Impacts of Graded Levels of Metabolizable Energy on Growth Performance and Carcass Characteristics of
2. Materials and Methods 1. Chickens, Diets and Management
The experimental conditions were approved by the Animal Care and Use Committee of the Institute of Animal Science, Guangdong Academy of Agricultural Sciences, China, with the approval number GAASISA-2015-011. The yellow-feathered male chickens (Lingnan breed, a meat-type breed that originated in South China) were obtained from a commercial hatchery (Guangdong Wiz Agricultural Science and Technology Co., Guangzhou, China) and were raised from day 1 to 8 weeks of age on a common, typical diet, provided ad libitum. One thousand two hundred birds were weighed at 8 weeks of age and randomly allocated to 30 equally-sized (4.55 m2) floor pens of 40 birds, having a similar average body weight (BW) (771.25±10.23 g). Five dietary treatments, each with six replicates, consisting of graded metabolizable energy (ME) levels (2900, 3000, 3100, 3200 and 3300 kcal ME/kg, calculated), were pelleted and provided ad libitum, as was water. These experimental diets (Table1) were formulated to provide the nutrient requirements of Chinese yellow-feathered broilers [18], except for the ME level. The gross energy of the diets was analyzed according to the guidelines of Association of Official Analytical Chemists [19], and the ME was determined and calculated according to the methods and the equation of Jiang et al., [20], which showed 2805, 2897, 2997, 3095 and 3236 kcal/kg, respectively. The 2997 kcal/kg was considered to be the control dietary energy level diet according to the previously determined value [18]. The birds were raised under artificial lighting providing 18 h light:6 h dark. Relative humidity and average room temperature were approximately 70.0% and 18◦C throughout the 7-week experimental period (9–15 weeks of age).
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Table 1.Composition and nutrient levels of the experimental diets (%, as fed basis).
Item Metabolizable Energy Levels (Kcal/kg, Calculated2)
2900 3000 3100 3200 3300
Ingredients
Corn (yellow) 70.26 70.26 70.26 70.26 70.26
Soybean meal 16.23 16.23 16.23 16.23 16.23
Corn gluten meal 3.8 3.8 3.8 3.8 3.8
Soybean oil 1.05 2.25 3.44 4.64 5.83
Limestone 1.12 1.12 1.12 1.12 1.12
Di-calcium phosphate 1.10 1.10 1.10 1.10 1.10
Salt 0.30 0.30 0.30 0.30 0.30
Vitamin-mineral premix1 1.00 1.00 1.00 1.00 1.00
L-Lysine HCL (78%) 0.27 0.27 0.27 0.27 0.27
DL-Methionine (99%) 0.09 0.09 0.09 0.09 0.09
Corn cob meal 4.78 3.58 2.39 1.19 0.00
Total 100.00 100.00 100.00 100.00 100.00
Calculated composition2
Metabolizable energy (Kcal/kg)3 2805 2897 2997 3095 3236
Crude protein (%) 16.00 16.00 16.00 16.00 16.00
Calcium (%) 0.80 0.80 0.80 0.80 0.80
Crude fiber 3.07 2.59 2.12 1.64 1.16
Total phosphorus (%) 0.56 0.56 0.56 0.56 0.56
Non-phytate phosphorus (%) 0.37 0.37 0.37 0.37 0.37
Lys (%) 0.85 0.85 0.85 0.85 0.85
Met (%) 0.37 0.37 0.37 0.37 0.37
Met+Cys (%) 0.65 0.65 0.65 0.65 0.65
1Supplied per kilogram of diet: VA 5000 IU, VD3 500 IU, VE 20 IU, VK 0.5 mg, VB1 2.4 mg, VB2 4.0 mg, VB6 3.5 mg, VB12 0.01mg, niacin 30 mg, D-calcium pantothenate 10 mg, folic acid 0.55 mg, biotin 0.15 mg, choline chloride 1200 mg, Fe 80 mg, Zn 65 mg, Cu 7 mg, Mn 60 mg, I 0.35 mg, Se 0.3 mg. The vitamins and minerals in the diet were supplied exactly as stated by the Ministry of Agriculture of the People’s Republic of China [18].2Values were calculated from data provided by the Feed Database in China [21].3Analyzed values.
