I've had an unbred (20 LC left) stallion with multiple unbred mares age 4-6 in my pasture for about a week, and not a single one has been sneaky stud pasture bred. I used to get pasture breeding of at least 1-2 per day.

How old are they?

I have since bred them, but the mares were ages 4-5 and the stallion was 10. After 2 weeks there ended up being 1 pasture breeding, but this is still a much lower rate than it had previously been.