| Three experiments were conducted to investigate the effects of dietary Zn source and level on growth performance, tissue Zn concentration, Zn metalloenzyme activity, metallothionein(MT) concentration, MT mRNA and Zn transporters-2(ZnT-2) mRNA level, so as to evaluate the optimal Zn level and relative bioavailabilities of different Zn sources for broilers fed a corn-soybean meal diet..Experiment 1. An optimal dietary Zn level for broiler chicks fed a corn-soybean meal dietExperiment 1 was carried out with a total of 720 day-old AA commercial male broilers to investigate the effects of dietary Zn level on growth performance, tissue Zn concentration, Zn metalloenzyme activity, metallothionein(MT) concentration, MT mRNA and Zn transporters-2(ZnT-2) mRNA level on D 7, 14 or 21, so as to choose suitable criteria to estimate the optimal dietary Zn level for broiler chicks fed a corn-soybean meal diet. Birds were randomly assigned by bodyweight to one of 8 treatments(six pen replicates of eight chicks each) in a completely randomized design. Dietary treatments included the basal diet(Control, 23.87mg/kg Zn) supplemented with either 0, 20, 40, 60, 80, 100, 120, or 140 mg/kg Zn as Zn sulfate. The results showed that: 1) Among the criteria investigated in this study, bone and pancreas Zn concentration, serum 5'-NT activity and pancreas ZnT2 mRNA level were the suitable criteria for estimate the optimal dietary Zn level for broiler chicks. Pancreas MT and MT mRNA increased linearly as dietary Zn level increased, they were good indicators of Zn status but weren't sutitable for Zn requirement estimation; 2) Among the criteria suitable for Zn requirement estimation, pancreas ZnT2 mRNA level was the most consistent criterion. Based on this criterion, the optimal supplemental Zn level for broiler chicks was 60mg/kg; 3) The optimal supplemental Zn level for broiler chicks on D 7, 14 or 21 were 60, 60 and 56mg/kg respectively, no great difference among them. Accordingly, the optimal supplemental Zn level for broiler chicks of 21d was 60mg/kg. Including the Zn within the basal diet, the estimate Zn requirement was 90mg/kg, double the recommendations of the NRC(1994).Experiment 2. Relative bioavailabilities of organic Zn sources with different complex strengths for broilers fed a conventional corn-soybean meal dietsAs the complex strength of organic Zn sources is the most important of all chemical characteristics, which might be related to theirZn utilization in the body, three organic Zn sources with different complex strengths were chosen for this study. They were Zn-AAC with the weak complex strength(Q_f 6.5, 11.93% Zn), Zn-Pro B with the moderate complex strength(Q_f 30.7, 13.27% Zn), and Zn-Pro A with the strong complex strength(Q_f 944.0, 18.61% Zn), respectively. Based on the results of experiment 1, experiment 2 was conducted using a total of 1092 day-old Arbor Acres(AA) commercial male broilers to investigate the effects of dietary Zn source and level on growth performance, tissue Zn content, MT concentration in pancreas, and its gene expression on D 6, 10 or 14, so as to choose suitable criteria to evaluate the relative bioavailabilities of these three organic Zn sources for broilers fed a conventional corn-soybean meal diet. Birds were randomly allotted by bodyweight to one of 13 treatments(six pen replicates of eight chicks each) in a completely randomized design involving a 4×4 factorial arrangement of treatments, and fed Zn-unsupplemented corn-soybean meal basal diets(Control, 27.82mg/kg Zn) or basal diets supplemented with 30, 60 or 90 mg/kg Zn as Zn sulfate, Zn-AAC, Zn-Pro B or Zn-Pro A. The results showed that: 1) Except growth performance, all other criteria investigated in this study were good indicators of Zn status, they were affected by Zn level, and increased linearly as dietary supplemental Zn level increased but there was only pancreas MT mRNA level affected by both Zn source and Zn level. The results indicate that pancreas MT mRNA level as a criterion for relative bioavailability assays of Zn sources was more sensitive than other indices; 2) There was a close correlativity between complex strengths of these organic Zn sources and their relative bioavailabilities for broilers. Based on slope ratios from the multiple linear regression of pancreas MT mRNA level on daily intake of dietary Zn, the relative bioavailabilities of Zn-AAC, Zn-Pro B or Zn-Pro A were 100, 121.1 and 72.3% if 100% was set for Zn sulfate, respectively on 6D; 100.5,123.5 and 77.9% on 10D, and 90.8,110.9和68.5% on 14D. Zn-Pro B source with the moderate complex strength was the most available, and the Zn-AA C source with the weak complex strength was similar to Zn sulfate, while the Zn-Pro A source with the strong complex strength was the least available(P<0.1). These findings have not been reported in literature before.Experiment 3. Relative bioavailabilities of organic Zn sources with different complex strengths for broilers fed a corn-soybean meal diet containing a high level of phytateExperiment 3 was carried out, based on the results of experiment 1 and 2, with a total of 1080 day-old AA commercial male broilers to investigate the effects of dietary Zn source and level on MT gene expression in pancreas, so as to evaluate the relative bioavailabilities of three organic Zn sources for broilers fed diets containing a high level of phytate. Birds were randomly assigned by bodyweight to one of 18 treatments(six pen replicates of eight chicks each) in a completely randomized design involving a 4×3×2 factorial arrangement of treatments. Two Zn-unsupplemented corn-soybean meal basal diets were a conventional corn-soybean meal basal diet(0.46% Phytate and 27.61mg/kg Zn), and fed a Zn-unsupplemented corn-soybean meal basal diet containing a high level of phytate(1.43% Phytate and 28.03mg/kg Zn). The basal diet supplemented with 0, 30 or 60 mg/kg Zn as Zn sulfate, Zn-AAC, Zn-Pro B or Zn-Pro A for a period of 6days. Based on slope ratios from the multiple linear regression of pancreas MT mRNA level on daily intake of dietary Zn, the relative bioavailabilities of Zn-AAC, Zn-Pro B or Zn-Pro A were 101.0, 128.0 and 70.0% if 100% was set for Zn sulfate, respectively on a conventional corn-soybean meal basal diet, and 105.0, 138.5 and 91.5% respectively on a high-phytate basal diet. The results showed that: 1) Pancrease MT mRNA level was affected by Zn source and Zn level, was a sensitive criterion for relative bioavailability assays of Zn sources; 2) Three organic Zn sources, especially Zn-Pro A showed increased relative bioavailabilities, suggesting that these organic Zn sources could partially or completely resist to the complexation from high phytate in the digestive tract, and then be absorbed more than Zn sulfate, which gave a further evidence that organic Zn sources are superior to Zn sulfate, and they are also different in bioavailability due to the difference of complex strength. These results have not been reported in literature before.In conclusion, the optimal Zn level for broiler chicks of 21 d was 90 mg/kg. Pancreas ZnT2 mRNA level was the most consistent criterion for optimal supplemental Zn level estimation. Pancreas MT mRNA level could be a new method for a quick and effective assessment in bioassays of Zn sources for broilers; There was a close correlativity between complex strengths of these organic Zn sources and their relative bioavailabilities for broilers, in which the organic Zn source with the optimal complex strength appeared to be utilized better than all other organic Zn sources with weak and strong complex strengths and Zn sulfate studied, because it could have a better resistance to the inhibition of Zn absorption by dietary high phytate. The results and findings in this thesis would be of very important theoretical and practical significances for developing and utilizing new and highly bioavailable organic Zn additives with the optimal complex strength, so as to promote the growth of broilers, and minimize the environmental pollution of Zn excreted by animal. |
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