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The objective was to determine the effect of GnRH as an aid to synchronise the time of ovulation in Merino ewes during the non-breeding and breeding seasons as determined by transrectal ultrasound. Oestrus was synchronized in 20 nulliparous Merino ewes (11–12 months old; 2 replicates of 10 animals) during spring 2003 and autumn 2004 at Camden, NSW, Australia, using FGA sponges for 12 days (30 mg Ovagest, Bioniche Pty. Ltd., Armidale, NSW) and an i.m. injection of 400 IU of PMSG (Pregnecol, Bioniche Pty. Ltd., Armidale, NSW). Ultrasound evaluations of ovaries were recorded on VHS tapes every 12 h for 36 h starting at sponge removal (SR), then half of the animals received an i.m. injection of 40 μg synthetic GnRH (Fertagyl, Intervet Australia Pty. Ltd, Bendigo, VIC) and ultrasound evaluations were conducted every 6 h until 60 h. The positions of the largest follicles were recorded on ovarian maps and their growth was monitored. Time of ovulation was defined as the time of disappearance of the largest follicle from the ovary. Ten days after ovulation, the position and diameter of the CL was confirmed by ultrasound. Comparisons were made between treated and control animals, and between breeding and non-breeding seasons, using t-tests. During the non-breeding season ovulation took place from 42 to 54 h (mean 48 ± 2.83 h) vs. 42 to 60 h (mean 52.2 ± 5.69 h) after SR in GnRH-treated vs. control animals (P &lt; 0.05), respectively. Ovulation was delayed in the breeding compared with the non-breeding season (P &lt; 0.05), starting from 48 to 60 h after SR for treated (52.8 ± 3.79 h) and control animals (57.0 ± 4.24 h; P &lt; 0.05). These results suggest that GnRH synchronized the time of ovulation compared with the controls but the time of ovulation was later in the breeding than in the non-breeding season.

338 SYNCHRONISATION OF OVULATION IN MERINO EWES WITH GnRH IN THE BREEDING AND NON-BREEDING SEASON