First off, I know this is a sensitive topic, so feel free to delete if it crosses the forum's threshold.
It's been kind of conventional wisdom that slaveowners in North America (and maybe other Anglo colonies) tried to breed the strongest slaves in a manner akin to breeding horses or dogs, Jimmy the Greek notoriously brought this up on TV and was fired. However, is there any real evidence for such practiced? I've always lived in the South, but I've never seen it mentioned in state history classes or local literature (not that I've made an exhaustive survey).
No, enslavers did not try to breed the people they enslaved like thoroughbred horses or pedigreed dogs. To the degree they select for physical traits, among others, they do so when enslaving new people rather than through trying to control which slaves had children together. On the reproduction side, the preference is for more enslaved babies in general. This occasionally extends to the point where enslavers order or encourage the rape of enslaved women by enslaved men.