Answer:. Maize is grown by the farmer.
Explanation:. Take the main verb from the active voice sentence and change it from present to past participle.
Add a form of the verb "to be" as the auxiliary verb.
"grows" becomes "is grown"
Switch the direct object of the original sentence to the subject: Maize is grown.
The subject of the original sentence can be switched to the end of the passive sentence by adding the preposition "by" and it becomes the object of the preposition:. Maize is grown by the farmer.