It is moreish as it has load of molasses and is highly palatable for the obvious reason that you want them to eat it.
They say 1 bucket should do 25 ewes a week, so on that basis your usage is probably about right.
I would be more worried about over feeding. Depending on your breed (Downs sheep tend to be too fat and jacob types too thin). Best thing is to condition score your sheep, so check their meaty coverage and then feed concentrates as needed thereby controlling their intake of easy energy. The last thing you want it lambs too big to be born and ewes to fat to bother pushing! Been there, done that, got the tshirt!