Effects of Different Pharmacological Zinc Withdrawal Strategies on Growth Performance of Nursery Pigs

Pharmacological levels of Zn from zinc oxide (ZnO) are commonly added to nursery diets to improve growth and reduce post-weaning diarrhea, but abrupt removal can impair feed intake and gut function. Gradual pharmacological withdrawal strategies may help maintain performance and gut stability; however, limited data are available comparing withdrawal approaches. A 55-d study was conducted to evaluate the effects of different pharmacological levels of Zn withdrawal strategies on growth performance of nursery pigs. At weaning, 360 pigs (initially 5.6 ± 0.88 kg) were allotted within two body-weight blocks to one of six dietary treatments in a generalized randomized block design. Each treatment had 12 replicate pens with 5 pigs per pen. Treatment diets were fed in Phase 1 (d 0-11), Phase 2 (d 11-24), and Phase 3 (d 24-45), followed by a common Phase 4 diet (d 45-55). Treatments included: (A) no pharmacological Zn, with 110 mg/kg Zn provided in the premix from zinc sulfate in all phases; (B) 3,000 mg/kg Zn in Phase 1, then 110 mg/kg for phases 2 and 3; (C) 3,000 mg/kg in Phase 1, 2,000 mg/kg in Phase 2, then 110 mg/kg in phase 3; (D) 3,000 mg/kg in phase 1, 2,000 mg/kg in phase 2, and 1,000 mg/kg for 7 d in phase 3 followed by 110 mg/kg for the remainder of phase 3; (E) 3,000 mg/kg in phase 1, 2,000 mg/kg in phase 2, and 1,000 mg/kg for 14 d in phase 3 followed by 110 mg/kg for the remainder of phase 3; and (F) 3,000 mg/kg in phase 1, 2,000 mg/kg in phase 2, and 1,000 mg/kg throughout phase 3. During phase 1, pigs fed pharmacological Zn had increased ADG (P < 0.001), ADFI (P = 0.005), and G:F (P = 0.008) compared with pigs without ZnO. In phase 2, ADG (P = 0.005) was lowest and ADFI tended to be lowest (P = 0.065) for the treatment previously fed 3,000 mg/kg and then reduced to 110 mg/kg of Zn during phase 2. No treatment differences were observed during the total 3-week phase 3 feeding period, nor the common period (P > 0.10). Body weight was greater (P < 0.05) on d 11 and 24, and tended to be greater (P < 0.10) on d 31 and 38 for pigs that remained on the higher Zn treatments; however, significance was lost due to increased variation even though the weight advantage remained. For the overall study (d 0 to 55), ADG, ADFI, and G:F were not different among treatments. In conclusion, ZnO benefits pigs during the early nursery phase; however, abrupt withdrawal of pharmacological levels of Zn appears to eliminate the long-term growth benefit.