RobertGeorge:
You need a long stay policy, with plenty of medical cover. The biggest cost in North America can be medical costs.
Use the moneysupermarket comparion check on competitive companies that offer this cover http://www.moneysupermarket.com/travelinsurance/
If you will be doing any work on the farm you should arrange the Insurance by phone as the companies will require details. The Insurers could charge an extra premium and there could be exclusions. Find out what they cover and the cost, but phone several to get the best cover for the premium you are willing to pay.
Huckster