Closed Flock (or Herd)
This usually relates to pedigree flocks/herds where replacement breeding stock is bred on-farm, ie. animals are not bought in from other breeders. The advantage of this system is that the farmer of a closed flock/herd can selectively breed for particular traits that suit his needs and the conditions of the farm. It also minimises the danger of importing disease onto the farm. It is standard practice on farms run on organic principles.