The affix khañ comes in the sense of 'a place of growing when that place is a field' after the name of any particular corn being in the Genitive 6th-Case.,
According to Bohtlingk, the Sutra should be thus translated. \To a corn-name is added ईन, to express the fields in which the corn grows\. According to Ballantyne, it is translated \when we speak of a place for grain, or a field of it, there may be the affix khan\.
The word धान्यानां 'of grains', shows that the word in construction should be in the genitive case. The plural number shows that the affix is not to be added to the word-form धान्य, but to words which denote various kinds of dhanya 1.1.63. भवन means 'the place where a thing is produced or grows (भवन्ति जायन्ते)' ॥
Thus मुद्गानां भवनं क्षेत्रं = मौद्गीनम्, कौद्रवीणम्, कौलत्थीनम् ॥
Why do we say \of grains\? There will be no affixing in a case like this तृणानां भवनं क्षेत्रं ॥
Why do we say \when it is a field\? Observe मुद्गानां भवनं कुशूलं 'a granary where kidney-beans grow'.,