A cattle guard assembly includes a cattle guard that is positionable in a depression in a roadway extending along a fence line. In this way the cattle guard inhibits livestock from crossing the fence line in the roadway. The cattle guard is comprised of a resiliently compressible material thereby resisting impact damage. Additionally, the cattle guard is comprised of a non-oxidizing material to resist corrosion. The cattle guard comprises a plurality of modular units thereby facilitating the cattle guard to have a selectable width for accommodating a variety of roadway widths. Each of the modular units is comprised of a molded, unitary structure such that said modular units resist breaking. Each of the modular units includes a base and a grate, and the grate is positionable in an existing cattle guard base for replacing an existing grate in an existing cattle guard.