The effect of melatonin implants during the seasonal anestrus on embryo production after superovulation in aged high-prolificacy Rasa Aragonesa ewes

The aim of this study was to assess the effect of melatonin implants administered in March on the ovarian cyclicity, ovulatory response and embryo production after repeated superovulation of selectedhigh-prolificacyRasa Aragonesaagedewes.During theseasonalanestrusof twoconsecutive years, 113 superovulatory treatments have been performed. Ewes were treated (M) or not (C) with melatonin implants in March (day 0). All of them received intravaginal progestogen sponges on day 24 (recovery 1) and 80 (recovery 2) after melatonin implants insertion in year 1, and on day 28 and 77 in year 2. The intravaginal sponges were removed after 14 days. Superovulatory treatments consisted of eight doses in decreasing concentrations (2 mL � 2 and 1 mL � 6) of oFSH (Ovagen TM ) administered twice daily starting 72 h before sponge removal. Seven days after the onset of estrus, embryoswererecoveredbylaparotomy.Melatoninincreasedcyclicityonlyinrecovery2year2(83% versus 42%; P < 0.05) but not in the other experimental periods. Among the 78% (88) ewes that ovulated and produced functional corpora lutea, melatonin implants tended to improve embryo viability in recovery 2 by increasing the number of blastocysts per superovulatory treatment (2.4 � 0.6 versus 1.1 � 0.4; P = 0.09), the rate of viability (67 � 9% versus 43 � 9%; P < 0.05), and freezability (55 � 9% versus 33 � 8%; P < 0.05). More specifically, melatonin induced a significant reduction of the number and rate of non-viable (degenerate and retarded) embryos in