Here is my list of differrent methods of how to break a broody (or not
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here is my list on ways to break a broody
You cant stop them going broody but there are several ways to deal with it.
1) let them have fertile eggs,
2) keep removing the eggs so she is sitting on nothing - can take weeks,
3) keep removing her from the nest box - again can take weeks,
4) put her in a cage with food and water - can take between 1 and 7 days,
5) dunk her bum in a bucket of cold water - this lowers her body temperature - not tried this method but she may need several dunkings (just make sure she dries off though).
So far I have tried methods 1 and 4 - much prefer method 1 though
Hopefully one will work for you