Maybe this is just one of those “urban legends” roaming the ‘net?
For those of you who have never traveled to the west, or southwest,
cattle guards are horizontal steel rails placed at fence openings, in dug-out places in the roads adjacent to highways (sometimes across highways), to prevent cattle from crossing over that area. For some reason the cattle will not step on the "guards," probably because they fear getting their feet caught between the rails. (Read on under comments)