Shatapatha Brahmana 4.5.1.9

4.5.1.9

atha yadāsāḥ pāṃsavaḥ paryaśiṣyanta | tato gardabhaḥ samabhavattasmādyatra pāṃsulam bhavati gardabhasthānamiva batetyāhuratha yadā na kaścana rasaḥ paryaśiṣyata tata eṣā maitrāvaruṇī vaśā samabhavattasmādeṣā na prajāyate rasāddhi retaḥ sambhavati retasaḥ paśavastadyadantataḥ samabhavattasmādantaṃ yajñasyānuvartate tasmādvā eṣātra maitrāvaruṇī vaśāvakLptatamā bhavati yadi vaśāṃ na vindedapyukṣavaśa eva syāt

Then the dust of the ashes which remained: therefrom the ass was produced,--hence when it is dusty anywhere, people say, 'A very place for asses, forsooth!' And when no sap whatever remained,--thence was produced that barren cow belonging to Mitra and Varuna; wherefore that (cow) does not bring forth, for from sap seed is produced, and from seed cattle. And because she was produced at the end, therefore she comes after the end of the sacrifice. Hence also a barren cow for Mitra and Varuna is the most proper here: if he cannot obtain a barren cow, it may also be a bullock.