C. F. Keil and Franz Delitzsch on Deuteronomy 25:4

Deu 25:4

The command not to put a muzzle upon the ox when threshing, is no doubt proverbial in its nature, and even in the context before us is not intended to apply merely literally to an ox employed in threshing, but to be understood in the general sense in which the Apostle Paul uses it in 1Co 9:9 and 1Ti 5:18, viz., that a labourer was not to be deprived of his wages. As the mode of threshing presupposed here - namely, with oxen yoked together, and driven to and fro over the corn that had been strewn upon the floor, that they might kick out the grains with their hoofs - has been retained to the present day in the East, so has also the custom of leaving the animals employed in threshing without a muzzle (vid., Hoest, Marokos, p. 129; Wellst. Arabien, i. p. 194; Robinson, Pal. ii. pp. 206-7, iii. p. 6), although the Mosaic injunctions are not so strictly observed by the Christians as by the Mohammedans (Robinson, ii. p. 207).