The effect of duckweed (DW) supplementation was evaluated on dry matter intake (DMI), presence and duration of estrus, percentage of ewes repeating estrus and pregnancy rate, as well as the concentration of progesterone ($P_4$) in multiparous crossbred ewes from Pelibuey, Dorper, and Katahdin breeds, fed with Taiwan grass hay (TWH). Eighteen ewes with $39.7{pm}4kg$ mean body weight, kept in individual pens, were randomly assigned to one of the following treatments: $T_1$: TWH, $T_2$: TWH plus 200 g DW, $T_3$: TWH plus 300 g DW. The ewes were synchronized with 40 mg fluorogestone acetate (FGA) and 400 UI equine chorionic gonadotropin (eCG). Data were analyzed as a completely randomized design using the GLM procedure. DW supplementation had no effect on dry matter intake (p>0.05); however, a slight decrease of TWH intake was observed as DW supplementation increased. No differences (p>0.05) were found in the beginning of estrus, percentage of ewes presenting it, its duration, or pregnancy rate. There were no differences (p>0.05) on $P_4$ concentration among treatments, or $treatment{ imes}period$ interaction (p>0.05). However the period was significant (p<0.01), since the $P_4$ levels increased as time increased after the removal of the FGA device and eCG application.