Effects of protein source and nutrient density in the diets of male broilers from 8 to 21 days of age on their subsequent growth, blood constituents, and carcass compositions

451

Abstract

The effects of protein source and amino acid (AA) and AME levels in the diets of male broilers from 8 to 21 d of age on subsequent growth and blood and carcass traits were investigated in the current study. Fourteen Ross × Ross 708 male broiler chicks were randomly allocated to each of 80 floor pens arranged in a randomized complete block design. Each diet contained 1 of 2 dietary protein sources (high inclusion of distillers dried grains with solubles or high inclusion of meat and bone meal), 1 of 2 AA densities (moderate or 10% higher), and 1 of 2 AME densities (2,998 or 3,100 kcal/kg). Experimental diets were fed from 8 to 21 d of age, and common diets from 1 to 7 and 21 to 55 d of age. The higher AME density in high inclusion of meat and bone meal diets increased serum triglyceride and cholesterol levels on d 20. The dietary inclusion of high inclusion of distillers dried grains with solubles or lower levels of AA increased high-density lipoprotein cholesterol on d 20. Feeding the high-AA-density diet decreased feed intake without affecting BW gain, which resulted in a lower feed conversion ratio (FCR). A high-AME-density diet lowered feed intake but increased BW gain, which resulted in a lower FCR from 8 to 21 d of age. Feed intake, BW gain, FCR from 21 to 54 d of age, and carcass weight on 42 and 55 d of age were not affected by treatments from 8 to 21 d of age. However, early dietary manipulation from 8 to 21 d of age affected fat and meat yield at 42 and 55 d of age. Moreover, a high-AME diet decreased feed cost per carcass weight gain from 8 to 55 d of age. In conclusion, high AA or AME densities during the grower phase, from d 8 to 21 of age, may improve growth during the grower feeding phase, but may also affect meat yield during the latter grow-out phases. Furthermore, high-AME diets from 8 to 21 d of age may save on feed costs for meat production.

 

 

Key words

amino acid
feed cost
growth performance
metabolizable energy
protein source

INTRODUCTION

Nutrient density is a key factor affecting the growth, carcass quality, and health of broilers, which in turn affects the cost effectiveness of their production (Brickett et al., 2007; Talpaz et al., 2013; Zhai et al., 2013). High nutrient density may decrease the feed intake (FI) and feed conversion ratio (FCR) of broiler chickens (Saleh et al., 2004; Zhao et al., 2009; Delezie et al., 2010). It has also been reported that a balanced energy-to-protein ratio is important to achieve optimum broiler carcass yield and meat quality (Jackson et al., 1982; MacLeod, 1997; Kidd et al., 2004; Kamran et al., 2008). High-amino acid (AA)-density diets may lower abdominal fat deposition (Corzo et al., 2005; Dozier et al., 2007) and increase meat yield, especially breast weight (Dozier et al., 2008). A high energy density in diets, however, may also increase lipid accumulation in the bodies of broilers (Deaton and Lott, 1985; Summers et al., 1992; Nahashon et al., 2005).

The ethanol fuel industry currently produces large quantities of distillers dried grains with solubles (DDGS), which is a by-product of processed corn that has had its starch component extracted. These DDGS are a plant protein resource and they may also contain various levels of corn oil, depending on the process of oil extraction to which they are subjected (Xu et al., 2009). Plant-source diets containing moderate fiber concentrations may improve early chick performance by reducing gizzard pH and by promoting nutrient utilization (González-Alvarado et al., 2007). The use of DDGS at a 6% level in broiler diets from 0 to 18 d of age has been shown to have no effect on growth (Lumpkins et al., 2004; Wang et al., 2007a, b; Youssef et al., 2008). Schilling et al. (2010) further reported that DDGS can be included in broiler diets at up to 12% without having any detrimental effects on meat quality.

Another widely used protein resource is meat and bone meal (MBM), which is an animal by-product meal that is high in Lys, Met, and Cys (Parsons et al., 1997). Meat and bone meal consists of meat, bone, and animal lipid (Dale, 1997), and contains almost 100% available P and 90% available Lys and Met (Sell and Jeffrey, 1996; Wang and Parsons, 1998); meat and bone meal contains 4 to 5% phosphorus (Knowlton et al., 2004; Jeng et al., 2006). The inclusion of MBM can provide both protein and phosphorus and can decrease feed costs by allowing for the addition of lower levels of inorganic phosphates.

Because protein sources may vary significantly in nutrient profile and structure, they may have differential effects on the nutrient utilization, metabolism, and subsequent growth of broilers. The present experiment was conducted to study the effects of dietary protein source, and AA, and AME levels from 8 to 21 d of age on the growth and blood serum constituents of broilers from 8 to 21 d of age, and on their later growth and meat yield at 42 and 55 d of age.

MATERIALS AND METHODS

Birds, Diets, and Housing

A total of 1,120 one-day-old male Ross × Ross 708 broiler chicks were obtained from a commercial hatchery. Fourteen chicks were randomly allocated to each of 80 pens in an environmentally controlled room. All the diets were formulated to meet nutritional requirements of broiler males exhibiting standard performances (Rostagno et al., 2011). Birds were fed the same feed from 1 to 7, 22 to 35, and 35 to 55 d of age (Table 1). From 8 to 21 d of age, birds were fed 1 of 8 treatment diets.

Table 1. Feed ingredient composition, nutrient contents, and feed price of diets1

