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Background: The protein/energy ratio is important for the production performance and utilization of available feed resources by animals. Increased protein consumption by mammals leads to elevated feed costs and increased nitrogen release into the environment. This study aimed to evaluate the effects of dietary protein/energy ratio on the growth performance, carcass traits, meat quality, and plasma metabolites of pigs of different genotypes. Methods: Bama mini-pigs and Landrace pigs were randomly assigned to two dietary treatment groups (Chinese conventional diet with low protein/energy ratio or National Research Council diet with high protein/energy ratio; n = 24 per treatment) in a 2 × 2 factorial arrangement. Blood and muscle samples were collected at the end of the nursery, growing, and finishing phases. Results: We observed significant interactions (P < 0.05) between breed and diet for total fat percentage, intramuscular fat (IMF) content, protein content in biceps femoris (BF) muscle, and plasma urea nitrogen (UN) concentration in the nursery phase; for average daily gain (ADG), average daily feed intake (ADFI), dry matter, IMF content in psoas major (PM) muscle, and plasma total protein and albumin concentrations in the growing phase; and for drip loss and plasma UN concentration in the finishing phase. Breed influenced (P < 0.05) growth performance, carcass traits, and meat quality, but not plasma metabolites. Throughout the trial, Landrace pigs showed significantly higher (P < 0.05) ADG, ADFI, dressing percentage, lean mass rate, and loin-eye area than did Bama mini-pigs, but significantly lower (P <0.05) feed/gain ratio, fat percentage, backfat thickness, and IMF content. Dietary protein/energy ratio influenced the pH value, chemical composition of BF and PM muscles, and plasma activities of glutamic-pyruvic transaminase and gamma-glutamyl transpeptidase, and plasma concentration of UN. Conclusions: Compared with Landrace pigs, Bama mini-pigs showed slower growth and lower carcass performance, but had better meat quality. Moreover, unlike Landrace pigs, the dietary protein/energy ratio did not affect the growth performance of Bama mini-pigs. These results suggest that, in swine production, low dietary protein/energy ratio may be useful for reducing feed costs and minimizing the adverse effects of ammonia release into the environment. Keywords: Dietary protein/energy ratio, Growth performance, Meat quality, Mini-pig, Plasma metabolites * Correspondence: email@example.com; firstname.lastname@example.org Equal contributors Key Laboratory of Agro-ecological Processes in Subtropical Region, Hunan Provincial Engineering Research Center of Healthy Livestock and Poultry, and Scientific Observing and Experimental Station of Animal Nutrition and Feed Science in South-Central, Ministry of Agriculture, Institute of Subtropical Agriculture, Chinese Academy of Sciences, Changsha, Hunan 410125, China Full list of author information is available at the end of the article © 2015 Liu et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Liu et al. Journal of Animal Science and Biotechnology (2015) 6:36 Page 2 of 10 Background Chinese Academy of Sciences . Ninety-six barrows The pig is one of the most economically important (including 48 Bama mini-pigs, a Chinese local breed species among domesticated livestock and is a major [average initial BW, 3.38 ± 0.96 kg], and 48 Landrace protein source for human consumption. A major ob- piglets [average initial BW, 7.68 ± 0.89 kg]) were used in jective of pig production is to increase skeletal muscle this study. growth and reduce excess fat accretion. Livestock pro- ducers use nutritional modifiers in an attempt to in- Study design crease protein accretion in the muscle, while they The experiment was a 2 × 2 factorial arrangement, with often simultaneously reduce fat deposition [1, 2]. The two breeds (Bama mini-pig vs. Landrace) and two diet- pattern of fat deposition in pigs in the growing- ary treatments (Chinese conventional diet [GB diet] and finishing phase affects carcass and meat quality . National Research Council [NRC] diet), resulting in a Genetic selection of pigs for leaner meat has also re- total of four treatments (Table 1). Piglets of each breed sulted in reduced intramuscular fat (IMF) content, were randomly assigned to one of two dietary treatment and