MELATONIN TREATMENT OF EMBRYO DONOR AND RECIPIENT EWES DURING ANESTRUS AFFECTS THEIR ENDOCRINE STATUS, BUT NOT OVULATION RATE, EMBRYO SURVIVAL OR PREGNANCY

Thirty-two Border Leicester x Scottish Blackface ewes that lambed in M arch were individually penned with their lambs from April 16th and giv en daily an oral dose of 3 mg melatonin at 1500 h (Group M). A further 32 acted as controls (Group C). Within each group half were used as e mbryo donors (Group D) following superovulation and half received embr yos (Group R) following an induced estrus. Prior to weaning on 21 May ewes received ad libitum a complete diet providing 9 megajoules (MJ) o f metabolizable energy and 125 g/kg crude protein. Thereafter each rec eived 1.6 kg of the diet daily. In early June each ewe received an int ravaginal device (300 mg progesterone) inserted for 12 d. Donors were superovulated with 4 im injections of porcine FSH 12 h apart, commenci ng 24 h before progesterone withdrawal. Ovulation in recipients was in duced with 800 IU PMSG injected im at progesterone removal. Donor ewes were inseminated 52 h after progesterone withdrawal. Embryos were col lected 4 d later and transferred to recipients. Melatonin suppressed p lasma prolactin (P<0.001) and advanced estrus (P<0.05) and timing of t he LH peak (P<0.05). These events also occurred earlier in donors than in recipients (P<0.01). Mean (+/-SEM) ovulation rates for melatonin-t reated and control donors were 5.5+/-0.71 and 4.7+/-0.66, respectively (NS). Corresponding recipient values were 3.3+/-0.40 and 3.4+/-0.39 ( NS). Mean (+/-SEM) embryo yields were 2.9+/-0.64 and 2.6+/-0.73 for me latonin-treated (n=15) and control (n=16) donors, respectively, and fo r the 12 ewes per treatment that supplied embryos, corresponding numbe rs classified as viable were 2.7+/-0.47 and 2.3+/-0.61 (NS). Following transfer, 57% of embryos developed to lambs when both donor and recip ient received melatonin, 86% when only the donor received melatonin, 9 1% when only the recipient received melatonin, and 67% when neither re ceived melatonin (NS). Thus, embryo survival following transfer was no t improved by treating recipients with melatonin. Gestation length and lamb birthweights were unaffected by melatonin. Unlike nonpregnant co ntrol ewes, melatonin-treated recipients that failed to remain pregnan t sustained estrous cyclicity following embryo transfer. (C) 1998 by E lsevier Science Inc.