8 to 21 d
Item hDDGS × MAA × MAME hMBM × MAA × MAME hDDGS × HAA × MAME hMBM × HAA × MAME hDDGS × MAA × HAME hMBM × MAA × HAME hDDGS × HAA × HAME hMBM × HAA × HAME 22 to 34 d 35 to 55 d
Ingredient (%)
 Corn 60.48 63.10 53.93 59.34 57.73 64.52 51.16 57.94 62.93 67.31
 Soybean meal 27.93 25.16 33.46 30.23 28.39 24.92 33.93 30.46 22.49 16.38
 DDGS 6.00 2.00 6.00 2.00 6.00 2.00 6.00 2.00 7.00 9.00
 MBM 2.00 6.00 2.00 6.00 2.00 6.00 2.00 6.00 4.00 3.00
 Poultry fat 0.620 0.000 1.660 0.000 2.940 0.750 3.970 1.800 1.510 2.617
 Dicalcium phosphorus 0.620 0.000 0.590 0.000 0.633 0.000 0.600 0.000 0.206 0.000
 Calcium carbonate 1.100 0.530 1.090 0.510 1.097 0.530 1.090 0.510 0.720 0.620
 Salt 0.350 0.330 0.330 0.330 0.346 0.346 0.341 0.330 0.274 0.242
 l-Lys hydrochloride 0.322 0.342 0.308 0.336 0.315 0.350 0.300 0.332 0.321 0.338
 Premix2 0.250 0.250 0.250 0.250 0.250 0.250 0.250 0.250 0.250 0.250
 dl- Met 0.163 0.174 0.189 0.196 0.165 0.173 0.190 0.197 0.149 0.116
 Sand 0.000 1.934 0.000 0.634 0.000 0.000 0.000 0.000 0.000 0.000
 l-Thr 0.096 0.104 0.097 0.107 0.094 0.105 0.096 0.110 0.079 0.075
 Monteban 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050
 Ronozyme 0.020 0.020 0.020 0.020 0.020 0.020 0.020 0.020 0.020 0.000
Nutrient content3 (%, unless otherwise noted)
 ME (kcal/kg) 2,998 2,998 2,998 2,998 3,100 3,100 3,100 3,100 3,100 3,199
 CP 20.80 20.80 22.88 22.88 20.80 20.80 22.88 22.88 19.80 17.30
 Crude fiber 3.11 2.87 3.09 2.92 3.05 2.91 3.02 2.86 3.15 3.23
 Crude fat 3.48 3.05 4.34 2.97 5.68 3.85 6.55 4.71 4.62 5.38
 Ca 0.82 0.82 0.82 0.82 0.82 0.82 0.82 0.82 0.76 0.58
 Nonphytate P 0.34 0.40 0.34 0.41 0.34 0.40 0.34 0.41 0.35 0.27
 Na 0.21 0.21 0.21 0.21 0.21 0.21 0.21 0.21 0.20 0.19
 Digestible Lys 1.17 1.17 1.29 1.29 1.17 1.17 1.29 1.29 1.08 0.94
 Digestible Met 0.46 0.46 0.50 0.50 0.46 0.46 0.50 0.50 0.43 0.37
 Digestible TSAA 0.73 0.73 0.80 0.80 0.73 0.73 0.80 0.80 0.70 0.62
 Digestible Thr 0.76 0.76 0.84 0.84 0.76 0.76 0.84 0.84 0.70 0.61
 Feed price ($/t) 305.9 297.4 318.9 307.4 319.3 306.1 332.3 319.2 304.7 302.3
1

hDDGS = high inclusion of distillers dried grains with solubles (DDGS); hMBM = high inclusion of meat and bone meal (MBM); MAA = moderate level of amino acids (AA); HAA = high level of AA; MAME = moderate level of AME; HAME = high level of AME.

2

Premix contained the following per kilogram of diet: retinyl acetate, 2.654 µg; cholecalciferol, 110 µg; dl-α-tocopherol acetate, 9.9 mg; menadione, 0.9 mg; vitamin B12, 0.01 mg; folic acid, 0.6 µg; choline, 379 mg; d-pantothenic acid, 8.8 mg; riboflavin, 5.0 mg; niacin, 33 mg; thiamine, 1.0 mg; d-biotin, 0.1 mg; pyridoxine, 0.9 mg; ethoxyquin, 28 mg; manganese, 55 mg; zinc, 50 mg; iron, 28 mg; copper, 4 mg; iodine, 0.5 mg; selenium, 0.1 mg.

3

Nutrient contents were calculated on a DM basis.

Feed ingredients, including corn, soybean meal, DDGS, and MBM, were analyzed for their total AA and CP compositions (AOAC International, 2006) and the nutrient matrix was used to formulate least-cost diets using linear programming (Table 1). Digestible AA values were calculated from digestibility coefficients (Ajinomoto Heartland, 2004) and the analyzed total AA content of the ingredients.

A randomized complete block design was used, with all 8 treatments being equally represented in each block. Ten blocks were identified by their locations in the broiler facility. The 8 treatment diets were arranged in a 2 (protein sources) × 2 (AA densities) × 2 (AME levels) factorial design. Each diet contained either high DDGS (hDDGS) or high MBM (hMBM), moderate or high AA densities, and moderate or high AME levels (Table 1). The hDDGS diets contained 6% DDGS and 2% MBM and the hMBM diets contained 2% DDGS and 6% MBM. Other diets included the hDDGS diet with high AA and AME densities (DHH); the hDDGS diet with high AA and moderate AME densities (DHM); the hDDGS diet with moderate AA and high AME densities (DMH); the hDDGS diet with moderate AA and AME densities (DMM); the hMBM diet with high AA and AME densities (MHH); the hMBM diet with high AA and moderate AME densities (MHM); the hMBM diet with moderate AA and high AME densities (MMH); and the hMBM diet with moderate AA and AME densities (MMM).

The moderate AA level was based on digestible AA requirements of male broilers with a standard performance from 8 to 21 d of age (Rostagno et al., 2011), and the high AA level was based on a 10% performance level above that for the moderate AA level. The calcium, sodium, total mineral, vitamin, and medication premix levels were the same among all 8 diets.

All birds had ad libitum access to fresh water and feed throughout the whole experiment. The photoperiod from 0 to 7 d of age was 23L:1D and from 8 to 54 d of age was 20L:4D. All methods and management procedures used in the current study were approved by the Institutional Animal Care and Use Committee of Mississippi State University.

Blood Characteristics

On d 20, 2 birds from each pen were randomly selected and blood samples were collected from the brachial vein in nonheparinized tubes. After allowing the blood to clot (2-h period), samples were centrifuged at 1,900 × g for 10 min at 4°C to in order extract serum. For determination of high-density lipoprotein cholesterol (HDLC) concentration, serum samples were pipetted into Vitros HDL tubes (DT HDL Cholesterol Kit, Ortho Clinical Diagnostics, Rochester, NY) and centrifuged at 1,500 × g for 10 min at 4°C. All serum and supernate (HDLC) samples were stored at −20°C and warmed to room temperature before testing. Serum cholesterol (CHOL), triglycerides (TRIG), total protein (TP), uric acid (UA), and HDLC concentrations were analyzed on a Kodak Ektachem DT-60 analyzer (Eastman Kodak Co., Rochester, NY). Serum or supernate samples (10 μL) were applied to the appropriate Vitros DT slide and the slides were then scanned by the analyzer.

Growth Performance and Carcass Traits

At 8, 21, 42, and 54 d of age, BW and FI were determined on a per pen basis. Body weight gain, FI, and FCR were determined within grow-out phases from 8 to 21, 22 to 41, and 42 to 54 d of age. Daily mortality was recorded and FCR was adjusted by accounting for the BW of the birds that died.

