Albert Barnes Commentary Job 39:12

"Wilt thou confide in him, that he will bring home thy seed, And gather [the grain] of thy threshing-floor?" — Job 39:12 (ASV)

Will you believe him? - That is, will you trust him with the productions of the field? The idea is, that he was an untamed and unsubdued animal. He could not be governed, like the camel or the ox. If the sheaves of the harvest were laid on him, there would be no certainty that he would convey them where the farmer wished them.

And gather it into your barn? - Or, rather, “to your threshing-floor,” for so the word used here (גרן gôren) means. It was not common to gather a harvest into a barn, but it was usually collected on a hard-trod place and there threshed and winnowed. For the use of the word, see Ruth 3:2; Judges 6:37; Numbers 18:30; Isaiah 21:10.