Zefra was spayed at about 18 months old. She went through 2 heat cycles.
I did not want to spay her but a situation arose in which I had no other choice.
I have heard people say that it changes the dog, etc. etc., but I have not noticed any difference in Zefra.
The people who work Zefra also agree that they do not see any change in behaviour or working ability. She is still a crazy female with an attitude.
Personally, I will not spay/neuter unless a medical emergency. Zefra was spayed so she was to remain in my home safe from other conditions/individuals.