There are MANY different kinds (brands ) of dewormers - two types - one gets only internal parasites and the other gets internal AND external (including GRUBS).
Unless you know the history of your cattle, YOU SHOULD NEVER use a dewormer that gets GRUBS in the spring/summer. If your cattle had grubs migrating thru their body, you COULD KILL YOUR CATTLE. Timing depends on your specific area.
Here in NY, we cannot de-grub after mid Nov.
I have used Safeguard in spring/summer and Dectomax or Ivomectin products in fall.
Do not wait for SYMTOMS - preventative treatment is advised. We wait until cows have been on grass for 4 weeks before we do spring deworming. And fall deworm close to first of November. We deworm calves in summer 8 wks after on grass and again in fall.
There are injectables and pour-ons. Some pour-ons need dry weather for 6 hours, some injectables sting like the dickings!