What is the meaning of the verse which Tirumazhisai alwar reminded to the Brahmins?

Question

Tirumazhisai Azhwar is one of the twelve azhwars (or alwar), a group of Vaishnava saints who sang devotional poems on Lord Vishnu. Those poems are collectively known as Naalayira Divya Prabhandham.

Tirumazhisai Alwar was the son of Bhrigu but raised by a childless tribal couple. He proclaimed that he doesn't belong to any of the four varnas (Avarna) i.e., Brahmin, Vaishya, Kshatriya and Shudra. But there was a couple of incidents other people saw him by the caste he was raised in.

As a part of his journey, Alwar was travelling to Thirukudanthai Sarangapani temple, Kumbhakonam which is also known as Aravamudhar Sannidhi. Alwar stopped at a verndah and sat down. A group of brahmins were reciting Vedas. Because they saw a person who is not a dwija, they stopped reciting (There is a smriti injunction that Vedas should not be chanted in front of a non dwija). Alwar noticed it and moved very distant from them. Later, the brahmins wanted to resume the chanting where they stopped. But they couldn't remember it and clueless. Alwar realised their cluelessness and turned back. He took a black grain, broke it with his nail and dropped it down. By this action, they took the hint and continued the chanting.

kṛṣṇānāṃ vrīhīnām nakhanirbhinnaṃ nakhanirbhinnaṃ

कृष्णानां व्रीहीणां नखनिर्भिन्नं नखनिर्भिन्नं

At this point, those brahmins understood the alwar knew the Vedas but he doesn't open his mouth. They asked for his pardon.

What is the meaning and context of the above verse Alwar hinted to the Brahmins? Where is this mantra located?

Answer 1

Partial answer providing location of verse in scriptures and English translation for the same; Answer that provides interpretation explaining the meaning and significance/reason for sacrifice is awaited.

This is from Taittariya Samhita of Krishna Yajurveda:

1.8 प्रपाठक: 8 राजसूयः
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1.8.9 अनुवाक 9
रत्निनां हवींषि

VERSE: 1
बार्हस्पत्यं चरुं निर् वपति ब्रह्मणो गृहे शितिपृष्ठो दक्षिणा । ऐन्द्रम् एकादशकपालम्̇ राजन्यस्य गृह ऋषभो दक्षिणा । आदित्यं चरुम् महिष्यै गृहे धेनुर् दक्षिणा नैर्ऋतं चरुम् परिवृक्त्य्àइ गृहे कृष्णानां व्रीहीणां नखनिर्भिन्नं कृष्णा कूटाँ दक्षिणा । आग्नेयम् अष्टाकपालम्̇ सेनान्यो गृहे हिरण्यं दक्षिणा वारुणं दशकपालम्̇ सूतस्य गृहे महानिरष्टो दक्षिणा मारुतम्̇ सप्तकपालं ग्रामण्यो गृहे पृश्निर् दक्षिणा सावित्रं द्वादशकपालम्

Here is the English translation:

PRAPKTHAKA VIII
The Rajasuya
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8. The offerings to various deities

i. 8. 9.

To Brhaspati he offers an oblation in the house of the Brahman (priest); the sacrificial fee is a white-backed (animal). (He offers) to Indra on eleven potsherds in the house of a Rajanya; the sacrificial fee is a bull. To Aditya (he offers) an oblation in the house of the chief wife; the sacrificial fee is a cow. To Nirrti (he offers) an oblation in the house of the neglected wife, made up of rice broken by the nails; the sacrificial fee is a black hornless (cow). To Agni (he offers) on eight potsherds in the house of the leader of the host; the sacrificial fee is gold. To Varuna (he offers) on ten potsherds in the house of the minstrel; the sacrificial fee is a great castrated (ox). To the Maruts (he offers) on seven potsherds in the house of the village headman; the sacrificial fee is a dappled (cow). To Savitr (he offers) on twelve potsherds [1] in the house of the carver; the sacrificial fee is a speckled (ox).

Going through Sanskrit Dictionary,

• It's commonly known that Kṛṣṇā means black.

• Vrīhī means grain of rice.

• nakha is the sanskrit word for nails and nirbhinna means broken/divided or pierced; so here "nakhanirbhinnaṃ" mean broken (or torn off) by the nails.

Thus, the action of breaking black grain of rice by nail would remind the verse of 8th Parapathaka of 1st Kanda of Taittariya Samhita.

P.S 8th Prapathaka of 1st Khanda is about Rajasuya and 9th Anuvaka is about offerings to various deities.