Four chicks from each pen were processed at 42 and 55 d. Birds were weighed individually and were cooped for 16 h before processing. Carcass weights and abdominal fat pad weights were obtained by automated processing. Carcasses were chilled by storage in an ice-water slurry for approximately 4 h. Wings, leg quarters (drumstick and thigh), and boneless and skinless breast and tenders weights were obtained by manually deboning individual carcasses.

Feed cost per carcass weight gain or BW gain was calculated. Ingredient prices were obtained from current ingredient market information (Feedstuff, Bloomington, MN). The price of each diet was calculated by Creative Formulation Concepts software (CFC, Pierz, MN). The cost of feed in each feeding phase was determined by multiplying FI by feed price. Carcass weight at 8 d was estimated as 60% of live BW.

Statistical Analysis

A randomized complete block design was used, with all 8 treatments being equally represented in each block. A 2 × 2 × 2 factorial arrangement of treatments was used, and all parameters were analyzed using SAS version 9.2 (SAS Institute, 2010). A 3-way ANOVA (PROC MIXED) was used to determine the significance of responses to dietary protein source, AA and AME levels, and their interactions. The dietary protein source and AA and AME levels were designated as fixed effects; block was a random effect. Linear correlations were conducted among the parameters 20-d CHOL, TP, UA, TRIG, and HDLC, 8- to 20-d BW gain, FI, and FCR, and 42- and 55-d carcass weight. Pearson product-moment correlation analysis of the PROC CORR procedure was used. Global effects, differences among least squares means, and correlations were considered to be significant at P ≤ 0.05.

RESULTS

Blood Constituents

Interactive effects of dietary protein source and AME level were noted on blood serum CHOL (P = 0.023) and TRIG (P = 0.013) concentrations (Table 2). Chicks fed hMBM diets with moderate AME density exhibited a lower serum CHOL concentration as compared with those fed hMBM diets with a high AME density or hDDGS diets with a moderate AME density. A lower AME density decreased serum TRIG concentrations in chicks fed hMBM diets. When birds were fed high-AME-density diets, a high inclusion of DDGS decreased serum TRIG concentrations. High AA levels in the diet increased serum UA concentrations (P = 0.004) and lowered HDLC concentrations (P < 0.001). Chicks fed an hDDGS diet exhibited higher serum HDLC concentrations than those fed an hMBM diet (P = 0.026). Pearson correlation analysis among blood serum constituents demonstrated that CHOL was positively correlated to TP (P = 0.02; Table 3) and HDLC (P = 0.001) and that TP was positively correlated to UA (P = 0.02).

Table 2. Effects of dietary protein source and nutrient density from 8 to 21 d on blood serum constituents at 20 d and growth performance from 8 to 21 d1,2

Item AA level AME level CHOL (mg/dL) TRIG (mg/dL) TP (g/dL) UA (mg/dL) HDLC (mg/dL) FI (g) BW gain (g) FCR
Protein source
 hDDGS 123.4 102.3 2.71 8.00 78.7a 1,004 751 1.339
 hMBM 119.3 104.6 2.68 7.83 72.7b 1,013 755 1.349
 SEM 1.86 3.18 0.031 0.216 2.26 5.4 4.3 0.0067
High 118.9 100.0 2.74 8.33a 71.0b 993b 755 1.322b
Moderate 123.8 106.9 2.65 7.41b 80.3a 1,023a 750 1.366a
SEM 1.83 3.18 0.035 0.216 2.19 6.4 4.9 0.0067
High 122.5 106.8 2.70 7.64 76.1 1,001b 759a 1.322b
Moderate 120.3 100.0 2.69 8.10 75.2 1,015a 747b 1.366a
SEM 1.68 3.18 0.032 0.209 2.22 5.5 4.3 0.0067
 hDDGS High 121.7 101.9 2.76 8.27 74.1 989 754 1.311
 hDDGS Moderate 125.2 102.8 2.66 7.55 83.3 1,018 748 1.367
 hMBM High 116.2 98.2 2.72 8.39 68.0 998 756 1.333
 hMBM Moderate 122.4 110.9 2.64 7.28 77.3 1,027 753 1.365
 SEM 2.58 4.34 0.046 0.300 2.49 7.6 6.0 0.0080
 hDDGS High 122.0ab 100.5b 2.73 7.48 77.4 996 757 1.315
 hDDGS Moderate 124.9a 104.2ab 2.68 8.34 80.0 1,011 744 1.363
 hMBM High 122.9a 113.2a 2.67 7.80 74.9 1,006 760 1.329
 hMBM Moderate 115.7b 95.9b 2.69 7.87 70.4 1,019 749 1.368
 SEM 2.40 4.30 0.040 0.283 2.77 6.5 5.5 0.0080
High High 121.5 101.9 2.74 8.29 72.7 989 761 1.304
High Moderate 116.4 98.2 2.73 8.36 69.4 998 749 1.34
Moderate High 123.4 111.8 2.66 6.99 79.6 1,014 756 1.341
Moderate Moderate 124.2 101.9 2.64 7.84 81.0 1,032 744 1.391
SEM 2.33 4.34 0.045 0.283 2.69 7.6 6.0 0.0080
 hDDGS High High 119.8 101.9 2.77 7.98 72.3 981 754abc 1.301f
 hDDGS High Moderate 123.6 101.9 2.74 8.56 75.8 996 753abc 1.322ef
 hDDGS Moderate High 124.2 99.1 2.69 6.98 82.4 1,011 761ab 1.330de
 hDDGS Moderate Moderate 126.3 106.5 2.62 8.12 84.2 1,026 735c 1.404a
 hMBM High High 123.3 101.9 2.72 8.61 73.0 996 768a 1.307ef
 hMBM High Moderate 109.2 94.5 2.73 8.17 63.0 1,000 745bc 1.358bc
 hMBM Moderate High 122.6 124.6 2.63 6.99 76.8 1,017 752ab 1.352cd
 hMBM Moderate Moderate 122.2 97.3 2.66 7.57 77.9 1,037 754abc 1.378b
 SEM 3.29 6.03 0.056 0.391 3.35 9.2 7.7 0.0101
Source of variation (P-value)
 Protein source 0.126 0.595 0.562 0.800 0.026 0.145 0.440 0.144
 AA level 0.065 0.105 0.081 0.004 <.001 0.004 0.501 <.001
 AME level 0.313 0.106 0.729 0.113 0.711 0.040 0.026 <.001
 Protein source × AA level 0.602 0.158 0.783 0.510 0.988 0.968 0.784 0.065
 Protein source × AME level 0.023 0.013 0.183 0.112 0.123 0.815 0.792 0.481
 AA level × AME level 0.133 0.459 0.925 0.109 0.213 0.536 0.997 0.261
 Protein source × AA level × AME level 0.056 0.106 0.549 0.647 0.091 0.396 0.017 0.003
a–f

Means in a column for a given major effect or interaction not sharing a common superscript are different (P ≤ 0.05).

