I groom my cows daily. I make useful observations as a result of that practice. Ivermectin will control ticks, I am very confident of that statement. If you use the pour-on, a minimum of a "double the recommended dose" is required. You will start seeing ticks again in about 6 weeks. Ocassionally, I see ticks survive in the face of a treatment but out of say 15 head, I will only find maybe one tick on all the cows in a month. (The ticks I usually see are the larger wood ticks, I would probably not see the smaller deer tick or the larval form of any of the ticks).
If you use the injectable treatments of Ivermectin like Ivomec, then the recommended does will do the job. Again, the period it is most effective is the first 6 weeks following application. These are personal observations, I have no documentation.
I started seeing a few ticks on my cows about 6 weeks ago. I was surprised the harsh winter did not delay them coming out. At about that time, I treated all my cattle with Ivomec. I have seen a total of about 3 ticks on my cows in the last 3 weeks.
I get the same type of response on my blue heeler. I only use the topical application in treating him but I double the dose. In about 6 weeks, I start seeing a few ticks on him.
I don't know if the double dose is safe so administering Ivermectin as I do may not be safe.
I use the injectable once a year. I use the pour-on for making follow-up treatments. I have used a combination of parasiticides as often as 4 times in 12 months. This is a combination of injectables, pour-ons and drench for only round worms and flat worms. I am confident, I have the most parasite-free cows in my county.
PS: Controlling ticks is very important in the south. The deer tick is the prime vector of anaplasmosis. If I am not seeing the wood tick as a result of my practice, I am assuming I am also killing the deer tick.
PS2: Applying the pour-on along the backbone more than once a year has caused me some concern. I have noticed that a couple of cows had a crusty build-up of either skin or debris along that line. I don't know if anyone else has seen that. It may just be the debris like dirt and skin accumulating in the medium the Ivermectin is in.