Your coverage for rented, borrowed, and occasional use, alongside your non-auto general liability and umbrella / excess liability coverages, should take care of your needs (assuming your farm is set up as a business & you are electing these pretty common products). You may also talk to your agent about off-road classification w/ on-road mileage limitations (travel between pastures, hauling harvest to nearby depot, etc.). I would ensure I was working with a seasoned business insurer/agent w/ extensive rural/ag/business diversity experience.
