In the following para, Carvaka derides the Agnihotra as merely a waste of money and useful only to keep the brahmins employed. It goes on to explain how the vedas are full of untruth, self contradictions and tautulogies.
I was shocked myselves initially about what all was involved in Aswamedha. Its a ceremony performed by a king for expanding territory, beget sons and for the well being of his country in general. When its performed to beget sons, a horse is slained and the chief queen is supposed to then "unite" with the dead horse. Apparently, this is what happens in the Bala Kanda of Ramayana when King Dasharatha performs the Aswamedha to beget sons. Chief Queen Kausalya is supposed to have "united" with the horse after the sacrifice. Valmiki's Ramayana mentions this as follows:
Kausalya walked reverently all around the horse and then with the greatest joy cut it with three knives. Her mind unwavering in her desire for righteousness, Kausalya passed one night with the horse. The priests-- the hotri, the adhvaryu and the udgatri-- saw to it that the second and the junior most of the king's wives, as well as his chief queen, were united with the horse. Then the officiating priest, who was extremely adept and held his senses in check, removed the fat of the horse and cooked it in the manner prescribed in the ritual texts.
Alternate Source: www.valmikiramayan.net . Go to verse 33 on the link provided.
B. R. Ambedkar, in his essay titled "Riddles in Hinduism", has also used this practise (among many other things) to criticise Vedas.