2.2. Growth Variables
The amounts of provided and refused feed were measured weekly on a replicate basis to calculate the average daily feed intake (ADFI), including adjustments for any dead birds. Mortality of birds was recorded daily. The initial BW, final BW (FBW), average daily body weight gain (ADG), and feed:gain ratio (g/g) (FCR) were measured on a per replicate basis. Metabolic BW was calculated according to the following equation: [(Initial weight+final weight)/2]0.75.
2.3. Sampling
At 15 weeks of age, after 12 h of feed-withdrawal, blood samples were collected in 5 mL heparinized tubes from the jugular vein of 12 birds per treatment (2/replicate) who had BW values within±10 g of the average; plasma was obtained by centrifugation at 1000×g for 15 min at 4◦C. The birds were slaughtered by approved methods for subsequent analyses. The right and left breast muscles were separately sampled, clear of observable connective tissues, and stored at−20◦C until analyses; the right breast muscle (Pectoralis majorandminor) was sampled for meat quality determinations, and the left muscle was used in measuring the chemical composition.
2.4. Carcass Trait Determinations
Dressing percentage (bled and defeathered carcass weight (CW), including head and feet, expressed as a percentage of BW), semi-eviscerated (CW minus weights of trachea, crop, esophagus, intestine, pancreas, spleen, gallbladder, gonads, contents of the proventriculus, and gizzard lining, expressed as a percentage of BW), and eviscerated proportions (semi-eviscerated weight minus neck, head, liver, heart, gizzard, shank, abdominal fat, and proventriculus, expressed as a percentage of BW) were
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calculated. In addition, the relative weights of de-boned thigh muscle, breast muscle, and abdominal fat, expressed relatively to BW, were calculated following the methods of the Chinese National Poultry Breeding Committee [22]. The breast and thigh muscles were placed in polyethylene bags and stored at−22◦C until chemical analysis.
2.5. Meat Quality Determinations
Meat pH, color (a* redness, b* yellowness and L* lightness), and drip loss were measured following the methods of Jiang et al. [23]. Meat pH was measured in the major rightPectoralisusing a portable pH meter (version HI8424; Beijing Hanna Instruments Sci. & Tech. Co., Ltd., Beijing, China).
Three readings of breast meat color were scored with a Chroma Meter (CR-410; Minolta Co., Ltd., Suita, Osaka, Japan) at different, but consistent, locations on the medial side of each muscle then averaged.
Meat color scores, using L* a* b* color scales, were measured; L* is lightness (0=black to 100=white), a* is green (a*) to red (+a*), and b* is blue (b*) to yellow (+b*). Drip loss was estimated following a method modified from Shang et al. [24]. Briefly, about 11 g (fresh weight) of regular-shaped muscle section (4 cm (length)×2 cm (width)×1.5 cm (thickness)) cut from the same location in the breast muscle was weighed and suspended on a steel wire hook, without any contact, in a plastic bag inflated with air and stored at 4◦C for 24 h. The muscle samples were re-weighed to evaluate the drip loss percentage, according to the following equation: [(initial weight−final weight)/initial weight]×100%.
Finally, the shear force of cooked breast muscles was measured according to the methods described by Jiang et al. [23], using an Instron Universal Mechanical Machine (Instron model 4411, Instron Corp, Canton, MA, USA).
2.6. Composition of Body, Breast and Thigh Muscles, and Deposition Rate of Energy and Protein
The frozen samples of left breast and thigh muscles were dissected into small pieces and finely homogenized in a blender at−10◦C. To measure the fat and protein content, deposition rate of energy and protein in the whole body, ten birds at the age of 8 weeks (at the beginning of this experiment) and two additional birds per replicate at the age of 15 weeks were selected and prepared according to the methods of Zhou et al. [25] and Xi et al. [26]. Contents of crude protein (CP), crude fat, and gross energy were analyzed according to the guidelines of AOAC [19]. The deposition rate of protein and energy was estimated following the methods of Xi et al. [26].
2.7. Blood Biochemical Variables
The plasma contents of uric acid (UA), triglycerides (TG), and cholesterol (CHOL) were measured colorimetrically using a spectrophotometer (Biomate 5, Thermo Electron Corporation, Rochester, NY, USA) and commercial kits (Nanjing Jiancheng Institute of Bioengineering, Nanjing, China).
