal growth, even though myofiber numbers are fixed and myonuclei do not divide [32, 33]. Providing broilers H AA dense diets increases breast meat yield, likely by increasing myofiber size via modulating protein synthesis and protein degradation.
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AMINO ACID INTERACTIONS
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Feeding H Lys and AA dense diets to broilers increases breast meat yield [5–9]. Dietary AA responses influencing breast meat yield may be additive among AA [21, 34–36], but other research found no interactions between Lys and Met [37]. Hickling et al. [34] reported that broilers fed diets with increasing Lys concentrations from 100 to 112% of the NRC [13] recommendation needed a dietary Met concentration at 112% of the NRC recommendation to optimize carcass and breast meat yields. Dietary Lys and Thr concentrations have also been shown to interact to optimize meat yield [35, 36]. From 18 to 54 d of age, optimum BW gain, fillet, drumstick, and thigh yields occurred with broilers fed diets containing Lys and Thr at 150 and 100%, respectively, of the NRC [13] recommendations [35]. However, decreasing dietary Thr concentration below 100% of the NRC recommendation in concert with 150% of the NRC Lys recommendation did not allow for optimum meat yield [35]. Kerr et al. [36] also reported a dietary Lys x Thr interaction for maximum breast meat and drumstick weights with 52-d-old broilers. These interactions inferred that dietary Thr need was 100% of NRC [13] recommendation when 105% of NRC dietary Lys was offered, but as dietary Lys was increased to 120% of NRC, 108% of NRC dietary Thr requirement was needed to maximize breast meat and drumsticks weights. Dietary Lys and Thr concentrations did not interact to influence thigh weight and yield or BW gain, which was inconsistent with a previous report [35]. Overall, dietary Lys x Met and Lys x Thr interactions were apparent to optimize meat accretion. Increasing dietary Lys without an increase in Met and Thr may limit protein synthesis, hence not allowing for maximum meat accretion.
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DIETARY AMINO ACID DENSITY RESPONSES
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In the United States, broiler feeding programs differ in AA density with bird size, product mix, and company philosophy as significant factors for the diversity of the concentrations of AA used among broiler companies. Genetic selection has resulted in the modern broiler having lower feed intake per unit of BW gain compared with commercial broilers of the past. Therefore, feed conversion and meat yield of the modern broiler should be optimized when fed diets formulated to H AA density. Dietary AA density research has been conducted recently with modern broiler chickens to document the importance of feeding diets H in AA density. With the emphasis placed on AA density research over the last 5 yr [5–8, 38–44], dietary AA density in the United States has increased each year from 2001 to 2005 for broilers grown to 2.7 kg or larger (Table 1
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Table 1. Dietary amino acid and crude protein density fed to broilers > 2.7 kg in the US broiler industry from 1995 to 20051
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Fast-Food and Tray-Pack Markets
Kidd et al. [41] reported the advantages of providing Ross x Ross 508 broilers H AA density diets from 1 to 49 d of age on BW, feed conversion, and breast meat yield. However, it was critical to provide high AA dense diets early in life (1 to 28 d) for improved performance and meat yield. Broilers fed H AA density diets throughout 49 d of age had a 1 percentage point increase (P 0.05) in breast meat yield over broilers provided H AA density diets until 28 d and moderate (M) or low (L) AA density diets from 29 to 49 d of age. Broilers fed low L AA density diets during the 49-d period had a 3 percentage point decrease (P 0.05) in breast meat yield compared with feeding broilers H AA density diets. Feeding H AA density diets throughout 35 d of age maximized breast meat yield at 35 d of age, but optimum 35-d cumulative feed conversion occurred when providing the H AA density diets until 28 d of age and feeding M or L AA diets from 29 to 35 d of age. Male broilers needed higher dietary AA percentages than female broilers. Male broilers optimized 35-d BW (1.8 kg) with dietary Lys of 1.38% (1 to 14 d), 1.20% (15 to 28 d), and 1.11% (29 to 35 d), respectively, in the starter, grower, and finisher periods, whereas females needed only 1.22% (1 to 14 d), 1.10% (15 to 28 d), and 0.94% (29 to 35 d), respectively, in the starter, grower, and finisher periods for 35-d BW (1.6 kg).
