While I think probiotics have a place, I don't think they are necessary
to give EVERY calf. Nutrition is essential to good quality colostrum and
good colostrum provides immunity for the calf.
If you have to doctor a calf more than once, I sure
advise giving probiotic then. It puts the good bacteria back in the
gut. Over treatment of antibiotic kills that good bacteria and
can cause the calf to scour. Probiotics can prevent that from happening.
If you get a critter that bloats, I mean any age critter, when you finish doctoring it, be sure to give probiotic then.
Bloat upsets the rumen and probiotic supports good rumen health which is very important.
Hope this helps!