The affix mayaṭ comes after the word go in the sense of 'its dung'.,
Thus गोमयम् 'cow dung'. 'Dung' is neither a modification nor a part of cow.
Therefore this separate sutra for गो ॥ Why do we say meaning 'its dung'? Observe गव्यं = milk. The affix यत् here comes in the sense of product or part. 4.3.160, though strictly speaking 'milk' also is not an 'avayava' or a 'vikara' of गो ॥,