They don’t need “incentive”. All they need is complete and utter loss of hope and humanity so that they eventually realize they should just “make the best of it”.

That or the threat of beatings.

There was a picture of a Congolese man who was forced to work on a Dutch rubber plantation with his entire family. This man was staring at the severed hand of his 5 year old daughter. She lost her hand because she failed to meet her quota. If that man suddenly became the most productive slave on the plantation… was that a good thing? Or was he forced into an inhumane situation that is absolutely inexcusable?

Farm work is a useful skill. The incentive was a whip.