US patent number 3719004 A covers the original cattle guard.

The claim is for:
“Cattle guard apparatus adapted for installation either on lever ground or across a ditch or trench, comprising an open rectangular frame formed by two longitudinal end members and two longitudinal side members disposed in the same plane and connected at their ends. . . ”

And it goes on for another few hundred words.  That is a mouthful to describe such a simple concept.

Interestingly the patent refers to another patent for a Portable vehicle operated gate (patent 3516202 A) that was filed in 1968.  This patent seems to be almost identical to the standard cattle guard or gate except with a spring loaded middle.  It appears that idea was to raise the grates so the animal would be more fearful of crossing.