2.8. Statistical Analysis
Each pen (replicate) served as the experimental unit. The effects of dietary ME levels were examined for each variable by ANOVA (JMP Ver. 8.0.2, 2009; SAS Institute Inc., Cary, NC, USA).
Whenever significant effects of treatment were detected, Duncan’s multiple range tests were used to compare the means. Where appropriate, orthogonal polynomial contrasts were used to estimate the linear and quadratic effects of the increasing levels of ME, and a probability level of 0.05 was applied to test significance (SPSS software version 17.0.1., IBM, Armonk, NY, USA). Based on the key indices (ADFI, feed:gain ratio, daily ME intake, uric acid, fat content of breast muscle, and fat content of thigh muscle), quadratic regression equations were used to determine the optimal dietary ME requirement of Chinese yellow-feathered chickens [27]. Data are expressed as means for each diet.
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3. Results
3.1. Growth Performance
Daily ME intake increased, but ADFI and FCR decreased as linear responses to the increment in dietary energy level. The FBW, ADG, metabolic BW, and mortality rate were not affected (p>0.05) by the dietary ME level, but the 3236 kcal/kg diet tended to have greater FBW and ADG than those of the lower ME levels (Table2).
3.2. Carcass Quality
The tested dietary ME levels did not exhibit any significant effect on the carcass quality traits in terms of dressing percentage, eviscerated and semi-eviscerated proportions, relative weights of breast muscle, thigh muscle, and abdominal fat (Table3).
3.3. Composition of Body, Breast and Thigh Muscles
As shown in Table4, the fat content in thigh muscle increased linearly (p<0.05) with the increase in dietary energy level, whereas the fat content in breast muscle showed a quadratic response (p<0.05), and the highest value was obtained with the level 2997 kcal/kg. The protein, fat and energy content in the whole body as well as the energy and protein deposition were not affected by the dietary ME level.
According to the regression model, the highest fat contents (%) in the breast and thigh muscles were obtained with diets containing 3047 and 3135 kcal/kg (Table5).
3.4. Breast Meat Quality
The results of breast meat quality as affected by the dietary ME level are shown in Table6. The 2897, 3095 and 3236 kcal/kg diets resulted in lower shear force values (p<0.05) than those of the control diet, and those of the 2805 kcal/kg diet had an intermediate value (p>0.05). The pH value, drip loss percentage, and meat color grades L*, a* and b* did not differ (p>0.05) among the tested diets.
3.5. Blood Biochemical Variables
The results shown in Table7indicated that plasma UA decreased linearly (p<0.01) with the increase in dietary ME level. The CHOL and TG concentrations were not affected by the diets.
The regression model indicated that the optimal plasma UA was obtained with a diet containing 3200 kcal/kg (Table3).
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Table2.EffectofdietarymetabolizableenergylevelonaveragedailymetabolizableenergyintakeandtheperformanceofChineseyellow-featheredchickensfrom 9–15weeksofage. VariablesDietaryMetabolizableEnergyLevels(Kcal/kg),AnalyzedContent SEMpLinearQuadratic 28052897299730953236 Finalbodyweight(g)1386.61375.091408.081385.391430.1216.850.0629 Averagedailygain(g)12.8212.5813.2712.7913.730.350.0629 Dailyfeedintake(g)78.15a78.32a77.59a75.28b76.06b0.590.00050.0000.002 Feed:Gainratio(g/g)6.10ab6.24a5.86bc5.90b5.56c0.130.00250.0010.002 DailymetabolizableenergyMetabolizableenergyintake(kJ/d)219.21d228.45c232.53bc233.52b246.06a5.69<0.00010.0000.000 Metabolicbodyweight(g)188.25189.04189.79188.60190.711.980.5248 Mortality(%)3.334.165.126.674.172.060.6668 Meanswithinarowwithdifferentsuperscriptsdiffersignificantly(p<0.05).SEM=pooledstandarderrormean.Metabolicbodyweight=[(Initialweight+finalweight)/2]0.75. Table3.EffectsofdietarymetabolizableenergylevelonthecarcassqualityofChineseyellow-featheredchickensat15weeksofage. VariablesDietaryMetabolizableEnergyLevels(Kcal/kg),AnalyzedContent SEMp 28052897299730953236 Dressingpercentage(%)89.6888.9388.9887.8988.841.010.1116 Semi-evisceratedproportion(%)83.1082.8082.3182.0182.161.270.5329 Evisceratedproportion(%)68.9968.5168.7467.6368.051.010.2321 Breastmuscle(%)15.6014.6214.9515.1914.230.630.0906 Thighmuscle(%)18.8719.0518.4219.1218.891.710.9304 Abdominalfat(%)1.100.871.291.711.630.290.0889 SEM=pooledstandarderrormean.
