For each crop, to maximize yield is to maximize profit. In fact, those are usually the same thing. Unless some requirement must be met, like moisture, protein or sugar content (i.e. corn or barley), in which case a given yield can mean different profits.

Farmers have poor leverage on produce retail prices, and depending on the soil and climate they have, the local markets and distributors, a limited range of crops is available. Once chosen, they simply maximize yield.

As for permanent crops, farmers simply maximize yield.