A mother can feed her baby after intercourse without ghusl?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

There is no harm for a woman who is in a state of greater impurity to breastfeed her child.

A person in a state of major impurity is like normal people in all matters except in matters which the law maker prohibits him/her to do, like praying, making Tawaf around the Kaaba, touching The Qur'an etc..

Breastfeeding is definitely not amongst the above matters, because if there is any harm for a woman in the state of greater impurity to breastfeed, then the law-Maker, The All-Wise, would have certainly clarified it, as He clarified to us all matters that we need in this world and in the Hereafter.

Allah knows best.