consumers perceive the meat thus obtained as groups (n = 24 per treatment). The NRC diet was formu- tougher, less moist, and poorly flavored. Thus, the lated to meet the nutrient requirements recommended livestock industry faces the challenge of increasing by NRC (2012)  and had a high protein/energy ratio, the IMF content of pork so that consumers may have whereas the GB diet was formulated per the recommen- a satisfactory experience, while simultaneously produ- dations of Chinese National Feeding Standard for Swine cing minimal visible fat, which is a deterrent to (GB, 2004)  and had a low protein/energy ratio health-conscious consumers. (Table 2 and Additional file 1: Table S1). The animals In China, several rich resources of indigenous pig breeds were individually housed in 0.6 m × 1.2 m pens with are available, with more than 30 breeds established. In hard plastic slatted flooring. Each pen was equipped with addition to their economic significance, pigs, particularly a stainless steel feeder and a nipple drinker. The animals miniature pigs (mini-pigs), are considered important ani- had free access to drinking water and feed. The room mal models for human disease and xenotransplantation re- temperature was maintained at 25–27 °C. All pigs were search because of their physiological and anatomical fed three times per day at 0800, 1300, and 1800 h. Diet- similarities to humans [4, 5]. Bama mini-pigs (Sus scrofa ary phase changes were noted on the day on which the domestica), a Chinese indigenous mini-pig breed which pigs were weighed; these changes were noted on the originated in Bama County, Guangxi Province, is a promis- same day for all treatment types. Every 2 weeks, feed in- ing animal model [6, 7] with an obese genotype. In con- take was recorded in order to determine average daily trast, Landrace, a representative lean genotype, is fast- gain (ADG), average daily feed intake (ADFI), and the growing breed and produces a relatively large amount of feed intake to body gain ratio (F/G). meat, and it is therefore more attractive to producers. Al- though muscle growth and meat quality considerably differ Sample collection between Western and indigenous Chinese pig breeds, it is According to the feeding standards, the BW range for not clear how nutrients mediate the effect of genetic back- nursery, growing, and finishing phases (ending with mar- ground on animal growth and meat quality. The growth ket weight) was defined as 8–20, 20–50, and 50–90 kg, and development processes of pigs, which involve changes respectively, for Landrace pigs, and 3–15, 15–35, and in body weight (BW) and shape as well as metabolic and 35–55 kg, respectively, for Bama mini-pig (Table 1). physiological functions, depend on factors such as the Nursery, growing, and finishing phases comprised feed- breed, nutritional status, and feeding condition of the ani- ing for 39, 44 and 30 days, respectively, for Landrace mal. Against this background, the objective of the present pigs, and 55, 48 and 21 days, respectively, for Bama study was to examine the effects of dietary protein/energy mini-pigs. At the end of each phase (i.e., when BW ratio on the growth performance, muscle development, reached 20, 50, and 90 kg for Landrace pigs, and 15, 35, and plasma metabolites (which are indicators of nitrogen and 55 kg for Bama mini-pigs), eight pigs from each metabolism) in Bama mini-pigs and Landrace pigs at dif- treatment were randomly weighed, bled, and sacrificed ferent phases of growth. for the evaluation of carcass characteristics and meat quality. Briefly, the animals were fasted for 12 h to Methods avoid the effect of feed intake on postprandial bio- Study animals chemical measurements and then pre-slaughter BW The experiment was carried out in accordance with the was measured. Thereafter, blood samples were col- Chinese guidelines for animal welfare and experimental lected into 10-mL centrifuge tubes containing sodium protocol, and was approved by the Animal Care and Use heparin (14.3 USP units/mL). Next, the pigs were Committee of the Institute of Subtropical Agriculture, held under general anesthesia and sacrificed by a Liu et al. Journal of Animal Science and Biotechnology (2015) 6:36 Page 3 of 10 Table 1 Animals and treatments Item Landrace pig Bama mini-pig Body weight GB diet group NRC diet group Body weight GB diet group NRC diet group Nursery phase 10–20, kg GB diet 1 NRC diet 1 8–15, kg