Conversely, Corzo et al. [40] determined that Arbor Acres Plus broilers fed H AA density diets (1.37% Lys) optimized growth rate and feed conversion from 1 to 14 d, but feeding M AA dense diets from 15 to 49 d of age (1.10% Lys, 15 to 28 d; 0.94% Lys, 28 to 35 d; 0.90% Lys, 36 to 49 d) was adequate for BW, feed conversion, and breast meat yield. Feeding H AA density diets early in development improves subsequent performance and meat yield [39, 40], which concurs with previous research that demonstrated feeding broilers H Lys diets during the starting period increased subsequent breast meat yield [14–16]. Broilers fed H AA diets early in development should have a favorable economic return on investment because feed intake is relatively low during this time of the production cycle.
Corzo et al. [43] evaluated dietary AA responsiveness with Ross x Cobb 500 broilers grown to 2.0 kg. Feeding H AA density throughout 35 d of age (1.49% Lys, 1 to 7 d; 1.33% Lys, 8 to 20 d; 1.24% Lys, 21 to 28 d; 1.10% Lys, 29 to 35 d) resulted in a 5-point improvement (P 0.05) of cumulative feed conversion (1.49 vs. 1.54) over broilers provided L AA density diets (1.35% Lys, 1 to 7 d; 1.35% Lys, 8 to 20 d; 1.10% Lys, 21 to 28 d; 0.95% Lys, 29 to 35 d). Dietary AA density did not influence carcass or breast meat yield at 35 d of age. Dietary AA density and gender did interact to influence feed intake and BW uniformity at 7 d. Male chicks fed H AA dense diets consumed less feed (P 0.05) and had more uniform BW (P 0.05), whereas dietary AA density did not affect feed intake and BW uniformity of female broilers at 7 d of age. Increasing AA density has also been shown to improve BW uniformity at 14, 28, 35, and 49 d of age [40], and the dietary AA percentage needed to optimize growth is influenced by strain and BW [40, 41, 43].
Debone Markets
Growth and meat yield responses to broilers fed H AA density diets grown to 2.7 kg have been well documented [9, 38–41, 43]. With the increasing number of broilers processed over 3.0 kg in the United States, defining dietary AA density needs of broiler chickens from 5 to 9 wk of age is of utmost importance. However, research is sparse evaluating dietary AA needs for this time period. Feeding diets formulated to contain AA density either at suboptimum or superfluous concentrations can become costly, particularly with large broilers, because 70% of the feed consumed occurs from 5 to 9 wk of age and the product mix is for the debone market.
Dozier et al. [7] evaluated dietary AA needs of Ross x Ross 708 broiler chickens from 36 to 59 d of age based on growth performance, meat yield, and economics with 2 identical trials. Six dietary treatments fed from 36 to 59 d of age were HH, HM, HL, MM, ML, and LL. Broilers were provided a 4-phase feeding program: starter (1 to 17 d), grower (18 to 35 d), withdrawal 1 (36 to 47 d), and withdrawal 2 (48 to 59 d). From 1 to 35 d of age, birds were fed common starter and grower diets formulated to H AA density. Dietary Lys, TSAA, and CP concentrations for the 6 experimental diets fed from 36 to 59 d of age were HH (36 to 47 d: 0.93% dig Lys, 0.73% dig TSAA, and 19.8% CP; 48 to 59 d: 0.89% dig Lys, 0.69% dig TSAA, and 18.0% CP), HM (36 to 47 d: 0.93% dig Lys, 0.73% dig TSAA, and 19.8% CP; 48 to 59 d: 0.80% dig Lys, 0.65% dig TSAA, and 17.3% CP), HL (36 to 47 d: 0.93% dig Lys, 0.73% dig TSAA, and 19.8% CP; 48 to 59 d: 0.72% dig Lys, 0.61% dig TSAA, and 16.0% CP), MM (36 to 47 d: 0.84% dig Lys, 0.70% dig TSAA, and 18.2% CP; 48 to 59 d: 0.80% dig Lys, 0.65% dig TSAA, and 17.3% CP), ML (36 to 47 d: 0.84% dig Lys, 0.70% dig TSAA, and 18.2% CP; 48 to 59 d: 0.72% dig Lys, 0.61% dig TSAA, and 16.0% CP), and LL (36 to 47 d: 0.75% dig Lys, 0.66% dig TSAA, and 16.7% CP; 48 to 59 d: 0.72% dig Lys, 0.61% dig TSAA, and 16.0% CP).
