Effects of antimicrobials and weaning on porcine serum insulin-like growth factor binding protein levels

The effects of subtherapeutic antimicrobial supplementation and weaning on serum levels of IGF-I and insulin-like growth factor binding proteins (IGFBP)-2, -3 and -4 were determined in crossbred weanling pigs. At weaning, pigs were allotted to a diet containing 21.8% crude protein and 1.15% lysine with or without Aureozol (110 mg/kg of Aureomycin chlortetracycline, 110 mg/kg of sulfathiazole, and 55 mg/kg of penicillin) for 4 wk. Insulin-like growth factor-binding proteins and IGF-I analyses were performed on blood samples that were drawn weekly. Weaning decreased serum IGFBP-3 levels in both control and Aureozol-treated groups on d 6 and d 14 (P < 0.05) relative to preweaning levels. The IGFBP-3 values returned to preweaning levels by d 21. Although the circulating levels of both the 43-kDa and the 39-kDa glycosylation variants of IGFBP-3 were affected by weaning, the level of the 39-kDa IGFBP-3 was affected relatively more than that of the 43-kDa IGFBP-3 (P < 0.05). Compared with circulating IGFBP-3 levels in control pigs, Aureozol-treated pigs had higher circulating IGFBP-3 levels on d 21 (43%, P < 0.05) and d 27 (46%, P < 0.05). In direct contrast to the effect of weaning on serum IGFBP-3 level, serum IGFBP-2 levels increased on d 6 and d 14 after weaning (P < 0.05) and decreased to preweaning levels by d 21. The IGFBP-2 levels continued to decline and were less than preweaning levels by d 27 (P < 0.05). Aureozol treatment had no effect on serum IGFBP-2 levels at any time. Serum levels of nonglycosylated IGFBP-4 were not affected by either weaning or Aureozol supplementation. Weaning decreased circulating IGF-I concentration on d 6 in both control and Aureozol-treated pigs (76 and 73%, respectively, P < 0.05) and on d 14 (62%, P < 0.05) and d 21 (32%, P < 0.05) in control pigs. Aureozol-supplemented pigs had higher serum IGF-I concentrations than control pigs on d 14 (82%, P < 0.05), d 21 (55%, P < 0.05), and d 27 (36%, P < 0.05). The Aureozol-fed pigs had a 14.2% increase in BW gain (P < 0.05) and a 59.6% increase in ADG (P < 0.05) compared with pigs fed the control diet. Both Aureozol-supplementation and weaning cause changes in serum IGFBP levels and IGF-I concentrations that might be involved in regulating rate and efficiency of growth.