ABSTRACTThe aim of this longitudinal study was to determine and compare the prevalences and genotypic profiles of antimicrobial-resistant (AR)Salmonellaisolates from pigs reared in antimicrobial-free (ABF) and conventional production systems at farm, at slaughter, and in their environment. We collected 2,889 pig fecal and 2,122 environmental (feed, water, soil, lagoon, truck, and floor swabs) samples from 10 conventional and eight ABF longitudinal cohorts at different stages of production (farrowing, nursery, finishing) and slaughter (postevisceration, postchill, and mesenteric lymph nodes [MLN]). In addition, we collected 1,363 carcass swabs and 205 lairage and truck samples at slaughter. A total of 1,090Salmonellaisolates were recovered from the samples; these were isolated with a significantly higher prevalence in conventionally reared pigs (4.0%;n= 66) and their environment (11.7%;n= 156) than in ABF pigs (0.2%;n= 2) and their environment (0.6%;n= 5) (P< 0.001).Salmonellawas isolated from all stages at slaughter, including the postchill step, in the two production systems.Salmonellaprevalence was significantly higher in MLN extracted from conventional carcasses than those extracted from ABF carcasses (P< 0.001). We identified a total of 24 different serotypes, withSalmonella entericaserovar Typhimurium,Salmonella entericaserovar Anatum,Salmonella entericaserovar Infantis, andSalmonella entericaserovar Derby being predominant. The highest frequencies of antimicrobial resistance (AR) were exhibited to tetracycline (71%), sulfisoxazole (42%), and streptomycin (17%). Multidrug resistance (resistance to ≥3 antimicrobials; MDR) was detected in 27% (n= 254) of theSalmonellaisolates from the conventional system. Our study reports a low prevalence ofSalmonellain both production systems in pigs on farms, while a higher prevalence was detected among the carcasses at slaughter. The dynamics ofSalmonellaprevalence in pigs and carcasses were reciprocated in the farm and slaughter environment, clearly indicating an exchange of this pathogen between the pigs and their surroundings. Furthermore, the phenotypic and genotypic fingerprint profile results underscore the potential role played by environmental factors in dissemination of ARSalmonellato pigs.