GB diet 1 NRC diet 1 Growing phase 20–50, kg GB diet 2 NRC diet 2 15–35, kg GB diet 2 NRC diet 2 Finishing phase 50–90, kg GB diet 3 NRC diet 3 35–55, kg GB diet 3 NRC diet 3 GB diet, Chinese conventional diet jugular vein injection of 4 % sodium pentobarbital Assessment of meat quality solution (40 mg/kg BW) . After the head, legs, Meat quality was examined at the end of experiment by tail, and viscera were removed, the carcass was split determining the pH, muscle color, drip loss, and cook- longitudinally. Longissimus dorsi (LD), biceps femoris ing yield. Initial pH (pH )and finalpH(pH ) 45min 24h th th (BF), and psoas major (PM) muscles from the right- values were measured in triplicate at the 6 to 7 rib side of each carcass were immediately sampled after position at 45 min and 24-h postmortem, respectively, slaughter, andstoredat −20 °C for biochemical ana- using a hand-held pH meter (Russell CD700; Russell pH lysis (including evaluation of dry matter [DM], crude Limited, Germany). Muscle color scores were assigned th protein [CP], and crude lipid contents). The LD to LD muscle at the 10 rib interface by using a Konica muscle on the right-side carcass was removed, and Minolta chromameter (CR410; Konica Minolta Sensing, approximately 2.5-cm-thick sections were cut from Inc., Tokyo, Japan) with an 8-mm measuring port, D65 the anterior end for assessment of meat quality be- illuminant, and 10 observers. Hunter lightness (L ), red- * * fore and after chilling the carcass for 24 h . Blood ness (a ), and yellowness (b )valueswererecordedin samples were subsequently centrifuged at 900 × g for triplicate. For evaluating drip loss, on the day of slaugh- 10 min at 4 °C to recover plasma. Plasma samples ter, approximately 100 g fresh LD muscle was weighed were stored at −80 °C until analysis. and placed in a Whirl-Pak bag, suspended in a 4 °C cooler for 24 h, reweighed, and drip loss was recorded. Percentage of cooking meat was measured by determin- Determination of carcass composition ing the weight of cooked LD muscle. The muscle sample Pre-slaughter BW, carcass weight, carcass length, backfat th was weighed and covered in a container before cooking. thickness, and loin-eye area at the 10 rib were measured Immediately after cooking for 45 min at 100 °C, the sam- immediately post-mortem according to the Chinese Guide- ple was removed from the container and dried with a lines on Performance Measurement Technology and Regu- paper towel, then reweighed. Cooking yield was expressed lations for Pigs . Carcass straight length was measured using the following formula: Cooking yield = (cooked from the first rib to the end of the pubic bone. Backfat weight/raw weight) × 100. thickness was measured using a vernier caliper, and the average measurements at three points: the first rib, last rib, and last lumbar vertebra were recorded. The left side of Chemical analysis of skeletal muscle each carcass was weighed and then physically dissected Chemical composition of the skeletal muscle was analyzed into skin, skeletal muscle, fat, and bone for evaluation of in duplicate according to AOAC methods (1997) . carcass characteristics. These components were weighed, DM content of muscle was determined gravimetrically by and the weights were multiplied by 2 to calculate the per- oven drying the samples at 110 °C for 24 h. CP and lipid centage of the whole carcass that each component consti- contents were measured using Kjeldahl and Soxhlet ex- tuted. Dressing percentage was calculated as carcass traction methods, respectively . weight divided by live BW. Analysis of plasma metabolites Table 2 Nutrient content of experimental diets Plasma activities of alkaline phosphatase (ALP), glutamic- Item NRC NRC NRC GB GB GB diet 1 diet 2 diet 3 diet 1 diet 2 diet 3 pyruvic transaminase (GPT), glutamic-oxaloacetic trans- Digestible energy, 14.22 14.21 14.22 13.46 13.40 13.40 aminase (GOT), lactate dehydrogenase (LDH), creatine MJ/kg phosphokinase (CPK), and gamma-glutamyl transpepti- Crude protein, % 20.06 18.01 15.11 18.03 16.05 13.46 dase (GGT), and plasma concentrations of albumin (Alb), total protein (TP), ammonia (AMM), and urea nitrogen Protein/energy 1.41 1.27 1.06 1.34 1.20 1.00 ratio (UN) were analyzed using a CX-4 Automatic Biochemical GB diet, Chinese conventional diet Analyzer (Beckman Inc., USA) and commercial kits Liu et al. Journal of Animal Science and Biotechnology (2015) 6:36 