| The hypertrophy and type conversion of muscle fibers is an important physiological process, being a link among all meat quality traits. With an aim to expose the formation mechanism of muscle fiber characteristics and meat quality traits and their nutritional modulation by dietary nutrient density in different broiler breeds, this comprehensive study includes 4 expeiments.Trial 1 investigated the developmental chages of muscle fiber characteristics underlying the formation of meat quality traits in 2 distinct broiler breeds of male chickens (Arbor Acres, AA, a commercial line, and Beijing-You, BJY, a Chinese nonimproved line). Male hatchlings (120 of each breed) were randomly assigned to 6 pens of 20 birds in each breed. The zootechnical parameters were recorded at 0, 7, 21, 35, 42, 63 and 91 d of age, and a muscle sample was obtained from 12 birds of each group, respectively, in the pectoralis major (PM) and biceps femoris (BF). Results showed that genetic improvement resulted in large differences of developmental pattern between breeds. AA broilers had better deposit capacity of protein and fat than did BJY chickens, leading to the allometric growth (overgrowth) of muscular mass and abdominal fat at the cost of internal organs. Altered nucleic acid concentrations of skeletal muscles were involved in the protein metabolism from different genotype challenges, although their value and accuracy in prediction of muscle development needed to be validated further. Muscle fiber properties were changed with age, whose impact on meat quality traits was breeds dependent.Trial 2 was conducted to evaluate the effects of varying nutrient density with constant ME:CP ratio on growing performance, carcass characteristics and blood responses in AA broilers and BJY chickens. Experimental diets were formulated with high, medium or low nutrient densities for 3 growing phases. Starter diets (1 to 21 d) contained 23, 21 and 19% CP with 3,059, 2,793 and 2,527 kcal/kg of ME; grower diets (22 to 35 d) were 21, 19 and 17% CP with 3,150, 2,850 and 2,550 kcal/kg of ME; and finisher diets (36 to 42 d for AA and 36 to 91 d for BJY) diets had 19, 17 and 15% CP with 3,230, 2,890 and 2,550 kcal/kg of ME. Male hatchlings (216 of each breed) were randomly assigned to 6 replicates of 12 birds in each treatment. Arbor Acres broilers had better (P < 0.001) body weight gain (BWG), feed conversion ratio (FCR) and carcass yield, but had greater (P < 0.001) abdominal and carcass fat deposition. In both breeds, the higher nutrient density increased (P < 0.05) BWG, protein efficiency ratio (PER) and energy efficiency ratio (EER), while decreasing (P < 0.05) feed intake and FCR. The breed differences were increased for FCR, PER and EER in the starter period and decreased for carcass chemical composition respectively by higher nutrient density. These findings indicate that: 1. genetic improvement has a significant effect on broiler responses to dietary nutrient density; 2. performance differences between breeds are lessened with diets of low nutrient density; 3. carcass quality differences are less when birds were fed diets of high nutrient density; 4. carcass composition are hardly modified by nutrient density and both breeds exhibit similar metabolite responses to dietary concentrations; 5. optimal diets are deduced for these breeds for the 3 growing phases.Trail 3 has evaluated the effects of varying growth rate, by feeding at different planes of nutrition with constant ME:CP ratio, on muscle characteristics and meat quality in AA broilers and BJY chickens. Experimental diets, differing on average by 2% CP, were formulated as trial 2 with high, medium, or low nutrient densities for 3 growing phases. Male hatchlings (216 of each breed) were randomly assigned to 6 pens of 12 birds in each treatment. Altered histological characteristics of muscle fibers, early postmortem muscle metabolism and meat quality were investigated in the PM and BF muscles. At their market age, AA broilers had significantly higher concentrations of plasma protein and lipid metabolites, ratios of white to red and intermediate fibers, pH, L* and b* values, and lower concentrations of plasma glucose metabolites, muscle fiber diameter, muscle contents of energy stores, a* value, drip loss (DL) and shear force (SF) than did the BJY (P < 0.01). Higher nutrient density increased size of the muscle fibers, decreased glycogen reserve and reduced the rate and extent of acidification in the AA, while accelerating transformation of red and intermediate to white fibers, enhancing energy stores and hastening the decrease in pH in the BJY (P < 0.05). In each breed, most meat quality variables (e.g. SF, DL and color) were consistent with the histological and biochemical changes caused by the feeding strategy. Together, dietary nutrient density can influence meat quality as a result of altered histological and initial energy/metabolic characteristics of the muscle. Many of the responses to diet differed between AA and BJY and between the 2 muscles studied.Based on previous experiments, trail 4 describes the long-term changes of mRNA expression levels for different myosin heavy chain (MyHC) genes using real-time quantitative PCR in AA broilers and BJY chickens, and explores the possible mechanism underlying the effect of dietary nutrient density on muscle development. Results suggested that the relative expressions of MyHC genes, reflecting the type composition of muscle fibers, exhibited a clear temporal-spatial dynamics. Significant differences were observed between breeds, especially at the inflection stages in growth curve. The remarkably decreased MyHC I fibers and inereased MyHC IIB fibers contributed to the fast muscle deposition as well as the accelerated body growth in AA broilers, while the higher proprtion of MyHC I and IIA fibers corresponded to better meat quality in BJY chickens. Dietary nutritional levels could affect muscle fiber characteristics of different broiler breeds as a result of altered expressions of growth axis genes, yet the causal mechanism needed to be investigated further.
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