In ancient times, one common method of threshing grain was to let a team of oxen (or horses) walk over it. As the animals stepped on the grain with their hooves, the grain kernels gradually became separated from the straw.
The law cited here is found in Deuteronomy 25:4. Its obvious aim was to ensure that an animal could eat of the grain that it was busy treading out. Paul does not deny this obvious application, yet he points beyond it to a greater, ultimate application. If God has determined that even an ox should eat of the fruit of its labour, then surely he means for humans to do the same.
Paul’s interpretation is confirmed by the context of this law in the book of Deuteronomy. It is set amid a series of laws that concern the provision for, and the protection of, human life. In his first letter to Timothy, Paul again cites this law with the same application in mind (1 Timothy 5:17–18).
9 For it is written in the Law of Moses, “You shall not muzzle an ox when it treads out the grain.” Is it for oxen that God is concerned?