1

Observed means were calculated from 10 replicate values using the pen as the experimental unit.

2

hDDGS = high inclusion of distillers dried grains with solubles (DDGS), the hDDGS diet contained 6% DDGS and 2% meat and bone meal (MBM); hMBM = high inclusion of MBM, the hMBM diet contained 2% DDGS and 6% MBM; moderate amino acid (AA) diets contain 1.17, 0.46, 0.73, and 0.76% of digestible Lys, Met, TSAA, and Thr, respectively; high AA diets contain 1.29, 0.50, 0.80, and 0.84% of digestible Lys, Met, TSAA, and Thr, respectively; moderate AME = 2,998 kcal/kg; high AME = 3,100 kcal/kg; CHOL = whole cholesterol; TRIG = triglyceride; TP = total protein; UA = uric acid; HDLC = high-density lipoprotein cholesterol; FI = feed intake; FCR = feed conversion ratio.

Table 3. Pearson correlation coefficients (r) and probability values (in parentheses) among blood serum constituents (CHOL = whole cholesterol; TP = total protein; TRIG = triglyceride; UA = uric acid; HDLC = high-density lipoprotein cholesterol) on d 20

Item TP TRIG UA HDLC
CHOL 0.26 (0.02) 0.15 (0.19) 0.12 (0.31) 0.36 (0.001)
TP 0.04 (0.72) 0.26 (0.02) −0.14 (0.23)
TRIG 0.14 (0.20) −0.14 (0.21)
UA −0.12 (0.27)

Growth Performance

From 8 to 21 d of age, protein source did not affect FI. A high AA density in the grower feed decreased FI (P = 0.004; Table 2) without affecting BW gain, which resulted in a lower FCR from 8 to 21 d (P < 0.001). A high AME density in the grower feed lowered FI (P = 0.04), but increased BW gain (P = 0.026), which also resulted in a lower FCR from 8 to 21 d of age (P < 0.001). Furthermore, a 3-way interaction (protein source × AA level × AME level) existed for BW gain from 8 to 21 d of age (P = 0.017). Chicks fed MHH diets gained more BW than those fed MHM and DMM diets, and chicks fed DMH or MMH diets gained more BW than those fed DMM diets. A 3-way interaction also existed for FCR (P = 0.003) from 8 to 21 d of age. Chicks fed DMM diets exhibited the highest FCR, and chicks fed MMM diets exhibited a higher FCR than those fed MMH, DMH, DHM, MHH, or DHH diets. Chicks fed MHM diets exhibited a higher FCR than those fed DMH, DHM, MHH, or DHH diets; chicks fed MMH diets exhibited a higher FCR than those fed DHM, MHH, or DHH diets; and chicks fed DMH diets exhibited a higher FCR than those fed a DHH diet. Feeding various diets with different nutrient densities from 8 to 21 d of age only affected FI, BW gain, and FCR during that specific feeding phase (Table 2) without having any associated effects on those same performance parameters between 22 to 41 or 42 to 54 d of age (Table 4).

Table 4. Effects of dietary protein source and nutrient density from 8 to 21 d on broiler performance from 22 to 41 and 42 to 54 d1,2

22 to 41 d 42 to 54 d
Item AA level AME level FI (g) BW gain (g) FCR FI (g) BW gain (g) FCR
Protein source
 hDDGS 3,530 1,878 1.846 2,773 1,167 2.433
 hMBM 3,521 1,873 1.844 2,813 1,227 2.342
 SEM 33.4 27.2 0.0080 48.5 32.7 0.0831
High 3,531 1,865 1.853 2,789 1,169 2.440
Moderate 3,521 1,887 1.837 2,796 1,225 2.336
SEM 35.9 29.8 0.0079 49.8 37.8 0.0935
High 3,520 1,867 1.844 2,738 1,155 2.434
Moderate 3,531 1,885 1.846 2,848 1,239 2.341
SEM 33.4 25.8 0.0076 48.5 34.9 0.0960
 hDDGS High 3,546 1,865 1.858 2,786 1,146 2.478
 hDDGS Moderate 3,513 1,892 1.833 2,761 1,189 2.388
 hMBM High 3,515 1,864 1.848 2,793 1,193 2.401
 hMBM Moderate 3,528 1,882 1.840 2,832 1,261 2.284
 SEM 39.1 36.1 0.0110 57.6 44.5 0.1071
 hDDGS High 3,531 1,867 1.852 2,709 1,140 2.463
 hDDGS Moderate 3,528 1,890 1.839 2,838 1,195 2.403
 hMBM High 3,508 1,866 1.836 2,768 1,170 2.405
 hMBM Moderate 3,535 1,880 1.851 2,857 1,284 2.279
 SEM 36.8 31.7 0.0104 56.5 42.0 0.1094
High High 3,526 1,851 1.850 2,732 1,126 2.480
High Moderate 3,535 1,879 1.851 2,847 1,212 2.399
Moderate High 3,514 1,882 1.833 2,744 1,183 2.388
Moderate Moderate 3,528 1,891 1.840 2,848 1,267 2.284
SEM 39.1 34.0 0.0104 57.6 46.1 0.1174
 hDDGS High High 3,528 1,848 1.863 2,686 1,105 2.497
 hDDGS High Moderate 3,564 1,882 1.853 2,885 1,186 2.459
 hDDGS Moderate High 3,535 1,886 1.841 2,731 1,174 2.429
 hDDGS Moderate Moderate 3,492 1,897 1.826 2,790 1,203 2.347
 hMBM High High 3,524 1,853 1.847 2,778 1,148 2.464
 hMBM High Moderate 3,506 1,876 1.849 2,809 1,238 2.338
 hMBM Moderate High 3,492 1,879 1.825 2,758 1,192 2.347
 hMBM Moderate Moderate 3,563 1,884 1.855 2,906 1,330 2.221
 SEM 44.8 42.9 0.0144 70.7 56.7 0.1387
Source of variation (P-value)
 Protein source 0.697 0.860 0.880 0.238 0.089 0.238
 AA level 0.778 0.575 0.173 0.376 0.288 0.376
 AME level 0.592 0.435 0.872 0.457 0.063 0.457
 Protein source × AA level 0.300 0.874 0.448 0.858 0.712 0.858
 Protein source × AME level 0.492 0.855 0.141 0.661 0.375 0.661
 AA level × AME level 0.914 0.676 0.555 0.878 0.976 0.878
 Protein source × AA level × AME level 0.060 0.961 0.377 0.886 0.452 0.886
1

Observed means were calculated from 10 replicate values using the pen as the experimental unit.

