Causes them to 'blow up' with gasses and die, when they start taking in more creep it's vital to cut the milk down and don't feed straights like barley because this will react too.
If you soak creep you will see it expands when wet, so milk and large quantities of creep spell disaster... I've just weaned 5 6 week old lambs, they are demolishing creep and didn't want them to bloat and die, it's a big disappointment when you've got them through the early stages to lose them fairly late on (OH's mum and sister have lost 2 now... they keep making the mistake of feeding barley mixed with the creep and don't wean until much later).