Abstract 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.
Magoga et al. (Wed,) studied this question.