Mowing your pasture regularly is definitely a great way to reduce tick abundance. If you or someone you know is experienced with performing prescribed burns, you might also consider doing those annually, but definitely something you need experience with before just trying on your own! I am not aware of anything you can put on the horses to prevent the ticks from biting. However, you should definitely perform daily tick checks if possible and propery remove any ticks you find (grasp as close to the skin as possible and pull straight out---some fine-tipped tweezers might help and be worth it with the number of ticks you are finding!)---if you can do that then you should be able to prevent most tick-borne pathogens from being transmitted to your horse as most require 24-48 hours of tick attachment to be transmitted. After removing a tick, you may wipe the site of attachment down with some type of antibacterial scrub---betadine, hydrogen peroxide, etc. It is a very small wound essentially so can get infected, etc. The yellow stuff you mentioned is probably nothing to worry about, but does probably indicate that it could use a rub down with something like this.