Not to change this thread to horses who are afraid of cows, but one more quick story. So, my younger horse is also part TB and when we first got him, he didn't seem to notice the cows until one day they were closer to the fence line. So, all of a sudden, he jumped sideways and acted terrified. So, I got off and walked him over to the fence to stand and look at the cows until he calmed down. All seemed fine.
So, another day, I went to go ride in the field again and we were riding along minding our own business and I was just thinking maybe the cow thing is "fixed" when.....those darn cows all turned and ran right up to the fence right at us. It was like a mini-stampede!!!! So, as you can guess - the problem with the cows is not fixed
If anything, that reinforced to him that cows really are scary.