2

hDDGS = high inclusion of distillers dried grains with solubles (DDGS), the hDDGS diet contained 6% DDGS and 2% meat and bone meal (MBM); hMBM = high inclusion of MBM, the hMBM diet contained 2% DDGS and 6% MBM; moderate amino acid (AA) diets contain 1.17, 0.46, 0.73, and 0.76% of digestible Lys, Met, TSAA, and Thr, respectively; high AA diets contain 1.29, 0.50, 0.80, and 0.84% of digestible Lys, Met, TSAA, and Thr, respectively; moderate AME = 2,998 kcal/kg; high AME = 3,100 kcal/kg; FI = feed intake; FCR = feed conversion ratio.

Carcass Composition

Dietary treatment from 8 to 21 d of age did not affect carcass, breast, wing, or leg quarter weights at 42 d (Table 5). However, chicks fed moderate-AA- and -AME-density diets had greater abdominal fat pad weights at 42 d of age than those fed high-AA- and moderate-AME-density diets, or moderate AA and high AME density diets. Chicks fed high-AA and -AME-density diets also had greater abdominal fat pad weights at 42 d of age than those fed high-AA- and moderate-AME-density diets.

Table 5. Effects of dietary protein source and nutrient density from 8 to 21 d on carcass and cut-up parts weight at 42 and 55 d1,2

d 42 d 55
Item (g) AA level AME level Carcass Fat Breast Tender Wings Quarters3 Carcass Fat Breast Tender Wings Quarters3
Protein source
 hDDGS 2,156 39.1 638 115 240 648 3,085 66.4 775 168 330 918
 hMBM 2,145 37.4 638 114 239 645 3,108 63.5 796 169 334 916
 SEM 17.0 0.92 8.1 1.7 1.9 6.9 43.3 1.87 15.1 2.6 4.1 11.1
High 2,154 37.8 640 115 239 649 3,095 66.3 786 170 332 920
Moderate 2,147 38.7 635 114 239 644 3,098 63.6 785 168 332 914
SEM 17.0 0.92 8.1 1.7 1.9 6.9 44.0 1.68 15.1 2.9 3.9 11.7
High 2,165 38.4 646 113 239 652 3,075 65.1 774 167 331 914
Moderate 2,147 38.1 629 116 240 644 3,118 64.8 797 170 333 920
SEM 21.0 0.91 10.6 1.7 2.4 8.4 42.8 1.82 15.8 2.6 3.9 11.3
 hDDGS High 2,143 39.7 635 117 239 646 3,106 69.8a 779 171 332 934
 hDDGS Moderate 2,148 38.5 641 114 238 644 3,064 63.1b 771 166 328 903
 hMBM High 2,161 36.0 642 114 239 649 3,084 62.8b 793 168 332 907
 hMBM Moderate 2,141 38.9 634 115 239 644 3,132 64.2ab 798 169 336 924
 SEM 19.0 1.30 9.1 2.1 2.2 7.6 47.9 2.32 16.8 3.4 4.5 13.7
 hDDGS High 2,169 39.1 640 114 241 650 3,068 66.3 762 167 329 919
 hDDGS Moderate 2,143 39.1 635 116 239 646 3,102 66.6 788 170 331 918
 hMBM High 2,152 37.7 643 113 238 648 3,082 63.9 786 167 333 907
 hMBM Moderate 2,139 37.2 632 116 240 641 3,134 63.1 805 171 335 924
 SEM 22.0 1.29 9.6 2.1 2.4 8.3 46.8 2.56 17.4 3.1 4.5 13.5
High High 2,168 40.2ab 642 114 239 656 3,065 65.9 773 167 331 923
High Moderate 2,141 35.5c 639 116 239 642 3,125 66.7 800 172 333 917
Moderate High 2,153 36.6bc 641 113 239 643 3,085 64.3 775 166 331 903
Moderate Moderate 2,141 40.8a 629 116 239 645 3,110 63.0 794 169 333 923
SEM 21.0 1.29 9.6 2.1 2.5 8.3 47.5 2.33 17.4 3.3 4.4 13.9
 hDDGS High High 2,174 41.7 648 115 239 654 3,056 67.4 753c 167 331 930ab
 hDDGS High Moderate 2,157 37.7 645 119 239 650 3,156 72.2 806ab 174 333 938ab
 hDDGS Moderate High 2,165 36.5 633 113 242 646 3,079 65.3 772abc 166 328 907bc
 hDDGS Moderate Moderate 2,128 40.5 626 114 238 642 3,048 60.9 771bc 166 328 898c
 hMBM High High 2,162 38.7 637 114 239 657 3,073 64.5 793abc 167 331 915abc
 hMBM High Moderate 2,124 33.3 633 114 239 635 3,095 61.2 794abc 169 333 900bc
 hMBM Moderate High 2,142 36.7 650 112 236 640 3,091 63.3 779abc 166 335 899c
 hMBM Moderate Moderate 2,154 41.1 632 117 241 648 3,173 65.0 817a 172 338 949a
 SEM 28.0 1.82 12.7 2.7 3.2 10.6 54.1 3.26 20.4 4.1 5.3 17.0
Source of variation (P-value)
 Protein source 0.603 0.214 0.998 0.626 0.656 0.641 0.437 0.277 0.055 0.858 0.304 0.771
 AA level 0.736 0.514 0.592 0.506 0.997 0.502 0.936 0.209 0.881 0.582 0.936 0.436
 AME level 0.313 0.826 0.338 0.170 0.945 0.398 0.090 0.901 0.139 0.152 0.476 0.348
 Protein source × AA 0.499 0.123 0.241 0.251 0.685 0.630 0.073 0.048 0.551 0.252 0.172 0.011
 Protein source × AME 0.724 0.827 0.679 0.927 0.313 0.842 0.719 0.832 0.768 0.867 0.829 0.349
 Protein source × AME 0.643 0.001 0.497 0.795 0.994 0.255 0.489 0.607 0.693 0.687 0.921 0.235
 Protein source × AA level × AME 0.264 0.715 0.727 0.155 0.299 0.255 0.057 0.078 0.032 0.230 0.824 0.033
a–c

Means in a column for a given major effect or interaction not sharing a common superscript are different (P ≤ 0.05).