Optimum cumulative feed conversion was obtained when feeding the HH regimen from 36 to 59 d of age resulting in 2, 3, 4, 4, and 7 point advantages (P 0.05), respectively, over HM, MM, HL, ML, and LL fed birds. In contrast to the younger broiler, BW gain was not affected by dietary AA density from 36 to 59 d of age. Reducing dietary AA density to LL regimen during 36 to 59 d of age limited (P 0.05) breast meat yield compared with the other dietary regimens. When varying breast meat prices and diet costs as outputs and inputs, the HH regimen had higher gross feeding margins per bird than the other dietary treatments. For example, feed costs (diet cost x feed consumed) were $1.180, $1.187, $1.161, $1.71, $1.153, and $1.149 per kg, respectively, for HH, HM, HL, MM, ML, and LL during the 59-d period. Based on feed costs noted, breast meat weight and breast meat prices (breast fillets and breast tenders prices were $3.31 and $3.97 per kg, respectively), increased gross feeding margins from the HH regimen of $0.015, $0.047, $0.007, $0.011, and $0.043 per bird were observed, compared with the HM, HL, MM, ML, and LL regimens, respectively. Breast meat price had a more pronounced effect on gross feeding margin than ingredient prices. Therefore, breast meat prices and ingredient cost must be considered collectively when establishing dietary AA minimums.
Dozier et al. [44] further evaluated dietary AA needs of Ross x Ross 708 broilers from 36 to 59 d of age. This study was conducted to avoid carryover effects that may have occurred in [7] from 36 to 59 d of age. Two separate experiments were conducted: experiment 1 evaluated responses from 36 to 47 d on 59-d performance, and experiment 2 assessed responses from 48 to 59 d of age. The 4 treatments in experiment 1 consisted of HH, HL, ML, and LL. The LL AA specifications from 36 to 47 and 48 to 59 d of age were increased compared with [7] to minimize adverse effects with breast meat yield. Dietary Lys, TSAA, and CP concentrations of the 4 experimental diets fed from 36 to 59 d for experiment 1 were HH (36 to 47 d: 0.92% dig Lys, 0.75% dig TSAA, and 19.0% CP; 48 to 59 d: 0.88% dig Lys, 0.70% dig TSAA, and 17.6% CP), HL (36 to 47 d: 0.92% dig Lys, 0.75% dig TSAA, and 19.0% CP; 48 to 59 d: 0.75% dig Lys, 0.63% dig TSAA, and 16.1% CP), ML (36 to 47 d: 0.85% dig Lys, 0.73% dig TSAA, and 18.0% CP; 48 to 59 d: 0.75% dig Lys, 0.63% dig TSAA, and 16.1% CP), LL (36 to 47 d: 0.78% dig Lys, 0.70% dig TSAA, and 16.8% CP; 48 to 59 d: 0.75% dig Lys, 0.63% dig TSAA, and 16.1% CP). Cumulative feed conversion was improved by 5 and 3 points (P 0.05) with the HH and HL regimens over the LL regimen, respectively. Dietary HH and HL regimens, respectively, provided 37 and 38 g more (P 0.05) breast meat per bird compared with the LL regimen, resulting in approximately a 0.6 percentage point increase in breast meat yield. The HL regimen had the highest gross feeding margin per bird based on 35 scenarios of varying feed cost and breast meat prices typical of market fluctuations occurring during a 12-mo period.
