The Collaborative International Dictionary of English v.0.48:

diestrum \diestrum\ n.
   same as diestrus.

   Syn: diestrus.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

diestrum
    n 1: (of animals having several estrous cycles in one breeding
         season) a state or interval of sexual inactivity or
         quiescence between periods of activity [syn: diestrus,
         diestrum]