How much affected would a farmer get from a tornado?

It depends on the tornado.

A weak tornado of EF0 or EF1 strength might knock down a fence or two, damage or destroy a barn, or blow apart a few hay bales.

A stronger tornado of EF2 or EF3 strength could easily destroy a barn and badly damage or destroy a farm house. Livestock may be killed. At this point a farmer's life is in notable danger.

A violent tornado of EF4 or EF5 strength can completely destroy an entire farm. In some EF5 tornadoes, farms have essentially vanished, leaving the concrete foundations of buildings as the only evidence that they ever existed. In some cases tornadoes such as this have killed entire families.