While you might use "he" in a casual way, it's not technically correct. If you need to be specific about a cow's gender, you can use:
* Heifer: A young female cow that has not yet given birth.
* Cow: A female cow that has given birth.
* Bull: A male cow.
* Steer: A male cow that has been castrated.
So, you can refer to a cow as "she" if you are certain it's a female.