THE EFFECT OF STAGE OF BREEDING SEASON AND ADMINISTRATION OF FERTIRELIN ACETATE (GNRH ANALOGUE) POST-MATING ON OVARIAN ACTIVITY AND FERTILITY IN EWE LAMBS

A study was conducted to determine the effects of stage of breeding season and treatment with GnRH analogue (Fertierlin acetate) on day 6; day 12 or days 6 &12 post-mating on ovarian activity and reproductive performance of ewe lambs. A total of 152 Ossimi ewe lambs were involved in this study [ 76 animals for first experiment (at the beginning of breeding season) and 76 animals for second experiment (at mid breeding season)]. In each experiment, following mating the ewe lambs were assigned to saline treated group (n = 20; 19 animals) or GnRH treated groups (I: n= 18, 19; II: n = 19, 19 and III: n= 19, 19 animals, respectively). Blood samples were collected at days 8; 12 and 14 post mating for hormonal assay. The non return rate and the lambing rate were recorded for ewe lambs. The result of this study indicated that irrespective of the reproductive state, the mean serum concentration of progesterone and estradiol, as well as the non return rate tend to be higher during mid- than during early breeding season. Furthermore, the breeding season has a significant (P<0.05) effect on lambing rate which was lower (35%) during early than (57.9%) during mid-breeding season. Fertirelin acetate treatment has a significant (P<0.05) effect on overall mean concentration of progesterone and estradiol at day 14 post-mating. The lambing rate of ewe lambs treated with fertirelin acetate at both days
6 &12 (group III) either during early or mid stage of breeding season were significantly (P<0.05) higher (63.1% and 78.9%) than those treated with saline solution (35% and 57.9%), respectively. Moreover, fertirelin acetate treatment at day 12 (group II) or at both days 6 &12 (group III) significantly (P<0.05) decrease the overall rates of non return and non pregnant ewe lambs (11.5% and 6.9% compared with saline treated group (40.1%), receptively. From the results of this study, it is concluded that: stage of breeding season affects both ovarian activity and lambing rate of Ossimi ewe lambs. Treatment with fertirelin acetate at days 6&12 post-mating improve lambing rate of ewe lambs and the mechanism by which treatment regime improve fertility was discussed. There was no effect when GnRH was given at day 6 or day 12 post-mating on lambing rate in Ossimi ewe lambs.