What Jeff Marchand said about cattle and sheep loving black locust and then loosing interest in it matches up exactly to the behaviour id expect. As I understand it Black Locust is an established silvopasture crop for feeding to animals (my interest is small ruminants). It has great crude protein levels, but this is counterbalanced by high tannin levels which cause issues with digestibility. The net effect is that is a useful feed crop, but animals may benefit even more from a crop that has a lower crude protein level to begin with, but better digestibility so they can utilize more of it. I'm planning on setting up a silvopasture system so have been reading up on this. I'd recommend Steve Gabriel's Silvopasture book and there are great academic journal papers on research gate and Google scholar. I have not come across any academic work suggesting that black locust is poisonous to small ruminants when supplied as a component in a mixed diet or free choice system. The high tannin levels sound like they could cause problems if you fed black locust exclusively.