We are polluted with Lone Star ticks in my area; see a few wood ticks but not many considering how many deer we have.
That is why I start my horses on processed powdered garlic every March and keep them on it until mid-November (I'm in Middle Tennessee with more warm months than the OP).
One horse does NOT get garlic because he has ulcer issues; I wouldn't recommend feeding it to horses that colic easy either.
Zonk fly spray seems to work pretty good for ticks but I have to spray the legs and bottoms of their tails every day. I spray the tail bottoms really heavy.
I have heard that putting powdered sulphur in a sock and whapping the sock against their legs works very well but I've never tried it.
That being said, MTG is sulphur based and could probably be lightly rubbed on their legs. I can't use it because it is so hot and humid here I am afraid it will burn their legs.
Thankfully three of my dogs are limited to a huge backyard, which Mr. WTW liberally sprinkled with Sevin granules.
My barn cats stay pretty much tick-free because I lightly rub Sevin Powder along their necks, toplines, and butt. It might help your dogs if they are patrolling your entire 40 acres
Hope this helps, I sure do understand your anguish.