Within breeder birds, there is always a natural variation of bodyweight, even at day-old. Usually it should be quite low variation at placement (bird quality). When the birds grow, the variation within a flock will increase further due to the different responses of individual birds to factors such as vaccination, disease, differing competitiveness for food, etc. This increased variation reduces overall flock performance and makes flock management much more difficult.

 Evaluation of working time requirements of different grading and vaccination processes  

Prof. Dr. Ludwig Theuvsen, Hauke Tergast, Jörg Hurlin