So it wasn't practical, but it was competitive?
Automation was coming on strong - the cotton gin, steam engines and soon to come combustion engines.
Cotton was a hand labor-intensive crop, and it wasn't automated until the 1940s, when the first practical cotton picking machines came onto the market and when herbicides developed as a by-product of WW2 research ended the need for hand chopping and weeding. Would slavery have persisted until until then? Coincidentally, that was about the time that sharecropping, the labor system that replaced bond slavery with debt peonage, faded away.
Moreoever, not every slave was working on a plantation. Many were household help in the towns and cities. Why would automation make owning your housekeeper less attractive?
As was tobacco, which to my knowledge was never automated.