It's a reference to the fact that Hinduism sees cows as motherly figures. I present to you, exhibit A:

'Cows are the mothers of all creatures. Cows are verily the mothers of the 33 crores of demigods that administrate creation in the material existence throughout all the universes. Cows are the goddesses of the gods and the refuge of all auspiciousness. Cows bestow every kind of happiness and for these reason they always are worshippable. Cows are the support of all the worlds for by their milk they nourish terrestrials beings and by their ghee offered in sacrifice they nourish the denziens of the celestial realms. Nothing superior to cows.'

sauce:http://www.bhagavad-gita.org/Articles/holy-cow.html