Jamieson, Fausset and Brown on Deuteronomy 25:4

4. Thou shalt not muzzle the ox when he treadeth out the corn--In Judea, as in modern Syria and Egypt, the larger grains were beaten out by the feet of oxen, which, yoked together, day after day trod round the wide open spaces which form the threshing-floors. The animals were allowed freely to pick up a mouthful, when they chose to do so: a wise as well as humane regulation, introduced by the law of Moses (compare 1Co 9:9; 1Ti 5:17, 18).