Page 4 of 10 (Leadman Biochemistry Technology Company, Beijing, Carcass quality China) according to the manufacturers’ instructions . Table 4 shows the effects of treatments on carcass charac- teristics. In all phases, Bama mini-pigs had significantly lower (P < 0.05) dressing percentage, carcass length, loin- Statistical analysis eye area, and skeletal muscle percentage (15–25 units dif- Data were analyzed by a mixed-effects model using the ference), but higher (P < 0.05) total fat percentage (1–20 SAS version 8.2 (SAS Institute Inc., Cary, NC, USA). units difference) than did Landrace pigs. Except during Diet, breed, and their interactions were included in the the nursery phase, the backfat thickness of Bama mini- statistical model. Effects were considered statistically sig- pigs was greater (P < 0.05) than that of Landrace pigs nificant at P < 0.05. Probability values between 0.05 and (2- to 5-fold difference after the nursery phase). In the 0.10 were considered to be trends. nursery and finishing phases, Bama mini-pigs fed the NRC diet showed significantly higher (P < 0.05) dressing per- Results centages than those fed the GB diet. In the nursery phase, Growth performance Landrace pigs fed the NRC diet had significantly greater All pigs showed healthy growth throughout the experi- (P < 0.05) loin-eye area and reduced backfat thickness mental period. ADG and ADFI of Bama mini-pigs were (P < 0.05) than those fed the GB diet. However, in the lower (P < 0.05), whereas F/G was higher (P < 0.05) when growing phase, Landrace pigs fed the GB diet had longer compared with Landrace pigs in the same phase and fed carcasses, more total skeletal muscle, and lower percent- the same diet (Table 3). The growth performance of age of fat than those fed the NRC diet (P < 0.05). Bama mini-pigs did not significantly differ (P > 0.05) be- tween dietary treatments in any of the three phases. Meat quality However, in the growing phase, Landrace pigs fed the Compared with Landrace pigs, Bama mini-pigs had GB diet had higher (P < 0.05) ADG and ADFI than lower (P < 0.05) pH and pH , but higher (P < 0.05) 45min 24h those fed the NRC diet, indicating breed × diet inter- a and cooking yield (Table 5). For Bama mini-pigs, the actions (P < 0.05). In contrast, in the finishing phase, NRC diet showed increased (P < 0.05) pH in the fin- 24h Landrace pigs fed the GB diet had lower (P < 0.05) ishing phase. A breed × diet interaction (P < 0.05) was ADG than those fed the NRC diet. observed for dripping loss. Table 3 Effects of dietary protein/energy ratio and breed on growth performance of pigs Item Landrace pig Bama mini-pig SEM P-value GB diets NRC diets GB diets NRC diets Breed Diet B × D Nursery phase (n = 24) a a b b Initial BW, kg 7.71 7.64 3.37 3.38 0.19 <0.001 0.88 0.83 a a b b Final BW, kg 22.95 23.06 17.29 17.91 0.81 <0.001 0.66 0.76 a a b b Average daily gain, g 390.80 395.40 253.10 264.10 15.36 <0.001 0.62 0.84 a a b b Average daily feed intake, kg 0.76 0.71 0.62 0.63 0.03 <0.001 0.42 0.41 b b a a Feed to gain ratio 1.97 1.87 2.53 2.38 0.07 <0.001 0.09 0.73 Growing phase (n = 16) a a b b Initial BW, kg 24.74 25.15 20.55 19.66 0.66 <0.001 0.73 0.34 a b c c Final BW, kg 61.82 56.37 38.81 38.95 1.70 <0.001 0.13 0.12 a b c c Average daily gain, g 842.80 709.60 380.60 401.80 31.39 <0.001 0.09 0.02 a b b b Average daily feed intake, kg 1.96 1.64 1.79 1.68 0.05 0.23 <0.001 0.04 a b a a Feed to gain ratio 2.33 2.43 4.77 4.65 0.27 <0.001 0.98 0.70 Finishing phase (n =8) a a b b Initial BW, kg 65.03 62.83 44.07 45.82 1.07 <0.001 0.87 0.15 a a b b Final BW, kg 91.03 92.03 51.40 52.76 1.46 <0.001 0.52 0.92 b a c c Average daily gain, g 866.70 973.30 348.80 330.50 41.87 <0.001 0.40 0.24 a a b b Average daily feed intake, kg 3.12 3.07 1.69 1.83 0.06 <0.001 0.59 0.22 b b a a Feed to gain ratio 3.66 3.19 5.62 5.79 0.49 0.001 0.80 0.61 a, b, c Mean values with unlike superscript letters were significantly different (P < 0.05) GB diets, Chinese conventional diets; B × D, breed × diet interaction Liu et al. Journal of Animal Science and Biotechnology (2015) 6:36 Page 5 of 10 Table 4 Effects of dietary protein/energy ratio and breed on carcass performance in pigs Item Landrace pig Bama mini-pig SEM P-value GB diets NRC diets GB diets NRC diets Breed Diet B × D Nursery phase a a b b Pre-slaughter BW, kg 19.39 19.15 12.00 14.40 0.84 <0.001 0.21 0.13 a a c b Left carcass BW, kg 6.25 6.17 3.20 4.19 0.32 <0.001 0.16 0.10 a a c b Dressing percentage, % 64.29 64.15 53.26 58.16 1.32 <0.001 0.08 0.06 a a b b Carcass length (bevel), cm 25.69 23.76 14.92 15.69 1.13 <0.001 0.61 0.24 a a b b Carcass length (straight), cm 24.04 21.72 13.56 14.35 1.03 <0.001 0.46 0.14 a b ab ab Backfat thickness, mm 7.78 5.64 6.81 7.19 0.52 0.75 0.78 0.14 2 b a c c Loin-eye area, mm 904.40 1,124.20 372.30 437.80 73.41 <0.001 0.06 0.30 a a b b Total skeletal muscle, % 61.23 65.75 47.70 46.38 1.58 <0.001 0.32 0.07 b b a a Total fat, % 6.47 3.63 19.36 23.54 1.56 <0.001 0.67 0.03 Growing phase a b c c Pre-slaughter BW, kg 58.51 49.21 33.62 34.36 1.87 <0.001 0.05 0.02 a b c c Left carcass BW, kg 20.98 17.66 10.91 11.23 0.71 <0.001 0.07 0.03 a a b b Dressing percentage, % 71.67 71.74 65.04 65.13 0.97 <0.001 0.94 0.99 a b c c Carcass length (bevel), cm 85.21 80.57 69.33 69.29 1.18 <0.001 0.08 0.08 a b c c Carcass length (straight), cm 82.07 76.57 59.00 61.29 1.12 <0.001 0.20 0.004 b b a a Backfat thickness, mm 6.07 6.54 30.70 33.36 1.33 <0.001 0.29 0.46 2 a a b b Loin-eye area, mm 1,919.70 1,724.20 696.70 671.70 120.20 <0.001 0.41 0.52 a b c c Total skeletal muscle, % 66.53 62.97 40.70 40.08 1.08 <0.001 0.09 0.22 c b a a Total fat, % 10.07 13.45 34.60 35.88 0.87 <0.001 0.02 0.28 Finishing phase a a b b Pre-slaughter BW, kg 86.79 92.03 49.84 49.19 2.15 <0.001 0.36 0.24 a a b b Left carcass BW, kg 32.39 34.79 17.14 17.69 0.90 <0.001 0.16 0.38 a a c b Dressing percentage, % 74.50 75.61 68.91 71.90 0.76 <0.001 0.03 0.29 a a b b Carcass length (bevel), cm 92.29 92.00 81.80 78.86 1.32 <0.001 0.30 0.39 a a b b Carcass length (straight), cm 87.40 87.83 79.90 77.00 1.22 <0.001 0.39 0.25 b b a a Backfat thickness, mm 22.69 23.52 40.40 43.29 1.42 <0.001 0.27 0.53 2 a a b b Loin-eye area, mm 3,303.90 2,836.60 1,018.50 812.40 155.78 <0.001 0.07 0.47 a a b b Total skeletal muscle, % 61.84 62.73 37.40 38.38 0.97 <0.001 0.41 0.98 b b a a Total fat, % 15.65 16.81 38.00 37.85 1.01 <0.001 0.67 0.57 a, b, c Mean values with unlike superscript letters were significantly different (P < 0.05). n= 8 GB diets, Chinese conventional diets; B × D, breed × diet interaction Table 5 Effects of dietary protein/energy ratio and breed on meat quality in finishing pigs Item Landrace pig Bama mini-pig SEM P-value GB diets NRC diets GB diets NRC diets Breed Diet B × D a ab b b pH value (45 min) 6.38 6.22 6.08 6.08 0.07 0.01 0.34 0.36 a a b a pH value (24 h) 5.45 5.49 5.29 5.45 0.04 0.06 0.05 0.27 L* 47.62 47.47 48.55 48.11 1.00 0.85 0.27 0.33 c bc ab a a* 12.87 13.80 14.30 15.29 0.30 <0.001 0.01 0.94 b* 5.62 5.68 6.11 5.70 0.39 0.57 0.69 0.60 a ab b ab Drip loss, % 1.99 1.53 1.16 1.62 0.20 0.12 0.99 0.05 Cooking yield, % 53.45 53.22 55.49 55.78 0.81 0.02 0.97 0.78 a, b, c Mean values with unlike superscript letters were significantly different (P < 0.05). n= 8 GB diets, Chinese conventional diets; B × D, breed × diet interaction Liu et al. Journal of Animal Science and Biotechnology (2015) 6:36 Page 6 of 10 Table 6 Effects of dietary protein/energy ratio and breed on muscle chemical composition in pigs, % Item Landrace pig Bama mini-pig SEM P-value GB diets NRC diets GB diets NRC diets Breed Diet B × D Nursery phase Longissimus dorsi muscle Dry matter 23.75 23.68 23.61 24.09 0.33 0.69 0.55 0.41 b b a a Intramuscular fat 1.22 1.06 2.82 3.54 0.26 <0.001 0.30 0.10 b a a a Crude protein 18.34 19.69 19.47 19.61 0.36 0.17 0.05 0.11 Biceps femoris muscle bc c a ab Dry matter 22.11 21.39 23.39 23.15 0.42 0.001 0.26 0.57 c c a b Intramuscular fat 0.86 0.99 3.25 2.45 0.22 <0.001 0.14 0.05 bc c ab a Crude protein 17.60 16.49 18.51 19.01 0.40 <0.001 0.46 0.05 Psoas major muscle Dry matter 22.21 22.42 21.77 22.37 0.35 0.50 0.27 0.59 b b a a Intramuscular fat 0.89 0.94 2.48 2.32 0.20 <0.001 0.78 0.62 b b ab a Crude protein 16.94 17.41 17.72 18.56 0.35 0.01 0.09 0.63 Growing phase Longissimus dorsi muscle b b a a Dry matter 25.07 25.05 29.11 27.37 0.63 <0.001 0.24 0.25 b b a a Intramuscular fat 1.16 1.50 3.73 3.50 0.34 <0.001 0.89 0.48 b b ab a Crude protein 21.27 20.77 21.98 22.73 0.39 0.007 0.79 0.18 Biceps femoris muscle b b a b Dry matter 23.15 22.89 26.79 23.76 0.75 0.01 0.05 0.10 b b a b Intramuscular fat 1.01 0.81 2.77 1.68 0.30 <0.001 0.05 0.17 b b a b Crude protein 19.80 19.24 22.13 19.35 0.71 0.13 0.04 0.16 Psoas major muscle c c a b Dry matter 23.67 23.68 26.78 24.97 0.36 <0.001 0.04 0.03 b b a b Intramuscular fat 1.06 1.19 4.66 