Unfortunately, we often get some misinformation from vets and physicians when it comes to the blacklegged tick and Lyme disease! First, the blacklegged tick is primarily a woodland habitat tick. It does not survive well in open fields where you likely have the hay bales. The blacklegged tick larvae (sometimes called seed tick stage) feed primarily on small rodents (mice, voles, rats, chipmunks, etc.) which are animals that commonly harbor the Lyme disease and serve as the initial place where the ticks pick up the disease. After feeding the larval ticks drop to the ground and molt into nymphs. These nymphs prefer larger animals, but they may also feed on small rodents. If the larva had picked up the Lyme disease, the nymph can transmit it. If the nymph doesn't have Lyme disease, it may pick it up if it feeds on an infected animal. The engorged nymph drops off the animal host, molts again and is now an adult. If the larva of the nymph stage got infected, the adult will be able to transmit the disease. The adult ticks prefer larger animals (deer, livestock, dogs, people, etc.).

The blacklegged tick takes two years to undergo this life cycle with the adults being formed in late fall. The nymphs and adults of this tick can be active anytime that temperatures get into the 50s and 60s degrees F. We have noted folks picking up this tick during hunting season in November and December as well as hikers that walk in woodland sites in January through March!

It sounds like you may have an ongoing population of the blacklegged tick and you'll have to do an all-out program of tick management! This means trying to reduce the small rodent population, treating all susceptible animals on a regular basis (dogs, livestock), and discouraging skunks, raccoons, opossum and deer from being active on your farm.  You may even have to treat pastures (usually only the edge 15-20 feet that border a woodlot) with something like carbaryl (Sevin) or bifenthrin (Talstar) which are very good at knocking down tick larvae, nymphs and adults. 

Most state departments of health have web pages on the blacklegged tick and Lyme disease. I note this one for Minnesota:

http://www.health.state.mn.us/divs/idepc/dtopics/tickborne/ticks.html