Effect of dietary supplementation of organic or inorganic zinc on carbonic anhydrase activity in eggshell formation and quality of aged laying hens.

This study evaluated the effects of different dietary levels and sources of zinc (Zn) on performance and carbonic anhydrase (CA) activity in eggshell formation and quality in aged laying hens. A total of 504 Hy-line Grey layers aged 59 wk were fed a basal diet (Zn, 28.4 mg/kg) for 4 wks, then randomly allocated to 7 groups that were fed a basal diet or a basal diet supplemented with inorganic (ZnSO4·H2O) or organic (amino acid metals, 9.58%) Zn at 35, 70, or 140 mg Zn per kg of feed for 6 weeks. Each group had 6 replicates of 12 hens. Results showed that egg weight decreased linearly with the supplemental level of organic Zn (P < 0.05). Dietary Zn supplementation had linear and quadratic effects on the CA activity in plasma (P < 0.05), and it was higher in the organic Zn-added groups at wks 2 and 4 (P < 0.05). Dietary Zn supplementation had a quadratic effect on the CA activity in the eggshell gland (P < 0.05). Shell thickness was greater in the organic Zn-added groups (P < 0.05), and its relationship with the supplemental level of Zn showed linearly and quadratically, increasing with the organic Zn and with the inorganic Zn at wk 4, while linearly increasing with the inorganic Zn at wk 6 (P < 0.05). At wk 4, the supplemental level of inorganic Zn had a linear effect on shell weight, and linear and quadratic effects on shell index and ratio (P < 0.05), while shell weight, the index, and ratio increased linearly and quadratically with the organic Zn level in the diet (P < 0.05), with more obvious effects in the organic Zn-added groups (P < 0.05). Overall, dietary Zn supplementation, up to 140 mg/kg feed, could increase eggshell thickness by enhancing CA activity in the plasma and eggshell gland of aged layers; thicker eggshells were found in the organic Zn-added groups, but the breaking strength did not increase despite the eggshell thickness increasing.