Why don't cattle like walking over cattle grids?

It's more of an optical illusion for livestock than a physical barrier, as they see these gaps in the ground that looks to them like a bottomless pit that is simply covered by these "thin" iron bars. However it is no longer an optical illusion if a careless cow that attempts to cross gets her foot stuck and ends up breaking a leg if she struggles too hard. The bars are smooth cylinders that make it more difficult for hooves to gain traction, making it even more likely for the cattle to fall through if they test the gate. Since cattle are very good at remembering bad experiences and transferring this to other herd members, they will refuse to cross once and for all.

They're called by different names in different parts of the world. Cattle grid, cattle guard, cattle gate, cattle stop, stock grid, vehicle pass or Texas gates.