The dietary treatments for experiment 2 were H, M, L, or suboptimum (S) AA density from 48 to 59 d of age: H (48 to 59 d: 0.87% dig Lys, 0.70% dig TSAA, and 17.9% CP), M (48 to 59 d: 0.81% dig Lys, 0.65% dig TSAA, and 16.9% CP), L (48 to 59 d: 0.75% dig Lys, 0.60% dig TSAA, and 15.8% CP), and S (48 to 59 d: 0.69% dig Lys, 0.55% dig TSAA, and 14.8% CP). Birds were fed H AA density diets from 1 to 47 d of age. Broilers provided the H diet had 2, 4, and 5 point advantages (P 0.05) in cumulative feed conversion over birds fed the M, L, and S diets, respectively. No treatment differences were observed for BW gain. The H fed birds had more (P 0.05) total breast meat weight (963 vs. 916 g) and yield (24.0 vs. 23.2%) over broilers given the S diet. Broilers fed the H diet had higher gross feeding margins with 35 different economic scenarios based on changing feed costs and breast meat prices than the other treatments.
Dozier et al. [5] evaluated dietary AA density and ME responses of broilers from 42 to 56 d of age with 2 different trials. Dietary AA density and AME did not interact to influence growth performance or meat yield, but main effects were observed. Dietary AA density treatments were H (0.98% Lys, 0.83% TSAA, and 18.0% CP) and L (0.88% Lys, 0.75% TSAA, and 16.2% CP). Feeding the H AA density diet improved (P 0.01) 42 to 56 d feed conversion by 10 and 7 points and increased (P 0.01) breast meat yield by 0.5 and 0.6 percentage points, respectively, in trials 1 and 2, over broilers provided the L AA density diet.
The previously mentioned research [5, 7, 44] addressed dietary AA needs from 36 to 59 d or 42 to 56 d of age to avoid the carryover effects of dietary AA density during the early growth periods. However, a broiler company desiring to optimize meat yield might feed an H AA regimen throughout the production cycle. Research [6, 8, 42] has addressed feeding dietary AA density diets throughout an 8-wk production period. Kidd et al. [8] evaluated dietary AA density needs of Ross x Ross 708 broilers from 1 to 55 d. A 5-phase feeding program was provided that included a prestarter (1 to 5 d), starter (6 to 14 d), grower (15 to 35 d), withdrawal 1 (36 to 45 d), and withdrawal 2 (46 to 55 d). Dietary treatments were combinations of H and M AA density diets fed throughout the 55-d period as MMMMM, HMMMM, HHMMM, HHHMM, HHHHM, and HHHHH. Diets formulated to H AA density were analyzed to contain Lys, TSAA, and CP for prestarter (1.38% Lys, 0.93% TSAA, and 22.6% CP), starter (1.36% Lys, 0.93% TSAA, and 22.4% CP), grower (1.23% Lys, 0.83% TSAA, and 20.1% CP), withdrawal 1 (1.13% Lys, 0.83% TSAA, and 20.5% CP), and withdrawal 2 (1.09% Lys, 0.74% TSAA, and 17.8% CP). Diets formulated to M AA density were analyzed to contain Lys, TSAA, and CP for prestarter (1.28% Lys, 0.87% TSAA, and 20.6% CP), starter (1.24% Lys, 0.85% TSAA, and 20.5% CP), grower (1.05% Lys, 0.75% TSAA, and 17.3% CP), withdrawal 1 (1.06% Lys, 0.76% TSAA, and 18.9% CP), and withdrawal 2 (1.01% Lys, 0.67% TSAA, and 16.5% CP). The HHHHH fed broilers had a 4- to 5-point improvement (P 0.05) in feed conversion vs. the other treatments, but final BW and total breast meat were not affected. However, a sample of birds was processed at 35 d, and broilers fed the HHH regimen had increases (P 0.05) of total breast meat yield ranging from 0.41 to 0.57 percentage points compared with the MMM, HMM, and HHM fed broilers. Feeding M AA density during the withdrawal 1 and 2 diets probably met the broilers AA needs, thus no differences in breast meat yield were detected at 55 d.
