You could start with the article by Lang et al on cup feeding.  It showed
that premature babies cup fed were more likely to be still nursing at
discharge from the NICU, and more likely to breastfeed longer.

Lang S, Lawrence CJ, L'E Orme R. Cup feeding: an  alternative method of
infant feeding. Arch Dis Child 1994;71:365-69

Jack Newman, MD, FRCPC