The Journal of Equine Veterinary Science recently published an article titled, "Post-thaw addition of caffeine and/or pentoxifylline affect differently motility of horse and donkey cryopreserved spermatozoa." The authors were Alessandra Rota; Chiara Sabatini; Anna Przyby; Alessia Ciaramelli; Duccio Panzani and Francesco Camillo, researchers and/or practitioners from Italy and Poland.
The summary noted, in part, that, "Caffeine and pentoxifylline added post-thaw were able to increase the proportion of motile spermatozoa only for stallions and not for donkeys. Whether the improvement in post-thaw motility of equine spermatozoa may have an effect on in vivo or in vitro pregnancy rates remains to be determined."
You can find more information information about this research on ScienceDirect.com.