| The experiment was conducted to investigate the effects of different supplemental manganese (Mn) levels on growth performance, incidence of leg abnormality, carcass traits, meat quality, heart Mn content, MnSOD mRNA levels, transcriptional factors specificity protein 1 (Sp-1) and activating protein 2(AP-2) of DNA-binding activities, MnSOD protein expression and MnSOD activities to find the most sensitive indices to determine the optimal dietary Mn level for 4 to 6- week-old broiler chickens. A total of 480 1-day-old male Arbor Acres chicks were allotted to sixty cages with eight chickens each, and fed the same diet (containing 127.33 mg Mn/kg) until 21 days of age. At 22 days of age, 384 birds were selected and randomly allotted by body weight to one of eight treatments for eight cage replicates of six chicks each in a completely randomized design, and fed a Mn-unsupplenented corn-soybean meal basal diet (containing 13.69 mg Mn/kg) or the basal diet supplemented with 20, 40, 60, 80, 100, 120 and 140 mg /kg Mn as Mn sulfate for a duration of 21 days respectively. The results showed that: 1) there were no significant differences (P>0.10) in ADG, ADFI, F/G and incidence of leg abnormality in broilers. However, the incidence of leg abnormality tend to decrease with the increasing supplemental Mn levels, and the average daily gain was the numerically highest in chicks fed diet supplemented with 80 mg Mn/kg. 2) There were no significant differences (P>0.10) in carcass traits and meat quality. However, the pH in breast and leg muscle increased with the increasing supplemental Mn levels, and the percentage of abdominal fat was the numerically lowest in chicks fed diet supplemented with 80 mg Mn/kg. 3) Supplemental Mn levels had significant (P<0.10) influence on heart Mn content which showed the linear correlation (P<0.05) with the diet Mn levels. It indicated that the heart Mn content can be used as index for evaluating the Mn nutritional status of broilers, but not be suitable for evaluating the Mn requirements for broilers. 4) Supplemental Mn levels had significant (P<0.10) influence on MnSOD mRNA levels of heart mitochondria which showed the quadratic correlation (P<0.05) with the diet Mn levels. It indicated that the Mn can affect the MnSOD gene transcription by regulating the MnSOD mRNA levels, and the MnSOD mRNA levels can be used as sensitive index for evaluating the Mn requirements of broilers. Based on the MnSOD mRNA levels the 87.8 mg/kg Mn in diet would be a satisfactory supplemental level for broilers. 5) Supplemental Mn levels had significant (P<0.10) influence on transcriptional factors Sp-1 and AP-2 of DNA-binding activities which showed the linear increase and decrease (P<0.05) with the diet Mn levels respectively. It indicated that Mn can affect the MnSOD gene transcription by the transcription factors Sp-1 and AP-2 of the DNA binding activity. And the Sp-1 was a positive transcription factor, while the AP-2 was a negative regulation factor. It suggested that the Sp-1 and the AP-2 can be used as index for evaluation of the Mn nutritional status of broilers, but not be suitable for evaluating the Mn requirements of broilers. 6) Supplemental Mn levels had significant (P<0.10) influence on MnSOD protein expression of heart mitochondria which showed the quadratic curve correlation (P<0.05) with the diet Mn levels. It indicated that the Mn can affect the MnSOD gene translation by regulating MnSOD protein expression and the MnSOD protein expression can be used as sensitive index for evaluating the Mn requirements of broilers. Based on the MnSOD protein expression the 90.5 mg/kg Mn in diet would be a satisfactory supplemental level for AA broilers. 7) Supplemental Mn levels had significant (P<0.10) influence on MnSOD activity of heart mitochondria which showed the broken line correlation (P<0.05) with the diet Mn levels. It indicated that the MnSOD activity can be used as sensitive index for evaluating the Mn requirements of broilers. Based on the MnSOD activity levels the 80.2 mg/kg Mn in diet would be a satisfactory supplemental level for broilers. 8) Based on these indexes the 100 mg/kg would be a satisfactory level of dietary Mn, and the recommended supplemental Mn is 80 mg/kg in practical corn-soybean meal diet (containing 20 mg Mn/kg) for 4-6 week-old broilers.Summarily, supplemental Mn levels had no significant (P>0.10) influence on growth performance, incidence of leg abnormality, carcass traits and meat quality, but significant (P<0.10) differences were observed in heart Mn content, MnSOD mRNA levels, the ranscriptional factors Sp-1 and AP-2, MnSOD activities and MnSOD protein expression among treatments. The heart Mn content and the ranscriptional factors Sp-1 and AP-2 were not suitable for evaluating the Mn requirements. However the MnSOD mRNA levels, the MnSOD protein expression and the MnSOD activities can be used to evaluate the Mn requirements, and the optimal level of dietary Mn is recommended to be 100 mg/kg in practical corn-soybean meal diet (containing 20 mg Mn/kg) for 4-6 week-old broilers on base of these indexes.
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