2.25 0.36 <0.001 0.01 0.005 Crude protein 19.82 20.17 21.07 20.84 0.51 0.10 0.91 0.62 Finishing phase Longissimus dorsi muscle c bc a ab Dry matter 26.16 26.56 28.74 28.26 0.58 0.004 0.95 0.50 b b a a Intramuscular fat 2.59 2.48 5.85 4.50 0.44 <0.001 0.15 0.22 Crude protein 21.32 22.02 21.73 21.93 0.47 0.76 0.40 0.64 Biceps femoris muscle b b b a Dry matter 24.97 25.18 25.43 28.93 0.77 0.02 0.04 0.07 c c b a Intramuscular fat 1.42 1.88 3.65 5.23 0.31 <0.001 0.01 0.12 ab ab b a Crude protein 20.84 21.19 19.41 22.85 0.81 0.90 0.05 0.10 Psoas major muscle Dry matter 25.47 24.70 26.42 26.04 0.57 0.10 0.40 0.77 b b a a Intramuscular fat 2.46 2.43 4.01 4.00 0.38 0.002 0.96 0.97 Crude protein 21.08 21.30 20.70 20.91 0.48 0.51 0.71 0.99 a, b, c Mean values with unlike superscript letters were significantly different (P < 0.05). n= 8 GB diets, Chinese conventional diets; B × D, breed × diet interaction Chemical composition of muscle was significantly higher (P < 0.05) in Bama mini-pigs Chemical compositions of LD, BF, and PM muscles were than in Landrace pigs. The DM contents of BF in the examined (Table 6). The lipid content of these muscles nursery phase; that of LD, BF, and PM in the growing Liu et al. Journal of Animal Science and Biotechnology (2015) 6:36 Page 7 of 10 Table 7 Effects of dietary protein/energy ratio and breed on the plasma biochemical parameters in pigs Item Landrace Bama mini-pig SEM P-value GB diets NRC diets GB diets NRC diets Breed Diet B × D Nursery phase b b a ab ALP, U/L 173.63 148.88 235.88 182.63 18.64 0.01 0.04 0.45 GPT, U/L 62.88 77.75 67.63 81.00 7.94 0.62 0.08 0.92 GOT, U/L 64.00 89.00 71.50 101.63 14.75 0.50 0.07 0.86 LDH, U/L 665.50 742.63 654.63 793.00 60.15 0.74 0.08 0.61 CPK, U/L 1,622.00 2,218.50 1,654.40 1,901.00 347.96 0.68 0.23 0.62 GGT, U/L 111.00 75.38 66.00 82.63 16.63 0.27 0.57 0.13 Alb, g/L 31.53 32.31 34.97 34.74 2.14 0.18 0.89 0.81 b ab ab a TP, g/L 61.92 68.39 64.67 71.51 2.72 0.29 0.02 0.94 AMM, μmol/L 213.31 113.75 167.10 129.14 34.25 0.66 0.05 0.38 a ab b ab UN, mmol/L 6.79 5.86 5.06 5.81 0.41 0.04 0.82 0.05 Growing phase a a b b ALP, U/L 207.67 192.33 123.67 90.75 18.00 <0.001 0.24 0.67 a b b b GPT, U/L 99.67 71.33 69.50 72.13 4.76 0.03 0.31 0.17 GOT, U/L 92.67 84.83 64.83 63.38 16.26 0.19 0.80 0.86 a a b b LDH, U/L 854.21 996.22 490.50 503.44 116.31 0.003 0.56 0.62 CPK, U/L 1,614.50 1,640.00 1,826.30 1,608.00 337.95 0.81 0.80 0.75 a b b b GGT, U/L 120.00 71.00 67.33 72.00 13.04 0.18 0.26 0.16 a b b b Alb, g/L 48.98 39.35 38.65 37.75 1.73 0.005 0.01 0.03 a b a a TP, g/L 87.28 69.82 84.80 85.02 3.52 0.12 0.04 0.03 AMM, μmol/L 88.02 112.68 96.18 139.16 11.63 0.32 0.06 0.60 UN, mmol/L 6.52 6.56 5.25 5.42 0.68 0.13 0.89 0.93 Finishing phase ab a b ab ALP, U/L 114.00 157.83 85.60 117.14 13.98 0.05 0.03 0.72 ab a b ab GPT, U/L 62.80 70.00 51.60 67.14 4.70 0.22 0.05 0.46 GOT, U/L 46.40 56.33 46.40 46.57 6.06 0.51 0.49 0.50 LDH, U/L 498.40 551.33 377.80 387.43 49.59 0.03 0.60 0.72 a a a b CPK, U/L 1,974.00 1,901.30 1,925.00 897.10 267.43 0.11 0.10 0.15 GGT, U/L 49.80 52.17 51.20 56.43 4.21 0.58 0.46 0.78 Alb, g/L 39.30 40.30 37.50 39.17 1.26 0.34 0.38 0.82 TP, g/L 75.88 76.67 82.12 81.94 2.60 0.08 0.92 0.88 AMM, μmol/L 88.98 97.60 72.64 87.01 11.87 0.35 0.43 0.84 a b ab a UN, mmol/L 5.12 4.03 4.47 5.29 0.28 0.18 0.38 0.05 a, b, c Mean values with unlike superscript letters were significantly different (P < 0.05). n= 8 GB diets, Chinese conventional diets; B × D, breed × diet interaction; ALP, alkaline phosphatase; GPT, glutamic-pyruvic transaminase; GOT, glutamic-oxaloacetic transaminase; LDH, lactate dehydrogenase; CPK, creatine phosphokinase; GGT, gamma-glutamyl transpeptidase; Alb, albumin; TP, total protein; AMM, blood ammonia; UN, urea nitrogen phase; and that of LD and BF in the finishing phase BF muscle in the nursery phase, and significantly were higher (P < 0.05) in Bama mini-pigs than in Land- higher (P < 0.05) contents of DM, lipid, and CP in the race pigs. In addition, compared with the Landrace BF and PM muscles in the growing phase, than those pigs, Bama mini-pigs had higher (P < 0.05) CP levels in fed the NRC diet. In the finishing phase, Bama mini- the BF and PM muscles in the nursery phase and in the pigs fed the GB diet had lower (P < 0.05) contents of LD muscle in the growing phase. Bama mini-pigs fed DM, lipid, and CP in the BF muscle than those fed the the GB diet had higher (P < 0.05) lipid content