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Table4.Effectofdietarymetabolizableenergylevelsonthecompositionsofbody,breastandthighmuscles,anddepositionratesofenergyandproteinin slow-growingChineseyellow-featheredchickensat15weeksofage. Variables
DietaryMetabolizableEnergyLevels(kcal/kg),Analyzed ContentSEMpLinearQuadratic 28052897299730953236 Compositionofbody Crudeprotein(%)65.3162.5662.4360.5860.778.900.4481 Fat(%)22.7325.0624.1925.8527.147.950.5656 Energy(kJ/g)23.2024.1824.0224.0524.050.440.4839 Intramuscularfatcontent(%) Inbreastmuscle0.94b1.12ab1.64a1.37ab1.31ab0.0560.04410.3860.016 Inthighmuscle5.12b5.56ab6.86a6.18ab6.64a0.360.02880.0250.033 Nutrientdepositionrate(%) Energy12.7114.8214.8015.4815.180.850.0755 Protein25.0325.5826.2125.0126.881.410.2732 Meanswithinarowwithdifferentsuperscriptsdiffersignificantly(p<0.05).SEM=pooledstandarderrormean. Table5.Dose-responseregressionsforChineseyellow-featheredchickensfeddietswithdifferentmetabolizableenergylevelsfrom9–15weeksofage. VariableModel1RegressionEquation2Response3pR2 Uricacid(mmol/L)QP1y=21.494x2−575.47x+398432000.0120.144 Fatcontentofbreastmuscle(%)QP1y=−0.572x2+14.587x− 91.43530470.0160.142 Fatcontentofthighmuscle(%)QP1y=−0.715x2+18.765x− 116.6231350.0330.116 1QP=quadraticpolynomial;QPmodel=Y=α+β×X+γ×X2,whereYistheresponsevariable,Xisthedietarymetabolizableenergy(ME),αistheintercept;βandγarethelinear andquadraticcoefficients,respectively.2Regressionequationsobtainedusingtheanalyzedmetabolizableenergyinthediets(2805,2897,2997,3095and3236Kcal/kg).3Theresponse wasobtainedby−β/(2×γ).
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Table6.Effectsofdietarymetabolizableenergylevelsonthebreastmeatqualityofslow-growingChineseyellow-featheredchickensat15weeksofage. Variables
DietaryMetabolizableEnergyLevels(Kcal/kg),Analyzed ContentSEMpLinearQuadratic 28052897299730953236 pH6.116.006.086.066.020.0090.736 Driploss(%)2.062.222.192.062.080.0760.948 Shearforce(kgf)3.06ab2.49b3.52a2.31b2.62b0.0010.0230.2410.487 Meatcolor L*value55.5455.9955.9154.0155.961.6490.399 a*value15.0215.2416.4615.7614.770.5720.136 b*value20.1620.5421.9819.1921.582.2780.279 Meanswithinarowwithdifferentsuperscriptsdiffersignificantly(p<0.05).SEM=pooledstandarderrormean. Table7.Effectsofdietarymetabolizableenergylevelsonplasmavariablesofslow-growingChineseyellow-featheredchickensat15weeksofage. Variables
DietaryMetabolizableEnergyLevels(Kcal/kg),Analyzed ContentSEMpLinearQuadratic 28052897299730953236 Cholesterol(mmol/L)3.123.193.223.093.130.070.9687 Triglycerides(mmol/L)0.320.330.3770.370.330.0020.3375 Uricacid(mmol/L)197.00a159.83ab156.13ab117.21b134.79b3.810.01090.0050.012 Meanswithinarowwithdifferentsuperscriptsdiffersignificantly(p<0.05).SEM=pooledstandarderrormean.
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4. Discussion