As I discuss in this question, one of the early movements that was important in the development of Vaishnavism was the ancient Pancharatra movement, whose sacred texts consisted of detailed procedures to worship the sage Narayana, an ancient incarnation of Vishnu who was the son of Yama god of death and twin brother of the sage Nara. Among the oldest Pancharatra texts are the Satvata Samhita, Paushkara Samhita, and Jayakhya Samhita. But there's another Pancharatra text even older than those, and it is found within the Shanti Parva of the Mahabharata! It's called the Narayaniya, and it's an 18-chapter religious dialogue between Yudishthira and Bhishma on the importance of worshipping Narayana/Vishnu, similar to the Bhagavad Gita, another 18-chapter religious discourse between Krishna and Arjuna found in the Bhishma Parva of the Mahabharata.
In any case, this chapter of the Narayaniya describes a dispute that a bunch of sages once had with the gods, over the meaning of the word "Aja". One of the Yagnas (fire-rituals) described in the Vedas is called the Ajamedha Yagna, in which you're supposed to sacrifice "Ajas". But the Sanskrit word "Aja" can either mean goat or vegetable seed, so there was an argument about which one you had to sacrifice; the gods said it meant goats and the sages said it meant vegetable seeds. The king Uparichara Vasu came by and said that the gods were right, so the sages cursed him for incorrectly siding with the gods.
The gods, once on a time, addressing many foremost of Brahmanas, said unto them that sacrifices should be performed by offering up Ajas as victims. By the word Aja should be understood the goat and no other animal.' The Rishis said, 'The Vedic Sruti declares that in sacrifices the offerings should consist of (vegetable) seeds. Seeds are called Ajas. It behoveth you not to slay goats. Ye deities, that cannot be the religion of good and righteous people in which slaughter of animals is laid down. This, again, is the Krita age. How can animals be slaughtered in this epoch of righteousness?' ... Learning what the opinion was that was entertained by the deities, Vasu, moved by partiality for them, said that sacrifices should be performed with animals. At this answer, all the Rishis, endued with the splendour of the Sun, became very angry. Addressing Vasu who was seated on his car and who had (wrongly) taken up the side of the deities, they said unto him,--Since thou hast (wrongly) taken up the side of the deities, do thou fall down from heaven.
The clear implication from the text is that the sages were right after all, and that Ajas does indeed refer to vegetable seeds.
But my question is about the sages' statement that "[t]he Vedic Sruti declares that in sacrifices the offerings should consist of (vegetable) seeds." Where in the Vedas is the statement that the offerings should be vegetable seeds rather than goats? And first of all, where in the Vedas is the description of Ajamedha Yagna itself? That might be the best place to look.