Bull is the emblem of the moral principle & Cow is the Representative of Earth.
The cow however is our Mother. Vedic philosophy teaches that there are seven mothers:
“We must have sufficient pasturing ground to feed the animals all round. We have to maintain the animals throughout their lives. We must not make any program for selling them to the slaughter houses. This is the way of cow protection. Krishna, by his practical example, taught us to give all protection to the cows, and that should be the main business..."
- Srila Prabhuapda