Shatapatha Brahmana 4.5.5.10

4.5.5.10

atha yadgāvaḥ | bhūyiṣṭhāni pātrāṇyanu prajāyante tasmādetāḥ sakṛtsaṃvatsarasya vijāyamānā ekaikaṃ janayantyo bhūyiṣṭhā bhūyiṣṭhāni hi pātrāṇyanu prajāyante

And because cows are produced after the most cups, therefore, though bringing forth once in a year, and producing one each time, (they are) most numerous, for they are produced after the most cups.