1 Samuel 6:7

Now therefore make a new cart
For there were no Levites, nor priests of the Lord to carry it upon their shoulders, as it was wont to be when carried, and therefore they ordered a cart to be made; and they might know the Levites were allowed wagons to carry some of their sacred things on, ( Numbers 7:1-8 ) and a new one for the honour of the ark, as David afterwards did, ( 2 Samuel 6:3 )

and take two milch kine, on which there hath come no yoke;
which also might be designed for the honour of the ark; but there was a further view in it, at least in the providence of God; since two such creatures, who had young, would be apt, if left to themselves, as these were, to return home to them, and not to proceed on a journey; and being unaccustomed to a yoke, would draw one way, and another another, in different ways; and not go on in a direct road, as such that are used to the yoke do:

and tie the kine to the cart;
in order to draw it:

and bring their calves home from them;
that they might not cry after them, which would cause them to turn back.