I am not suggestion doing this but have to share...
My friend was attending a clinic for cutting. Her QH was terrified of the cows! Blowing hot air and all. So the clinician told her to get off and he took the horse and turned her out in a pen with some cows. She ran around for about 2 minutes, stopped, took a breathe and was fine ever since with cows. My friend got back on her horse and continued with the clinic like nothing happened.
I also knew a horse that would be scared of paints/pintos because they looked like cows. He was a TB by the way.
Cows can look scary especially when the are all grouped together in one big mass. If your horse is not familar with their smell they can be intimidating. I would take your time and not push your horse.