According to the opinion of sphoṭāyana , there is the substitution of avaṅ for the o of go when it is followed by any vowel.,
The anuvritti of अति ceases, that of आर्च manifests itself. Thus गो + अग्र = गवाग्रम्, so also गवाजिनम्, गवौदनम्, गवोष्ट्रम्, or in the alternative we have गोऽग्रम्, गोऽजिनम्, गवोदनम्, गवुष्ट्रम् ॥ The substitute अवङ् is accutely accented on the first syllable. This accent will be the original accent in the Bahuvrihi compound 6.2.1. Thus गावा अग्रमस्य = गवाग्रः ॥ In other places, however, this accent will be superseded by the samasanta Udatta accent 6.1.223 The mention of Sphotayana is for the sake of respect, for the anuvritti of विभाषा was already understood in it. This is a vyavasthita vibhasha, hence in गवाक्षः the अवङ् substitution is compulsory and not optional.,