in the NRC diet. Liu et al. Journal of Animal Science and Biotechnology (2015) 6:36 Page 8 of 10 Plasma metabolites as Landrace pig, which can utilize high nutrient diets with- Plasma biochemical analytes of Bama mini-pigs and out increased lipid deposition. Landrace pigs are listed in Table 7. In the nursery phase, Meat quality is one of the most important economic no breed × diet interactions were observed (P > 0.05) for traits of farm animals, and it determines the suitability plasma biochemical analytes except for UN (P = 0.05). of meat for further processing and storage, including re- For pigs fed the GB diet, the plasma ALP activity was tail display. The main desirable attributes are pH, color, lower (P < 0.05), but UN concentration was higher (P < drip loss, fat content, and composition . In our 0.05) in Landrace pigs than in Bama mini-pigs. Within study, Bama mini-pigs exhibited more carcass fat, higher each breed, diet did not influence (P > 0.05) the concen- IMF content, and greater backfat thickness, but reduced trations of plasma metabolites. dressing percentage, lean content of carcass, drip loss, In the growing phase, breed × diet interactions were and LD muscle area than did Landrace pigs in the same observed (P < 0.05) for plasma concentrations of TP and phase. These findings agree with previous reports of Alb. Plasma activities of ALP and LDH in Landrace pigs the superior quality of local Chinese pigs [22, 23]. were higher (P < 0.05) than those in Bama mini-pigs. Several studies have shown different dietary protein/ Landrace pigs fed the GB diet had higher (P < 0.05) plasma energy ratios in animal feeds. Some of these studies activities of GPT and GGT, and higher (P < 0.05) concen- examined the growth performance, body composition, trations of TP and Alb than those fed the NRC diet. and metabolism of aquatic livestock , and others were In the finishing phase, no interactions were observed related to obesity and health of humans [25, 26]. Dietary (P > 0.05) between breed and diet for any of the plasma protein/energy ratio has a significant influence on the fat biochemical analytes, except for UN (P = 0.05). Landrace deposition and chemical composition of muscle. Barea et pigs fed the NRC diet had lower (P < 0.05) plasma UN al.  reported that gain:feed and gain:metabolizable en- concentration than those fed the GB diet. Bama mini- ergy intake were improved by decreasing the ideal CP pigs fed the GB diet had higher (P < 0.05) plasma CPK content of the diet. In their study, when a diet providing activity than those fed the NRC diet. 95 g ideal CP per kg DM was fed, protein deposition reached a maximum value of 71 g/day. Hamill et al.  reported a two-fold increase in IMF content of the muscu- Discussion lus semimembranosus of Duroc gilts fed a low-protein diet The growth performance of pigs and their meat quality de- compared to those fed a high-protein diet. Moreover, they pend on the interactive effects of genotype, rearing condi- demonstrated, via transcriptome analysis, that regulation tions, pre-slaughter handling, and carcass/meat processing of IMF accumulation in response to dietary protein re- [15–17]. The present study focused on evaluating the ef- striction is associated with modulation of gene pathways fects of breed and dietary protein/energy ratio on growth involved in lipid synthesis and degradation. A high IMF performance, carcass composition, and meat quality. We content, also called “marbling fat”, is associated with found significant interaction effects of breed and diet on improved eating quality of meat . The threshold level growth performance and carcass composition. Bama mini- of IMF in meat that results in a pleasing eating experience pigs grew more slowly than Landrace pigs, and their car- is 1.5% IMF, with 2–3 % IMF considered necessary for casses were composed of less lean meat and more fat than optimum eating quality . In our study, IMF contents those of Landrace pigs. These findings indicate that there of all muscle samples examined from Bama mini-pigs are obvious differences in carcass composition between were greater than 1.5 %. However, the IMF contents varied breeds, as previously reported [18, 19]. Furthermore, the across the different muscle samples, and IMF contents in backfat in the native breed was much thicker than that in BF of Bama mini-pigs differed across phases. the Landrace breed. This was expected, as