Shatapatha Brahmana 14.1.3.11

14.1.3.11

bālvajībhī rajjubhirvyutā bhavati yajñasya śīrṣacinnasya raso vyakṣarattata etā oṣadhayo jajñire tenaivainametadrasena samardhayati kṛtsnam karoti

It is wound all over with cords of Balvaga grass (Eleusine indica). When the sacrifice had its head cut off, its vital sap flowed out, and thence these plants grew up: with that life-sap he thus supplies and completes it.
12 And as to why he places it north (of Soma's seat),--Soma is the sacrifice, and the Pravargya is its head; but the head is higher (uttara): therefore he places it north (uttara) of it. Moreover, Soma is king, and the Pravargya is emperor, and the imperial dignity is higher than the royal: therefore he places it north of it.