Hello,
Calf-Manna (from what I know) is a very rich feed (lots of protein). Therefore, you should lower the amount of feed you give them. Example; say you feed a 16% (protein) grain and you feed 1 lb of it (just an example). Then when you switch to Calf Manna (I don't know exact protein level, but I think it is higher than 16%) you want to feed a lower amount so that the goat gets about the same amount of protein. So if Calf Manna is 32% protein you should only feed a maximum of 1/2 lb of feed.
An overload of protein will surely upset their stomachs.
Hope this helps!
Thank you,
Patrick