Dozier et al. [6] evaluated M and H AA density in 3- and 4-phase feeding schedules fed during a 56-d production cycle. The 4 dietary treatments were implemented, consisting of either H or M AA density that were provided for the duration of 3 phases (1 to 17, 18 to 35, and 36 to 56 d) or 4 phases (1 to 17, 18 to 35, 36 to 46, and 47 to 56 d). The H AA density in the 3-phase schedule contained Lys, TSAA, and CP for starter (1.33% Lys, 1.03% TSAA, and 23.6% CP), grower (1.18% Lys, 0.92% TSAA, and 21.5% CP), and withdrawal (1.05% Lys, 0.84% TSAA, and 19.8% CP). The M AA density in the 3-phase schedule contained Lys, TSAA, and CP for starter (1.23% Lys, 0.95% TSAA, and 21.9% CP), grower (1.09% Lys, 0.84% TSAA, and 19.8% CP), and withdrawal (0.97% Lys, 0.77% TSAA, and 18.3% CP). Diets formulated to H AA density in the 4-phase schedule contained Lys, TSAA, and CP for starter (1.33% Lys, 1.03% TSAA, and 23.6% CP), grower (1.18% Lys, 0.92% TSAA, and 21.5% CP), withdrawal 1 (1.12% Lys, 0.87% TSAA, and 20.8% CP), and withdrawal 2 (0.97% Lys, 0.78% TSAA, and 18.7% CP). Diets formulated to M AA density in the 4-phase schedule contained Lys, TSAA, and CP for starter (1.23% Lys, 0.95% TSAA, and 21.9% CP), grower (1.09% Lys, 0.84% TSAA, and 19.8% CP), withdrawal 1 (1.04% Lys, 0.81% TSAA, and 19.3% CP), and withdrawal 2 (0.90% Lys, 0.72% TSAA, and 17.6% CP).
Broilers fed diets formulated to H AA density in the 3-phase schedule had increased 56 d BW (3,014 vs. 2,921 g; P 0.054), breast fillets (515 vs. 489 g; P 0.063), breast tenders (114 vs. 107 g; P 0.044), and total breast meat (629 vs. 596 g; P 0.051) over broilers provided M AA density diets in the 3-phase schedule. Feeding the H AA regimen with the 4-phase schedule increased 56-d BW over the M AA density diets (2,997 vs. 2,909 g; P 0.065) in the 4-phase schedule. This study was conducted during hot weather and broilers fed H AA diets had heavier BW than M AA fed birds, regardless of feeding schedule. This finding is in disagreement with other dietary AA density research conducted during the finisher period, where no differences in BW were observed due to dietary AA density [5, 7, 8, 44]. One possible explanation for the increased BW [6] was that the birds may have been limiting in AA intake due to a heat stress.
Corzo et al. [42] evaluated responses of broilers fed M AA diets (intended to resemble some of United States broiler complexes at that time) compared with feeding diets formulated to H AA density throughout a 56-d production cycle. Diets consisted of starter (1 to 14 d), grower (15 to 28 d), finisher (29 to 42 d), and withdrawal (43 to 56 d). Diets formulated to H AA density in the 4-phase schedule contained Lys, Met, and CP for starter (1.31% Lys, 0.34% Met, and 22.3% CP), grower (1.21% Lys, 0.33% Met, and 19.4% CP), finisher (1.04% Lys, 0.28% Met, and 17.2% CP), and withdrawal (0.89% Lys, 0.28% Met, and 15.8% CP). Diets formulated to M AA density in the 4-phase schedule contained Lys, Met, and CP for starter (1.27% Lys, 0.35% Met, and 21.1% CP), grower (1.06% Lys, 0.31% Met, and 18.0% CP), finisher (0.95% Lys, 0.26% Met, and 16.2% CP), and withdrawal periods (0.92% Lys, 0.29% Met, and 15.6% CP). Feeding H AA density diets improved (P 0.05) feed conversion by 5 points and increased (P 0.05) total breast meat yield by 0.5 percentage point over broilers fed the M AA density diets.
