Growth performance and antibiotic tolerance patterns of nursery and finishing pigs fed growth-promoting levels of antibiotics

The objective of this study was to evaluate the impact of growth-promoting levels of antibiotics in diets for nursery and finishing pigs on growth performance and antibiotic tolerance patterns. Gilts (n = 200, initial body weight was 6.2 +/- 0.003 kg), were allotted based on body weight to one of four treatments in a 2 x 2 factorial randomized complete block design. Nursery treatments consisted of feeding no antibiotics (CON) or an antibiotic diet (ANTI) containing chlortetracycline (CTC; 55 mg/kg). At the end of the nursery phase, one-half of the pigs receiving CON were switched to a diet containing antibiotic (virginiamycin: VIR, 11 mg/kg) and one-half of the pigs receiving ANTI were switched to CON for the remainder of the trial. This created four treatments for the finishing phase, consisting of: control in nursery and finishing (CC), antibiotic in nursery, control in finishing (AC), control in nursery, antibiotic in finishing (CA), or antibiotics throughout (AT). The pigs were weighed at the diet changes during the nursery (weeks 1, 3, and 5) and finishing (weeks 7, 9, 13, 17, and 20) phases. Fecal samples were collected at all diet changes for isolation of fecal coliforms and Enterococcus and subsequently tested for tolerance to CTC and VIR. After 1 week, CON pigs weighed less (7.09 vs. 7.28 kg) and had lower ADG (149 vs. 180 g/day) and ADFI (174 vs. 192 g/day) than ANTI pigs (P<0.05). No performance differences were observed during the remainder of the study. At the initiation of the study (week 0), the ability of coliforms to grow in the presence of CTC and VIR, respectively, were 68 and 73% and increased to 90 and 96% at week 19 (time effect, P<0.001). At week 17, tolerance of coliforms to CTC was greater for CA (98%) than AC (86%, time x treatment effect, P<0.004). Enterococcus tolerance to CTC at week 7 was lower for CC (55%) compared to AT (76%), AC (74%) and CA (83%, time x treatment effect, P<0.001). At week 9, Enterococcus tolerant to CTC and VIR, respectively, was lower for CC (15 and 18%) than AT (31 and 40%), AC (35 and 35%), and CA (44 and 43%, time x treatment effect, P<0.001). Antibiotic growth promoters had little impact on growth performance in clean, isolated facilities with high labor inputs. The tolerance of bacteria to antibiotics fluctuated over time and persisted regardless of the use of antibiotic growth promoters. (C) 2010 Elsevier B.V. All rights reserved.