REPRODUCTIVE RESPONSE AND LH-SECRETION IN EWES TREATED WITH MELATONIN IMPLANTS AND INDUCED TO OVULATE WITH THE RAM EFFECT

Two experiments were conducted to examine the effects of treating seasonally anoestrous ewes with melatonin before ram introduction on reproductive response, and on LH secretion in anoestrous ewes induced to ovulate by rams. In Experiment 1, a total of 667 ewes from three flocks involving Merino (Flock 1, N = 149), Merino entrefino (Flock 2, N = 325) and Rasa Aragonesa (Flock 3, N = 203) breeds were used. Within each flock, ewes isolated from rams since the previous lambing were assigned at random to receive melatonin implants of Regulin (75, 175 and 105 in Merino, Merino entrefino and Rasa Aragonesa flocks, respectively) or to serve as untreated controls (74 in Merino, 150 in Merino entrefino and 98 in Rasa Aragonesa Rocks). Fertile rams were introduced into all flocks 5 weeks after implantation in March (Flocks 1 and 2) or April (Flock 3), and remained with the ewes for a 50 day mating period. Percentage of ewes with luteal activity at ram introduction did not differ between melatonin treated and control ewes in any flock. There were no significant differences in either the mean interval from ram introduction to lambing or the distribution of lambing. Implantation with melatonin resulted in an improvement of prolificacy in all three flocks, although this only reached statistical significance in the Merino flock (1.15 vs. 1.03 in treated and control ewes, respectively, P < 0.05), Fertility was increased significantly (P < 0.05) in the Merino entrefino Rock (64.5% in treated vs. 51.3% in control ewes). In Experiment 2, two trials were undertaken utilizing a total of 63 ewes. Trial 1 involved 24 mature Manchega ewes and Trial 2 involved 39 Merino ewe lambs. Half of the animals in each trial received a Regulin implant on 28 February (Trial 1) or 12 March (Trial 2) and the remaining half acted as controls. Rams were introduced 5 weeks after implantation and remained with the ewes for a 25 day period, In both trials, anoestrous ewes at ram introduction were bled at 20 min intervals for 3 h before and 5 h after ram introduction and then at 3 h intervals over the next 24 h for assessment of plasma concentrations of LH. Secretion of LH before or following introduction of rams was not affected by melatonin. Both treated and control anoestrous ewes in each trial responded to introduction of rams with an increase in the frequency of the LH pulses (P < 0.05), but no significant changes were detected in pulse amplitude or mean levels of LH. A preovulatory surge of LH was detected between 8 and 26 h after ram introduction, but neither mean interval from ram introduction to the peak of LH surge, nor the magnitude of the LH peak, was influenced by melatonin treatment. Results from this study show that: (1) melatonin implants administered during early seasonal anoestrus have the potential to improve reproductive performance in Spanish breeds of sheep, but the response is conditioned by breed, management system and environmental factors; (2) melatonin did not modify the secretion of LH in anoestrous ewes induced to ovulate by the ram effect under our experimental conditions.