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DIETARY AA DENSITY RESPONSES INFLUENCED BY GENETIC STRAINS
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Differences in dietary AA density responses among published research [40, 41, 43] may be related to strain sources. The response to dietary AA/CP density [18, 45, 46] and dietary Lys [19, 20, 47–49] differs among strain sources. Interactions of Lys x CP x strain source also have been documented [18]. These AA response differences among strains likely relate to muscle development and feed intake. A high-yielding strain was shown to contain more breast muscle total RNA and protein on a weight basis and total DNA content over a low-yielding strain [50]. Muscle growth is largely related to the number of nuclei or total DNA [51]. Hence, strains exhibiting rapid muscle growth should have balanced H dietary AA needs for muscle accretion. Feed intake is another factor influencing needs of dietary AA percentages among strains. For example, 2 strains may be similar in terms of the breast meat accretion but differ in feed intake. Therefore, the dietary AA minimums would be higher for broilers having the lower feed intake. Nutritionists should account for genetic differences in meat yield and feed intake when formulating AA minimums.
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REGRESSION ANALYSES
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The importance of formulating diets to adequate AA minimums has been mentioned in previous sections. However, it would be useful to have prediction equations to estimate daily dietary AA needs during the life cycle of the broiler. Estimating daily dietary AA percentages would allow nutritionists to develop AA minimums for different feeding schedules.
Data from 7 peer-reviewed manuscripts [6–8, 41–44] published from 2003 to 2007 conducted at Mississippi State University and USDA-ARS were summarized to estimate critical AA needs throughout a 49-d production period. These select studies were chosen because actual AA values instead of calculated values were used. In addition, analyzed total AA values were used instead of calculated digestible AA values to minimize variability using digestible values from tables. Diets providing the statistically best BW, feed conversion, or both during the starter, grower, withdrawal 1, and withdrawal 2 periods were used. Feed intake was expressed on a daily basis. Data were averaged across gender, strain source, and ambient temperature due to the limited number of studies used in the analysis.
Regression analyses were conducted based on age to predict dietary AA percentage (Table 2). Dietary AA percentages were estimated for a 4-phase feeding schedule during a 49-d production period. Total TSAA, Lys, and Thr were estimated as 0.94, 0.85, 0.78, and 0.72%, 1.36, 1.19, 1.08, and 0.99%, 0.84, 0.77, 0.72, and 0.68%, respectively, for starter (1 to 14 d), grower (15 to 27 d), finisher (28 to 40 d), and withdrawal periods (41 to 49 d). Estimates for Lys are in close agreement for values used in commercial practice to optimize meat yield. However, dietary Lys estimates from regression analysis are higher than requirements/needs conducted with dose titration research [12, 17, 22], but is in agreement with research reported by de Leon et al [21]. Ingredient sources may influence requirement estimates as alternative ingredients are used in dose titration assays to create diets deficient with the test AA. Total dietary TSAA estimates reported herein are similar to other published research [52, 53] and higher than research by Baker et al. [54]. Kalinowski et al. [52, 53] reported total TSAA requirements for slow- and fast-feathering broilers in starter phase (1 to 21 d) for slow- and fast-feathering broilers as 0.89 and 0.94%, respectively, and TSAA requirements during the grower phase (21 to 42 d) as 0.83 and 0.88%, respectively. Baker et al. [54] determined a total TSAA requirement as 0.73% for 21- to 42-d-old broilers. The TSAA requirement reported by Kalinowski et al. [53] and Baker et al. [54] varied considerably. The research design to estimate the TSAA requirements differed between Baker et al. [54] and Kalinowski et al. [53], which have produced conflicting results. In addition, Kalinowski et al. [53] was reported in 2003, whereas Baker et al. [54] was published in 1996; thus, the genetic potential and feed intake per unit gain of the broilers used probably differed between the 2 reports [53, 54]. Total Thr estimates from the regression equation were in good agreement with published research. The NRC [13] published dietary Thr requirements as 0.80, 0.72, and 0.68%, respectively, in the starter (1 to 21 d), grower (21 to 42 d), and finisher (42 to 56 d). Kidd et al. [55] reported a Thr requirement as 0.65 and 0.75%, respectively, based on growth performance and breast meat yield for 30- to 42-d-old broilers. Dozier et al. [56] reported an optimum Thr requirement for BW gain, feed conversion, and breast meat yield with male broilers from 42 to 56 d of age of 0.68, 0.67, and 0.70%, respectively. In close agreement, Kidd et al. [57] determined the Thr requirement for male broilers from 42 to 56 d of age as 0.67% of the diet.
