Is there any documentary evidence, primary sources or otherwise that slaveowners participated in selective breeding of the slaves they owned?
This is related to my own research, and I have not found any evidence of it. I have also discussed the subject with other scholars in the field, and they have not heard of any such evidence. If such accounts exist, no one that I am aware of has ever found them.
However, there are a lot of sources where people say that it is happening, or that their relative knows somebody who does it. It is largely assumed at the time that this is happening. The consensus school of history, especially Ulrich B. Philips, considered these accounts to mainly be hearsay and slander. Later historians of the Revisionist school, especially Kenneth Stampp, saw these accounts and assumed them to be true. It was not until later historians tried to find direct evidence for this that the claim was disputed.