Whether your dog is motivated by prey drive or herding instinct doesn't matter very much to the scared cows, and it doesn't matter much for your training goals either. Regardless of why your dog wants to chase cows, your training goal is to teach her to resist that impulse continuously unless asked for, until eventually she is desensitized to the cows.
If your dog is very aggressive when chasing cows, jumping on them or biting, or if she has injured livestock in the past, it may be wise to use a muzzle until training has progressed.
A good harness and long line are essential for all training techniques. If your dog is too powerful for you to easily control with a harness, a chest lead harness or a head halter may be necessary.