The affix kap comes after the verb duh 'to milk' when in composition with a word ending in a case-affix and the letter gha replaces the final.,
As कामदुघा धेनुः 'a milch cow, giving abundance of milk'; so also अर्थदुघा, धर्मदुघा ।,
दुहः कब् घश् च
Adhyāya 3 · Pāda 2 · Rule 70