Effects of exogenous melatonin on in vivo embryo viability and oocyte competence of undernourished ewes after weaning during the seasonal anestrus M.I. Vázquez a, *, J.A. Abecia a , F. Forcada a , A. Casao a a Departamento de Producción Animal y Ciencia de los Alimentos, Facultad de Veterinaria, Universidad de Zaragoza, Miguel Servet 177, Zaragoza, Spain (C.P. 50013) Abstract This study investigated the effects of exogenous melatonin on embryo viability and oocyte competence in post-partum undernourished ewes during the seasonal anestrus. At parturition (mid-Feb), 36 adult Rasa Aragonesa ewes were assigned to one of two groups: treated (+MEL) or not treated (-MEL) with a subcutaneous implant of melatonin (Melovine®, CEVA) on the day of lambing. After 45 d of suckling, lambs were weaned, ewes were synchronized using intravaginal pessaries, and fed to provide 1.5 (Control, C) or 0.5 (Low, L) times daily maintenance requirements. Thus, ewes were divided into four groups: C-MEL, C+MEL, L-MEL, and L+MEL. At estrus (Day=0), ewes were mated. At Day 5 after estrus, embryos were recovered by mid-ventral laparotomy and classified based on their developmental stage and morphology. After embryo collection, ovaries were recovered and oocytes were classified and selected for use in in vitro fertilization (IVF). Neither diet nor melatonin treatment had a significant effect on ovulation rate and on the number of ova recovered per ewe. Melatonin treatment significantly improved the number of fertilized embryos/corpus luteum (CL) (-MEL: 0.35  0.1, +MEL: 0.62  0.1; P = 0.08), number of viable embryos/CL (-MEL: 0.23  0.1, +MEL: 0.62  0.1; P  0.01), viability rate (-MEL: 46.6%, +MEL: 83.9%; P  0.05), and pregnancy rate (-MEL: 26.3%, +MEL: 76.5%; P  0.05). In particular, exogenous melatonin improved embryo viability in undernourished ewes (L-MEL: 40%, L+MEL: 100%, P  0.01). Neither nutrition nor exogenous melatonin treatments significantly influenced the competence of oocytes during IVF. Treatment groups did not differ significantly in the number of healthy oocytes used for IVF, number of cleaved embryos, or number of blastocysts and, consequently, the groups had similar cleavage and blastocyst rates. In conclusion, melatonin treatments improved ovine embryo viability during anestrus, particularly in undernourished post-partum ewes, although the effects of melatonin did not appear to be mediated at the oocyte competence level. © 2010 Elsevier Inc. All rights reserved. Keywords: Embryo; Oocyte competence; Melatonin; Undernutrition; Post-partum ewes; Anestrus 1. Introduction As in other livestock, sheep production is strongly conditioned by embryo growth and development, which is an important factor that reflects productive success. Typically, embryo production in vivo is used to increase the number of lambs produced by individual females, which often is combined with multiple ovula- tion and embryo transfer (MOET) programs. Embryo production in vitro is an alternative method that pro- vides an inexpensive, abundant source of oocytes (col- * Corresponding Author. Tel.: +34-976-761000 (ext. 4150). E-mail address: isavazq@unizar.es (M.I. Vázquez). Available online at www.sciencedirect.com Theriogenology 74 (2010) 618 – 626 www.theriojournal.com 0093-691X/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.theriogenology.2010.03.007