Four experiments were conducted todetermine the interactive effects of pharmacologicalamounts of Zn from ZnO and Cu from organic (Cu-AA complex; Cu-AA) or inorganic (CuSO4) sources ongrowth performance of weanling pigs. The Cu was fedfor 4 (Exp. 1) or 6 (Exp. 2, 3, and 4) wk after weaning,and Zn was fed for 4 (Exp. 1) or 2 (Exp. 2, 3, and 4) wkafter weaning. Treatments were replicated with 7 pensof 5 or 6 pigs per pen (19.0 d of age and 5.8 kg of BW, Exp. 1), 12 pens of 21 pigs per pen (about 21 d of age and 5.3 kg of BW, Exp. 2), 5 pens of 4 pigs per pen (20.3 d of age and 7.0 kg of BW, Exp. 3), and 16 pens of 21 pigs per pen (about 21 d of age and 5.7 kg of BW, Exp. 4). In Exp. 1 and 2, Cu-AA (0 vs. 100 mg/kg of Cu) and ZnO (0 vs. 3,000 mg/kg of Zn) were used in a 2 × 2 factorial arrangement. Only Exp. 1 used in-feed antibiotic (165 mg of oxytetracycline and 116 mg of neomycin per kilogram feed), and Exp. 2 was conducted at a commercial farm. In Exp. 3, sources of Cu (none; CuSO4 at 250 mg/kg of Cu; and Cu-AA at 100 mg/kg of Cu) and ZnO (0 vs. 3,000 mg/ kg of Zn) were used in a 3 × 2 factorial arrangement. In Exp. 4, treatments were no additional Cu, CuSO4 at 315 mg/kg of Cu, or Cu-AA at 100 mg/kg of Cu to a diet supplemented with 3,000 mg/kg of Zn from ZnO and in-feed antibiotic (55 mg of carbadox per kilogram of feed). In Exp. 1 and 2, both Zn and Cu-AA improved ADG and ADFI. No interactions were observed, except in wk 1 of Exp. 2, where Zn increased the G:F only in the absence of Cu-AA (Cu- AA × Zn). A naturally occurring colibacillosis diarrhea outbreak occurred during this experiment. The ZnO addition reduced the number of pigs removed and pig-days on antibiotic therapy. In Exp 3, ADFI in wk 2 was improved by Zn and Cu with no interactions. In wk 1, G:F was reduced by ZnO only in the absence of Cu (Cu × Zn). Feeding Zn decreased fecal microbiota diversity in the presence of CuSO4 but increased it in the presence of Cu-AA (Cu source × Zn). In Exp. 4, Cu supplementation improved the overall ADG and G:F. The CuSO4 effect on G:F was greater than the Cu-AA effect. Our results indicate that pharmacological amounts of ZnO and Cu (Cu-AA or CuSO4) are additive in promoting growth of pigs after weaning.

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