Good Morning and welcome to Just answer. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
Since the calves are not running a fever, this is probably a viral respiratory infection that is not responding to penicillin. This will suppress their immune system allowing bacterial invaders to come in making this even worse. In this case, I would use a much stronger and more broad spectrum antibiotic such as Naxcel or Baytril and I would also give them a cough suppressant if at all possible to help them through this condition. If they are becoming dehydrated, supplement them with an electrolyte solution daily that can be purchased at the farm supply.
could it be a sammonella infection? I purchased three animals at auction, put with 4 others. about a week later all started to cough. then became ill.