Rashi on Deuteronomy 21:4/רש"י על דברים כ״א:ד׳

1אל נחל איתן UNTO A VALLEY WHICH IS איתן — which is hard; i.e. one that has never been tilled (Sifrei Devarim 207:2; Sotah 45b).

2וערפו AND THEY SHALL STRIKE OFF [THE HEIFER’S] NECK — i.e. one breaks its neck with a hatchet. The Holy One, blessed be He, says, as it were, Let a heifer which is only one year old and which therefore has brought forth no fruits (no offspring) have its neck broken at a spot (the untilled valley) which has not brought forth fruits, to expiate for the murder of him whom they did not permit further to beget children (Sotah 46a).