Katha-1-1-3-pītodakā

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

How he thought is explained; the epithet Pîtôdakâh and those which follow describe the cows to be given as rewards. Pítôdakâh, by which all water has been drunk; jagdhatrinâh, by which all grass has been eaten. Dugdhadôhâh, from which all milk has been milched. Nirindriâh (?), not capable of breeding. The meaning is—cows old and useless; giving such cows to the Ritviks for their rewards, the person performing the sacrifice attains those worlds which are joyless, i.e., devoid of happiness.

Links
Various resources on the Kathopanishad
Upanishad Series – Katopanishad Commentary by Swami Chinmayananda