Anti-Cavalry Barrier....

[Robert Vermaat]

a horse will do as it is trained, and people who don't believe that, have never worked training horses to jump through fire, haze cattle or jump blind obstacles. An untrained horse won't do stuff a trained animal will do, just like the difference between a civilian, a recruit and a seasoned, trained soldier.

Great point.
Which is what I wanted to say - you can train a horse to do the 'unnatural thing' (whereas a bull will attack head-on), but it's by no means easy (as Alex implied). And you certainly don't sqander such a valuable horse by driving it unto it's death for the off-chance that a line might break.
Which, I add again, I've not read was done in battle.