It is imperative that when you cull a sow there is a replacement gilt ready to take the sow's place and that breeding targets are met.
This spreadsheet indicates the age and bodyweight of the replacement gilt, depending on when you decided to cull the sow.
Note the decision to cull the sow does not mean that the sow is culled that day - it is only that you have decided that at the next possibly opportunity the sow should be culled. The sooner you make the decision to cull a sow, the more time you allow for gilts to be prepared.
Culling on the day of weaning, with no previous planning - there had better be plenty of prepared gilts available - or that lack of planning has just cost the farm 800kg of pig meat! (The output of one farrowing place sold)
Gilt selection protocols
Pig Flow models