I've always avoided bakery waste for the Kunekunes Sylvia, because they are so prone to putting on weight and the way I look at it, bread and buns are the equivalent of junk food for us (they love it, but it's not that good for them

)
Because mine only get 1lb of hard food each per day (plus fruit & veg, grass/hay) I think the most I'd be tempted to replace would be about a quarter of the hard feed, not sure what the ratio is for bread to pig nuts

Maybe 2 to 1 ? Just keep a close eye on them incase they start getting 'porky'

I wouldn't feed it to the weaners though - keep them on their full ration of pig nuts - they need it for minerals etc to ensure good growth and health.
Hope that helps a wee bit ?
Karen x