Did US Slaveholders ever try breeding their slaves for specific traits? If so, how did it work out?

Before we get in here, I want to lay down the content warnings. There will be talk about rape. Also it's gross to talk about breeding people like horses, but I can't not do that and also answer the question.

So then, the simple answer is no. Enslavers understood about selective breeding from livestock, of course, but I've never seen any evidence that anyone tried to breed a "better" slave. The closest analog you could find would be nothing to do with breeding at all: enslavers could increase the sale value of the people they enslaved by seeing to it that they developed valuable skills. For men, that would usually be some kind of trade: carpentry, blacksmithing, and so forth. For women, you would have more traditionally "feminine" skills like dressmaking. An enslaved person with such a skill would sell for more than one without and might have a longer time generating profit than a field hand. But you can't breed someone to be a carpenter; it's a learned ability.

Which is not to say that enslavers aren't interested in procreation of their human property. The value of enslaved women included estimates of their "increase". (The same term is used for livestock.) An enslaved woman with many reproductive years ahead of her could have many children, who would inherit her status. Thus, while she may not be as suitable for hard labor -though we have records of enslaved women being top pickers in cotton fields- she could make up for that deficit, and then some, by having children who her enslaver would then enslave automatically. The key thing here is numbers rather than traits. More people to enslave = more money, whether one tortures them for profit oneself or sells them off for others to enslave.

For the most part, this was all done by just putting people together and letting nature take its course but I do know of one incidence of an attempted forced reproduction by enslaver orders. In other words: enslaved men were told to rape an enslaved woman. This is probably not the only time that happened, but it also doesn't appear to be anywhere near normal.

The first known shipment of enslaved Africans to New England is in 1638. The Desire took a cargo of enslaved Native Americans down to the West Indies and came back with them. The same year we have our first known English enslaver in New England, Samuel Maverick, set up shop. He owns Noddle's Island in the middle of Boston Harbor. In this year he enslaved three people, two women and a man. Maverick is that kind of guy, so he sees dollar signs. He tells at least one of the women to get to it. She refuses. This is how Wendy Warren relates it in New England Bound:

Maverick ordered that “negro man,” whom he also owned, to impregnate her, or try to, by force; the man obeyed. In other words, when the woman would not willingly couple with the man, Maverick ordered her raped. After the attack, the distraught woman came to the window of another Englishman she could hardly have known well— John Josselyn, a temporary houseguest of Maverick’s on Noddle’s Island— and complained in a “very loud and shril” voice about the assault. The woman “took” the attack, Josselyn noted with concern, “in high disdain beyond her slavery,” and it was “the cause of her grief.”

Her story demands a pause: only eight years after the founding of the famed Massachusetts Bay Colony, less than two decades after the celebrated Pilgrims founded the equally mythologized Plymouth Colony, an enslaved African woman on an island in Boston’s harbor was anguished because another slave had raped her upon their mutual owner’s orders, so that he might own a “breed of Negroes.” In taking the time to write down her story, Josselyn showed himself to be an at least momentarily sympathetic listener. But the very next paragraph of his narrative recounted, in humorous tones, his first encounter with a ferocious North American wasp. It seems that little was done in response to the woman’s complaints.

Warren, Wendy. New England Bound: Slavery and Colonization in Early America (Kindle Locations 153-164). Liveright. Kindle Edition.

It wouldn't surprise me if other people at Maverick's idea and tried it, but if they did then it's rare enough that I've never seen it in antislavery literature. When enslavers want to ensure that the women they enslave have plenty of children, or just for pleasure, they're not shy about being rapists themselves; the availability of enslaved women for rape is sometimes touted as one of the benefits of slavery because otherwise men would go after white women. Enslavers (and their relatives, kids, etc) rape enslaved women so commonly that white women in elite households gossip about how the pale enslaved kids all look like You Know Who and white abolitionists activists make it one of their major lines of attack.