Question: query regarding use of anthalmintic for a calf 
Answer:  it is advisile to use anthalmintic at the age of 4 month  for the calf - since rumen development start & complete at the age of 4 month after calving the organogenesis [ fore stomah ] is going on after birth , , the developing cells of stomach are very delicate and use of such drug before complete formation of stomach may effect the epithelial layer[ mucosal layer ] of stomach , hence digestion maaaaa impaired in future resulting decreased digestion & absorption& similarly effect the production [  milk in lactating cows & in male poor growth ] female , i.e, if digestion is impaired  the production will also impaired in future , but it is not rigid for all the cases -- when parasitic load is thought to be maximum in calf -feed additive --containing antibiotic  & anthalmintic agent [ minimal dose ] can be given to the calf upto 4 month of age to inhabit the growth of parasite