1

Observed means were calculated from 10 replicate values using the pen as the experimental unit.

2

hDDGS = high inclusion of distillers dried grains with solubles (DDGS), the hDDGS diet contained 6% DDGS and 2% meat and bone meal (MBM); hMBM = high inclusion of meat and bone meal, the hMBM diet contained 2% DDGS and 6% MBM; moderate amino acid (AA) diets contain 1.17, 0.46, 0.73, and 0.76% of digestible Lys, Met, TSAA, and Thr, respectively; high AA diets contain 1.29, 0.50, 0.80, and 0.84% of digestible Lys, Met, TSAA, and Thr, respectively; moderate AME = 2,998 kcal/kg; high AME = 3,100 kcal/kg.

3

Quarters = thighs + drumsticks.

Dietary treatments from 8 to 21 d of age did not affect carcass, tender, or wing weights (Table 5). However, broilers fed hDDGS diets with a high AA density deposited more fat by 55 d of age than did those fed hDDGS diets with a moderate AA density or hMBM diets with a high AA density (P = 0.048). Three-way interactive effects (protein source × AA level × AME level) were found for breast and leg quarter weights at 55 d of age (P = 0.032 and P = 0.033, respectively). Chicks fed an MMM diet grew more breast muscle than those fed DMM or DHH diets (P = 0.032). Chicks fed a DHM diet also grew more breast muscle than those fed a DHH diet, and birds fed an MMM diet gained more leg quarter muscle than birds fed DMH, MHM, DMM, or MMH diets. Birds fed DHH or DHM diets gained more leg quarter muscle than those fed DMM or MMH diets.

No dietary treatment effects were noted for feed cost per BW gain between 42 and 55 d of age (Table 6). Nevertheless, a 3-way interaction existed for feed cost per carcass weight gain through 42 d of age (P = 0.017). Birds fed MMH grower diets cost less to feed as compared with those fed DHM or MMM diets, and birds fed DMM grower diets also cost less to feed than those fed DHM diets. The feeding of a higher AME density grower diet reduced feed cost per carcass weight gain through 55 d of age (P = 0.049).

Table 6. Dietary protein source and nutrient density effects on feed cost of carcass weight at 42 and 55 d1,2

d 423 d 554
Item (cents/kg) AA level AME level Feed cost per carcass gain Feed cost per BW gain Feed cost per carcass gain Feed cost per BW gain
Protein source
 hDDGS 75.63 52.39 82.25 59.52
 hMBM 74.99 52.43 83.26 60.36
 SEM 0.520 0.381 0.785 0.610
High 75.82 52.32 82.85 60.22
Moderate 74.80 52.51 82.66 59.66
SEM 0.520 0.347 0.750 0.604
High 74.76 52.32 81.72b 59.69
Moderate 75.86 52.51 83.79a 60.20
SEM 0.510 0.356 0.738 0.589
 hDDGS High 76.47 52.35 82.43 59.92
 hDDGS Moderate 74.79 52.43 82.07 59.12
 hMBM High 75.17 52.29 83.27 60.53
 hMBM Moderate 74.81 52.58 83.25 60.20
 SEM 0.735 0.490 1.107 0.844
 hDDGS High 75.21 52.34 81.39 59.34
 hDDGS Moderate 76.06 52.45 83.11 59.71
 hMBM High 74.32 52.30 82.05 60.04
 hMBM Moderate 75.65 52.57 84.47 60.69
 SEM 0.721 0.504 1.036 0.813
High High 75.22 52.12 82.33 60.38
High Moderate 76.42 52.52 83.37 60.07
Moderate High 74.31 52.52 81.11 58.99
Moderate Moderate 75.29 52.49 84.20 60.33
SEM 0.721 0.451 1.036 0.803
 hDDGS High High 75.10abc 52.14 82.20 60.11
 hDDGS High Moderate 77.84a 52.56 82.26 59.74
 hDDGS Moderate High 75.31abc 52.53 80.58 58.58
 hDDGS Moderate Moderate 74.28bc 52.34 83.56 59.67
 hMBM High High 75.34abc 52.09 82.46 60.66
 hMBM High Moderate 75.00abc 52.48 84.09 60.39
 hMBM Moderate High 73.30c 52.50 81.64 59.41
 hMBM Moderate Moderate 76.31ab 52.65 84.85 61.00
 SEM 1.045 0.637 1.459 1.121
Source of variation (P-value)
 Protein source 0.386 0.944 0.366 0.333
 AA level 0.174 0.670 0.859 0.501
 AME level 0.131 0.687 0.049 0.523
 Protein source × AA level 0.378 0.815 0.880 0.773
 Protein source × AME level 0.735 0.866 0.716 0.846
 AA level × AME level 0.884 0.539 0.288 0.261
 Protein source × AA level × AME level 0.017 0.786 0.807 0.887
a–c

Means in a column for a given major effect or interaction not sharing a common superscript are different (P ≤ 0.05). All the feed cost, carcass gain, and BW gain were started from 8 d.

1

Observed means were calculated from 10 replicate values using the pen as the experimental unit.

2

hDDGS = high inclusion of distillers dried grains with solubles (DDGS), the hDDGS diet contained 6% DDGS and 2% meat and bone meal (MBM); hMBM = high inclusion of MBM, the hMBM diet contained 2% DDGS and 6% MBM; moderate amino acid (AA) diets contain 1.17, 0.46, 0.73, and 0.76% of digestible Lys, Met, TSAA, and Thr, respectively; high AA diets contain 1.29, 0.50, 0.80, and 0.84% of digestible Lys, Met, TSAA, and Thr, respectively; moderate AME = 2,998 kcal/kg; high AME = 3,100 kcal/kg.

3

Feed cost per carcass gain at d 42 = [feed intake (FI) from d 8 to 21 × feed price + FI from d 22 to 34 × feed price + FI from d 35 to 42 × feed price]/(carcass weight at d 42 − BW at d 8 × 60%). Feed cost per BW gain at d 42 = (FI from d 8 to 21 × feed price + FI from d 22 to 34 × feed price + FI from d 35 to 42 × feed price)/(BW at d 42 − BW at d 8). Feed prices were determined by dietary formulation.

