Impact of Trace In-feed Antibiotics on Growth Performance and Diarrhea Incidence in Weanling Pigs

Abstract The persistence of antibiotic residues in the environment has raised concern regarding potential exposure of livestock to trace levels of antimicrobial compounds through contaminated feed ingredients. However, the biological effects of such low-level exposure remain unclear. The objective of this study was to evaluate the effects of dietary exposure to trace amounts of in-feed antibiotics on growth performance and diarrhea incidence in weanling pigs. It was hypothesized that sub-therapeutic antibiotic exposure would alter growth and gut health responses during the post-weaning period. A total of 180 pigs (PIC 800 x Yorkshire; initial body weight BW = 6.33 ± 0.69 kg) weaned at 20 ± 1 days were allotted to five dietary treatments in a randomized complete block design, with initial BW as blocking factor (6 pigs/pen; 6 pens/treatment; pen as the experimental unit). The dietary treatments were (1) Control (CON): basal nursery diet; (2) Trace carbadox (TAC): CON + 0.5 mg/kg carbadox; (3) Recommend carbadox (RDC): CON + 50 mg/kg; (4) Trace tiamulin (TAT): CON + 2 mg/kg tiamulin; and (5) Recommend tiamulin (RDT): CON + 200 mg/kg tiamulin. Pigs were fed the experimental diets for 28 days in two phases (phase 1: day 0 to 14; phase 2: day 15 to 28). BW and feed intake were recorded weekly to determine average daily gain (ADG), average daily feed intake (ADFI), and feed efficiency. Diarrhea scores were visually assessed twice daily using a 1 to 5 scale (1 = normal; 5 = watery diarrhea). Data were analyzed using the MIXED procedure of SAS, with diet as the fixed effect and block as the random effect; diarrhea frequency was analyzed using Chi-square tests. Final BW on day 28 was greater (P 0.05) in RDC compared with CON and TAC. During phase 1, RDC, TAT, and RDT exhibited higher (P 0.05) ADG than CON, while from day 21 to 28, RDC maintained higher (P 0.05) ADG compared with CON. Overall ADG across the 28-day period was greater (P 0.05) in RDC compared with CON and TAC. From day 7 to 14, ADFI was increased (P 0.01) in RDC and RDT than CON and TAT. During day 15 to 21 and day 21 to 28, ADFI was highest (P 0.01) in RDC compared with all other groups, a pattern that persisted for phase 2 (day 15–28) and overall ADFI. TAC and TAT generally exhibited higher diarrhea incidence than the control or recommended-dose groups. The frequency of severe diarrhea (score ≥ 4) was greater (P 0.05) in TAT compared with TAC and RDT. Overall, pigs fed recommended antibiotic levels demonstrated superior growth performance, whereas trace antibiotic exposure increased diarrhea occurrence without improving feed efficiency.