Impacts and characteristics of antimicrobial resistance of Escherichia coli isolates by administration of third-generation cephalosporins in layer hatcheries.

•In poultry, third-generation cephalosporins are administered in hatcheries to control the early mortality.•Third-generation cephalosporin-resistant E. coli at 1, 2, and 4 weeks was higher in ceftiofur-treated flocks than in gentamicin-treated flocks.•Third-generation cephalosporin-resistance gradually decreased from 83.3 % at 1 week of age to 4.4 % at 30 weeks of age in ceftiofur-treated flocks.•CTX-M-14, CTX-M-15, CMY-2, and TEM-1, were detected in third-generation cephalosporin-resistant E. coli from layer farm.•Plasmid replicon typing revealed that CTX-M-14, CTX-M-15, CMY-2, and TEM-1were located on F, F and FIB, I1 and K, and I1 and FII, respectively.