4

Feed cost per carcass gain at d 55 = (FI from d 8 to 21 × feed price + FI from d 22 to 34 × feed price + FI from d 35 to 55 × feed price)/(carcass weight at d 55 − BW at d 8 × 6%). Feed cost per BW gain at d 55 = (FI from d 8 to 21 × feed price + FI from d 22 to 34 × feed price + FI from d 35 to 55 × feed price)/(BW at d 55 − BW at d 8). Feed price was determined by dietary formulation.

Serum CHOL concentration at 20 d of age was positively correlated to abdominal fat pad and leg quarter weights at 42 d of age (P = 0.05 and P = 0.04, respectively; Table 7), and serum HDLC concentration at 20 d of age was positively correlated to abdominal fat pad and wings weights at 42 d of age (P = 0.006 and P = 0.006, respectively). Serum TRIG concentration at 20 d of age was negatively correlated to carcass, wings, and leg quarter weights at 55 d of age (P = 0.03, 0.04, and 0.04, respectively).

Table 7. Pearson correlation coefficients (r) and probability values (in parentheses) between blood serum constituents (CHOL = whole cholesterol; TP = total protein; TRIG = triglyceride; UA = uric acid; HDLC = high-density lipoprotein cholesterol) at 20 d and carcass characteristics at 42 and 55 d

Item Carcass Fat Breast Tender Wings Leg quarters
d 42
 CHOL 0.13 (0.26) 0.22 (0.05) 0.06 (0.61) −0.003 (0.98) 0.09 (0.44) 0.24 (0.04)
 TP 0.03 (0.81) 0.005 (0.97) 0.02 (0.86) 0.08 (0.51) −0.05 (0.65) −0.04 (0.71)
 TRIG 0.05 (0.65) −0.14 (0.20) −0.12 (0.29) −0.12 (0.30) 0.07 (0.54) 0.16 (0.16)
 UA 0.07 (0.53) 0.008 (0.94) 0.14 (0.23) 0.002 (0.99) 0.06 (0.58) −0.15 (0.20)
 HDLC 0.20 (0.08) 0.31 (0.006) 0.10 (0.36) 0.09 (0.42) 0.30 (0.006) 0.17 (0.13)
d 55
 CHOL 0.12 (0.29) 0.03 (0.81) 0.09 (0.43) 0.05 (0.68) 0.11 (0.32) 0.05 (0.69)
 TP 0.08 (0.47) 0.008 (0.94) 0.09 (0.42) −0.12 (0.30) 0.06 (0.56) 0.02 (0.89)
 TRIG −0.25 (0.03) −0.01 (0.90) 0.13 (0.24) −0.19 (0.10) −0.23 (0.04) −0.23 (0.04)
 UA −0.03 (0.08) 0.01 (0.92) 0.02 (0.87) −0.11 (0.32) 0.01 (0.91) −0.09 (0.46)
 HDLC −0.043 (0.71) −0.07 (0.55) 0.02 (0.88) 0.008 (0.94) −0.14 (0.23) 0.07 (0.52)

DISCUSSION

Blood Constituents

Higher serum concentrations of CHOL and TRIG at 20 d were induced in broilers by increasing the AME of the hMBM diets. A previous study has shown that TRIG and CHOL are associated with the fatty acid metabolic pathway, and that TRIG concentration is positively associated with energy intake (Bacon et al., 1981). However, CHOL or TRIG levels were not affected by the AME level in the hDDGS diet. The lack of response of TRIG to the high energy level may be due to the high fiber content of the hDDGS diet. Previous studies have shown that high fiber content may decrease lipid digestibility by reducing bile acid in chyme (Smits and Annison, 1996; Smits et al., 1998). Accordingly, the absorption of poultry fat in the gut of the broilers fed the DDGS diet may have been altered due to its high fiber content.

Consistent with previous works by Collin et al. (2003) and Zhao et al. (2009), higher dietary AA levels increased serum UA concentrations in the current study. Serum UA concentration was correlated to TP. This was expected, as UA is the major end product of protein metabolism in avian species (Sturkie, 1986), and the excess nitrogen from the high-AA-density diets is excreted through its assimilation in UA.

Higher serum HDLC concentrations were observed in broilers fed hDDGS diets when compared with those fed hMBM diets. This may also be a result of the high fiber content of DDGS. Research on humans has shown that high dietary fiber may increase propionate levels in the colon, which may result in higher serum HDLC levels (Venter et al., 1990). Cecal microbes in chickens may use nonstarch fibers in DDGS to produce propionate, thereby increasing HDLC levels. Higher serum HDLC concentrations were also observed in broilers fed diets with a lower AA density. This was likewise demonstrated in a study using rats (Madani and Belleville, 2000). High-density lipoproteins transport cholesterol and fat to the liver from cells in peripheral tissues, including those in artery walls. High serum HDLC concentrations may prevent coronary heart diseases in human (Gordon et al., 1977; Després et al., 2000). Distillers dried grains with solubles or a lower protein density in the diet may have the potential to improve the cardiovascular function of birds by increasing HDLC level.

Growth Performance

In the current study, high nutrient density diets lowered the FI of the birds. This is consistent with earlier findings by other researchers (Dale and Fuller, 1979; Hernández et al., 2004; Zhao et al., 2009). However, the actual AA (12.8 vs. 12.0 g of digestible Lys, 4.9 vs. 4.7 g of digestible Met, 7.9 vs. 7.5 g of digestible TSAA, and 8.3 vs. 7.8 g of digestible Thr of broilers fed high- or moderate-AA diets, respectively) intakes were similar between the groups fed either the high- or moderate-AA diets. And the actual AME (3,103 vs. 3,043 kcal for birds fed high or moderate AME diets, respectively) intakes were similar between groups fed either the high- or moderate-AME diets. By consuming smaller amounts of high nutrient density diets, the birds consumed similar amounts or even more of the key nutrients than those birds fed lower nutrient density diets. The current results showed that dietary protein source did not affect FI, which is consistent with previous research showing that chickens exhibit no preference between MBM and DDGS diets (Cantor and Johnson, 1983; Rochell et al., 2012). If DDGS comprise 25% of diets, chicks will consume more feed due to the low energy content of DDGS (Wang et al., 2007b). In the current trial, hDDGS diets, which contained 6% DDGS, did not affect FI.