Dietary Lys, Thr, and TSAA requirements reported from previous research may have used broiler strains that had higher feed intake per unit of BW gain, and genetic potential to accrete less lean tissue than strains used with the regression analyses. Hence, these factors may have suggested a lower requirement from previous dose titration experiments, particularly for Lys, than from regression equations reported herein. Furthermore, these AA density studies used in the regression analyses were not compromised in CP, and may have increased AA requirements above that reported in dose-response studies. With dietary AA density data used to generate the regression equations, diet formulation was governed by dietary Lys and the estimates for some of the less limiting AA may be in excess, which makes establishing ratios to Lys difficult because these studies were not intended to establish ratios to Lys.
Expressing dietary AA needs on a daily intake basis is useful in determining dietary AA percentages when management factors influence feed intake. Genetic strains having similar potential for white meat yield may differ in feed intake. However, dietary AA needs may be similar on an intake basis. Thus, nutritionists should alter dietary AA percentages if strains differ in feed intake to avoid suboptimum or superfluous AA minimums. Estimates for daily dietary AA intake determined by regression analyses are presented in Table 3. Increases in daily dietary AA consumption were more pronounced from 7 to 14 and 14 to 21 d of age than during subsequent periods.
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Table 3. Dietary amino acid consumption (mg/d) for Ross high-yielding broiler chickens based on regression equations1,2
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Daily dietary AA consumption per unit of daily BW gain predicted by regression analyses is presented in Table 4. The efficiency of AA for BW gain decreases as broilers age. For example, daily Lys intake per unit of BW gain was 31% lower at 7 than 14 d of age, whereas Lys intake per unit of BW gain was only 5% lower at 42 d compared with 49 d of age. These results support findings that BW gain is a more sensitive response criterion for younger than older broilers [7, 8, 41, 44]
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Table 4. Dietary amino acid consumption per unit of gain (mg of amino acid intake ÷ daily BW gain) for Ross high-yielding broiler chickens based on regression equations1,2
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CONCLUSIONS AND APPLICATIONS
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- Providing broilers H AA density through 4 wk of age should be cost effective because feed intake is relatively low during this period compared with finishing and withdrawal periods.
- Dietary AA density affects BW gain of broilers from placement to 5 wk of age, but BW gain is not a sensitive response criterion for dietary AA density from 5 to 9 wk of age.
- Feeding broilers H AA density diets from placement until 5 wk of age increases breast meat yield by 1.0% over broilers provided M AA density diets, whereas 0.5% increase in breast meat yield occurred when H AA density diets were fed from 5 to 8 or 9 wk of age compared with feeding M AA density diets.
- Breast meat price influences gross feeding margin more than ingredient cost within practical economic fluctuations that occur at a broiler production complex. Both breast meat price and ingredient cost should be considered for establishing AA minimums, particularly late in development.
- Prediction equations to estimate dietary AA percentages were developed as a guide for nutritionists, which would be very useful for feeding programs differing in duration of days fed during the starter, grower, finisher, and withdrawal periods.
- Daily dietary Lys percentages were estimated as Y = 9 x 10–5x2 – 0.014x + 1.44 based on published dietary AA density research.
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FOOTNOTES
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1 Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA. 
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REFERENCES AND NOTES
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A. M. Waguespack, S. Powell, T. D. Bidner, R. L. Payne, and L. L. Southern
Effect of incremental levels of L-lysine and determination of the limiting amino acids in low crude protein corn-soybean meal diets for broilers
Poult. Sci.,
June 1, 2009;
88(6):
1216 - 1226.
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