Landrace pigs Activities of metabolic enzymes, including ALP, GPT, and GOT, in the blood change in response to the growth are the result of several years of genetic selection through Mendelian genetics and molecular genetics approaches for and development of animals . Researchers have widely traits including increased growth rate and reduced fat con- divided opinions on the relationship between ALP, GPT, and GOT and growth performance (especially for ADG) tent . Within each breed, pigs fed the NRC diet had considerably higher dressing percentage than those fed the and carcass traits. ALP plays an important role in lipid GB diet. However, in Landrace pigs, GB diet promoted metabolism; hence, increasing the plasma activity of this enzyme may be useful for promoting ADG . GPT and carcass length and lean percentage, especially during the growing phase. The accelerated development of bones and GOT play a critical role in transamination and reflect the muscle may have resulted from compensatory growth due status of protein synthesis and catabolism . An in- crease in the plasma activities of these enzymes can im- to low nutrition. Landrace pigs fed the NRC diet deposited less fat than Bama mini-pigs. This may result from the high prove amino acid metabolism. LDH catalyzes the reversible capacity for muscle growth of fast-growing genotypes, such transformation of pyruvate to lactate and plays a principal Liu et al. Journal of Animal Science and Biotechnology (2015) 6:36 Page 9 of 10 role in anaerobic cellular metabolism . GGT catalyzes Competing interests The authors have declared that no competing interests exist. the transfer of the gamma-glutamyl group from glutathione to acceptor amino acids. In this study, plasma activities of Authors’ contributions ALPand LDHwerehigherinLandracepigsthaninBama YYL and XFK carried out the animal experiments and data analysis, and drafted the manuscript. FNL and YLY designed the study and revised the mini-pigs, especially in the growing phase. Plasma activities manuscript. GLJ and XX participated in the animal trial. BET, JPD, and XJY of GPT and GGT in Landrace pigs fed the GB diet were helped with data collection and analysis. All authors read and approved the higher than those fed the NRC diet. All of these findings final manuscript. were coincident with growth performance within the same Acknowledgements phase. The present work was jointly supported by the National Basic Research Nitrogen is an indicator of protein status  and has Program of China (No. 2012CB124704 and 2013CB127305), and K.C. Wong been used to determine protein requirements and lean tis- Education Foundation, Hong Kong. suegrowthrates in pigs.Blood UN,as the ultimate and Author details major nitrogenous product of protein and amino acid ca- 1 Key Laboratory of Agro-ecological Processes in Subtropical Region, Hunan tabolism, is synthesized in the body via the ornithine cycle Provincial Engineering Research Center of Healthy Livestock and Poultry, and Scientific Observing and Experimental Station of Animal Nutrition and Feed [36, 37]. Plasma UN concentration reflects the balance sta- Science in South-Central, Ministry of Agriculture, Institute of Subtropical tus of amino acids, and is often used as an indicator of kid- Agriculture, Chinese Academy of Sciences, Changsha, Hunan 410125, China. ney and liver function, as well as an indicator of relative Hunan Animal Science and Veterinary Medicine Research Institute, Changsha, Hunan 410131, China. College of Animal Science and hydration status in animals. Low blood UN indicates a good Technology, Hunan Agricultural University, Changsha, Hunan 410128, China. balance of amino acids, and suggests relatively low urea 4 Southern Research and Outreach Center, University of Minnesota, Waseca, synthesis and hydration in the liver and relatively high diet- MN 56093, USA. University of Chinese Academy of Sciences, Beijing 100049, China. ary protein efficiency . In the present study, we found that Landrace pigs fed the NRC diet had lower plasma con- Received: 4 February 2015 Accepted: 21 July 2015 centration of UN, whereas Bama mini-pigs fed the same diet had higher plasma concentration of UN, than did those References fed the GB diet. These findings suggest an interaction effect 1. Dunshea FR, D'Souza DN, Pethick DW, Harper GS, Warner RD. Effects of between breed and diet. 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