Chicks fed diets with a higher AME gained more BW, which was in agreement with early experiments showing that high-energy diets improved broiler BW gain (Grover et al., 1972; Reece and Mcnaughton, 1982; Zhao et al., 2009), and the BW gain of chicks fed an hDDGS diet was the same as those fed an hMBM diet. Consistent with previous research by Zhai et al. (2013), BW gain was not affected by dietary AA density alone. However, chicks fed hMBM diets with high AA and AME densities gained the most BW, whereas chicks fed hDDGS diets with moderate AA and AME densities grew more slowly from 8 to 21 d of age. This suggests that protein source or AA density do not exert independent effects on BW, but that they rather interact with AME level in affecting growth. A 3-way interaction for 8- to 21-d FCR was observed. In this interaction, the high dietary nutrient density groups (hDDGS and hMBM diets) resulted in a lower FCR. These results are in agreement with studies in which increases in dietary nutrient density decreased FCR (Saleh et al., 2004; Zhao et al., 2009; Delezie et al., 2010). Interestingly, among the observed effects of the 8 treatment groups, the groups of birds that exhibited the highest and lowest FCR were these that were fed hDDGS diets. This result indicates that broiler chickens fed DDGS exhibit a more sensitive FCR response to nutrient density than those fed an MBM diet.

The current results showed that a high nutrient density in grower diets from 8 to 21 d of age improves FCR only during the grower feeding phase and does not persist in to the latter phases. Previous studies have shown that young birds with a poor performance due to early nutrient restriction exhibit compensatory growth during the latter grow-out period. Indeed, BW eventually reaches normal levels within the latter growing phases (Summers et al., 1990; Zubair and Leeson, 1996; Cristofori et al., 1997; Pinheiro et al., 2004; Zhan et al., 2007; Zhai et al., 2013).

Carcass Traits

Except for abdominal fat pad weight, the weights of the carcass parts examined at 42 d of age were not affected by the dietary treatments that were imposed from 8 to 21 d of age. Nevertheless, the long-term effect that dietary treatment had on abdominal fat pad weight extended through 55 d of age. This suggests that an early dietary regimen may have a long-term effect on fat deposition. It is well accepted that body fat content varies depending on energy intake (Keren-Zvi et al., 1990; Cabel and Waldroup, 1991; Summers et al., 1992); those results are consistent with the current trial. When chickens fed high-AA-diets are fed a higher dietary AME level (3,100 vs. 2,998 kcal/kg) from 8 to 21 d of age, fat deposition at 42 d of age is increased. In addition, dietary protein level may also affect chicken fat deposition. Broiler chickens fed a 26% CP diet deposited less fat than those fed a 22% CP diet (Marks and Pesti, 1984), and chicken fed a 22.5% CP diet deposited less fat than those fed a 19% CP diet (Aletor et al., 2000). Those results are also consistent with the current trial, in which a higher AA density level in the diets (22.88%) reduced fat pat weight at 42 d of age when compared with a lower AA density in the diets (20.80%) of birds fed moderate AME levels. However, when chickens were fed hMBM diets, fat pad weight at 55 d of age was increased in the birds fed a high-AA-level diet. Those results suggest that fat deposition may have been affected by the interaction of ingredient type and nutrient density in the diets.

Interestingly, although early nutrition manipulation did not affect total carcass yield at 55 d of age, it did affect breast and leg quarter yield. When broiler chickens were fed MMM diets from 8 to 21 d of age, the highest breast and leg quarter weights were produced from 43 to 55 d of age. The broiler chickens fed lower nutrient densities at an early age appeared to exhibit compensatory growth with a subsequently expected increase in breast and leg quarter meat yield from 43 to 55 d of age. It is suggested that the morphology, mucin production, and enzyme excretion of the digestive systems of the broilers may have adjusted to the different nutrient densities of the diets, as indicated in earlier research (Bedford, 1996; Horn et al., 2009). The birds fed a lower density diet at an early stage may have been allowed more opportunity to develop digestive systems that become capable of absorbing more nutrients more efficiently during the latter phases of their growth. With the accumulated effects of compensatory growth, the MMM group of broilers produced the most breast and leg quarter meat by 55 d of age.

Dietary treatment did not affect feed cost based on BW gain; however, feed cost based on carcass weight at 55 d of age was decreased by an increase in the energy level of the diet fed from 8 to 20 d of age. Higher energy diets are more expensive on a feed-weight basis (Table 1); however, because they lead to a significant decrease in FI, the total cost of feed to achieve a given carcass weight by 55 d of age is less. This suggests that high-energy diets not only improve FCR, but that they also lower broiler meat production costs.

The current results showed that serum CHOL and HDLC level at 20 d of age were positively correlated to abdominal fat weight at 42 d of age. This is consistent with a previous study showing that abdominal fat pad weight was positively correlated with serum CHOL (Musa et al., 2007). In addition, serum CHOL concentrations in the present study were positively correlated with HDLC. These effects lasted through 42 d of age, which suggests that higher serum CHOL and HDLC levels in young broilers may serve as predictors of an increase in subsequent fat deposition. Nevertheless, serum TRIG at 20 d of age was not correlated with abdominal fat pad weight at 42 or 55 d of age, indicating a lack of association between serum TRIG and abdominal fat deposition. Although serum TRIG concentration at 20 d of age was not correlated with carcass weight or the weight of its cut-up parts at 42 d of age, a higher serum TRIG was associated with lower carcass, wings, and leg quarter weights at 55 d of age. A lower level of muscle production is indicative of lower protein anabolism or higher protein catabolism. Sanz et al. (2000) reported that a higher serum TRIG is indicative of lower lipid utilization in the tissues of birds. Because muscle tissue consists mainly of protein and lipid, a lower utilization of lipid may lead to a higher utilization of protein, which results in a decrease in the deposition of muscle protein.

In conclusion, high AA or AME densities in grower diets from 8 to 21 d of age may improve the growth performance of broilers during the grower feeding phase without affecting growth performance during the latter feeding phases, from 22 to 55 d of age. However, early diet manipulation (from 8 to 21 d) may affect meat yield during the latter grow-out phases (from 22 to 55 d). Grower diets from 8 to 21 d of age with a high AME density may increase carcass production profit margins. In addition, dietary regimens involving protein source and AA or AME densities during the grower phase from 8 to 21 d of age may alter serum CHOL and HDLC concentrations, as well as fat accumulation by 42 and 55 d of age. Furthermore, blood constituents at 20 d of age are associated with subsequent fat deposition and meat production at 42 and 55 d of age.

Acknowledgments

We express our appreciation for the expert technical assistance of Donna Morgan and Sharon Womack of the Mississippi State University Poultry Science Department.

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This is journal paper no. J-12468 from the Mississippi Agricultural and Forestry Experiment Station, Mississippi State University.

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