Weather its injectable , pour on or in feed.  If you use a lower than
recomended dose, will it be inefective and therefore help build
an immunity in the worms and bugs your trying to control?

Thinking along the lines of cutting a rate on Roundup in crops.
Now we have roundup resistant weeds.

Asked our vet, all he said, you need more cows, you have to much time
on your hands. lol :)   
BTW he couldn't (or wouldn't) answer my question.