| This study was conducted with male broilers of Arbor Acres at the age of 22 days to investigate the effects of intravenously injected different zinc sources and levels on growth performance, zinc concentration of tibia ash and pancreas, pancreatic MT and MT mRNA level of the broilers,and to elicit the differences in metabolizable mechanism and bioavailability among zinc sources.Considering that the birds younger than 21 days of age were difficult to be operated with intravenous injection, so this operation was started on d 22. The broilers were fed the basal corn-soybean meal diet containing 27.54 mg/kg Zn during 1 to 21 days of age. The diet was formulated to meet or exceed all other nutrient requirements recommended by NRC (1994) for 1~21 day-old broilers, in order to enhance their sensitivity to Zn. A total of 432 broilers on 22 days of the age were assigned to one of 9 treatments (eight cage replicates of six chicks each) by bodyweight according to a completely randomized design involving a 4×3 factorial arrangement of treatments. The Zn sources were Zn sulfate, Zn-AAC with the weak chelation strength, Zn-Pro B with the moderate chelation strength and Zn-Pro A with strong chelation strength, respectively. The three injected Zn levels were the dosages calculated according to the appropriate supplemental Zn level in our previous study(the optimal dietary Zn level of broilers was 90 mg/kg, in which the basal diet contained about 30 mg Zn/kg and the appropriate supplemental Zn level was 60 mg Zn/kg), daily feed intake, and Zn absorbility 0, 6% and 12%, respectively. All the groups of the birds injected with different Zn sources shared a control group injected with 0.9% NaCl solution. The experiment was lasted for 12 days. All the chicks were allowed to access to the same corn-soybean meal basal diet (containing 27.54 mg Zn /kg) and tap water (no detectable Zn) ad libitum. The results showed that injected Zn source significantly affected (P<0.08) the tibia ash Zn concentration even on d 6 after injection. There were no differences (P>0.25) among Zn sources in growth performance during d 1 to 6 and d 7 to 12 after injection, pancreas Zn concentration, pancreatic MT protein and MT mRNA level on d 6 after injection, and pancreas Zn concentration, tibia ash Zn concentration, pancreatic MT protein and MT mRNA level on d 12 after injection. Injected Zn level affected (P<0.10) ADG and FI during d 1 to 6 after injection, pancreas Zn concentration, tibia ash Zn concentration, pancreatic MT protein and MT mRNA level on d 6 and 12 after injection. No differences were observed (P>0.14) among injected Zn level in ADFI during d 1 to 6 or ADFI, ADG and FI during d 7 to 12 after injection. All of the above indices were not affected (P>0.43) by the interaction between Zn source and injected Zn level. On d 6 after injection, the birds injected with a solution containing organic Zn source with strong chelation strength had lower (P<0.08) tibia ash Zn concentration than those injected with solution containing organic Zn source with moderate or weak chelation strength, and showed the tendency to be low (P>0.14) compared with those injected with solution containing inorganic zinc sulfate. The chicks injected with solution containing organic Zn source with strong chelation strength tended to have consistently low (P>0.22) MT concentration and low MT mRNA level compared with other groups on d 12 after injection. The control group had lower (P<0.04) ADG than the group injected with 6% Zn absorbility solution, and had higher (P<0.02) FI than the group injected with 6% or 12% Zn absorbility solution. There were no differences (P>0.16) among the groups injected with solution dissolved Zn in ADG and FI. Pancreas Zn concentration, tibia ash Zn concentration, MT concentration and MT mRNA level on d 6 and 12 after injection increased linearly (P<0.006) as intravenously injected Zn level increased. So far, these findings have not been reported in literature.Of all the results attained in the present experiment, such as growth performance, pancreatic zinc concentration, MT concentration and MT mRNA level, the tibia ash Zn concentration on d 6 after injection was the most sensitive indicator for the discrimination of bioavailability among zinc sources. The Zn source with strong chelation strength was the least available for the utilization of tissue in the broilers. The findings herein are very important for developing the new and highly available organic Zn supplements, so as to improve the growth performance of broilers, minimize the environmental pollution of Zn excreted and protect the ecological balance, etc.
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