I wouldn't use cattle grates for keeping horses in. Horses often try to jump it and break legs or panic when they step into it and then break legs struggling. I think cattle fare better as I remember helping to arrange a hoist for a few when I worked for a vet, (something had panicked them and they didn't stop in time). Besides, a cow KNOWS she can't jump, so her lack of depth perception will make her pause before the grate. The grate/horse entanglements I saw, however, always ended with an ugly death for the horse.