ककुदस्यावस्थायां लोपः

Operational rule vidhi

The final letter of ककुद is dropped in a बहुव्रीहि समास when it denotes age or condition of animals.

Word by word (padaccheda)

Target ककुदस्य ṣaṣṭhī · ekavacanaRight context अवस्थायाम् saptamī · ekavacanaSubstitute / term prathamā · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

Dotted words are technical terms. Hover or tap one to see its card.

Full reading

ककुदस्य अवस्थायाम्

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

As a rewrite rule (sketch from case endings)

ककुदस्य → / अवस्थायाम्

Replace kakudasya with lopaḥ when avasthāyām follows. Inherited words (anuvṛtti) may add further conditions.

Commentary

The final letter of ककुद is dropped in a बहुव्रीहि समास when it denotes age or condition of animals.
The word अवस्था means the changes which things undergo by lapse of time, such as age &c. Thus असंजातं ककुदमस्य = असंजातककुत् = बालः "the young of an animal", पूर्णककुत् = मध्यमवया "middle-aged animal"; उन्नतककुत् = वृद्धावया "old". स्थूलककुत् = बलवान् 'strong', यष्टिककुत् = नातिस्थूलो नातिकृशः ॥

Why do we say when denoting a condition? Observe श्वेतककुदः ॥

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.