chickens
Flock Management Styles
All in  all out
All In All Out is when you buy all your chickens at once, run them as a flock for a predetermined period of time (usually around two to three years) then cull (kill) them all and replace them with a completely new group of chickens. 

The main advantage is that it eliminates the risk of introducing parasites and disease when adding new birds to the existing flock, as is done with the Staggered Replacement management system. The main disadvantage is that egg production is not even.  It starts off very high but gradually drops as the chickens, who are all the same age, grow older.

advantages/disadvantages summary

Advantages

  • Eliminates the risk of introducing parasites and disease when adding new birds to the existing flock.
     
  • Gives you an opportunity to fallow the chicken run, which is a good way of getting rid of existing diseases and parasites, most of which can't survive for more than a few weeks without chickens around to act as hosts.
     
  • Avoids the inevitable interruptions to the social structure of the flock, sometimes known as the pecking order, when you introduce new young chickens to an established flock.

Disadvantages
  • If you lose some of your flock due to predators or disease you may not get enough eggs for your needs forcing you to break the all in all out cycle by buying replacement chickens or to bring forward your all out culling date.
     
  • While egg production will start off high it will gradually fall as the chickens grow older.