Albert Barnes Commentary Deuteronomy 25:4

"Thou shalt not muzzle the ox when he treadeth out [the grain]." — Deuteronomy 25:4 (ASV)

Compare the marginal references. In other kinds of labor the oxen were usually muzzled. When driven to and fro over the threshing-floor in order to stamp out the grain from the chaff, they were to be allowed to partake of the fruits of their labors.