The Collaborative International Dictionary of English v.0.48:

Redtop \Red"top`\ (-t?p`), n. (Bot.)
   A kind of grass (Agrostis vulgaris) highly valued in the
   United States for pasturage and hay for cattle; -- called
   also English grass, and in some localities herd's grass.
   See Illustration in Appendix. The tall redtop is Triodia
   seslerioides.
   [1913 Webster]