Don't neglect the vaccination and dosing basics
It might be worth considering a non-milk withhold pour-on now for milkers, especially first calved heifers. Research has shown that treated cows will maintain their yields better in the face of a strong worm burden or challenge at pasture.
Don't forget to take care of the basics - ie clostridial/ blackleg vaccination and worming protocols.
With regard to worming young calves on their first grazing season, don't leave it too long, especially with dairy calves.
By treating inside the first month, you not only protect the calves but also reduce pasture contamination with eggs.
If for whatever reason it's put on the long finger and the calves are coughing a lot - especially when being herded -these calves may be clinically affected with hoose (get your vet to examine them as it may be viral pneumonia).
I find dosing with a levamisole-based product may be a gentler safer way to worm these calves and will cause less anaphylaxis and shock by means of a more gradual kill of the hoose. Bear in mind it won't last as long as ivermectin-based products giving less than three weeks protection.