Abstract
The present study investigates, at farm level, the effect of the time-span between sampling and the last time a particular antimicrobial growth promoter (AGP) was included in the feed on the probability of selecting an AGP-resistant Enterococcus faecium isolate from a broiler flock. The probability that a randomly selected E. faecium isolate was resistant to avilamycin, erythromycin or virginiamycin was 0.91, 0.92 and 0.84, respectively if the isolate originated from a broiler flock fed either avilamycin- or virginiamycin-supplemented feed. As the time-span between sampling and the last AGP consumption increased, the probability of isolating an E. faecium isolate resistant to a particular AGP decreased (probability <0.2 within 3-5 years after last exposure to AGPs). The decrease in probability over time showed little farm-to-farm variation. The number of times a particular AGP was given to previous flocks reared in the same house had no effect on the probability of isolating a resistant isolate.