الفهرس 
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Abstract
This study was carried out at Sakha Research Station, Kafr El-Sheikh Governotate, belonging to Animal Production Research Institute, Ministry of Agriculture, in cooperation with Department of Animal Production, Faculty of Agriculture, Mansoura University, during the period from October, 2009 to June, 2010. Experiment 1:The aim of this study was to evaluate the time of uterine involution, and provide information on oestrus, ovarian activity changes from parturition to next oestrus and mating in ewes. Ewes in the experiment were divided into two similar treatment groups (10 ewes each) according to age, body weight and physiological condition. Experiment 2:The aim of this study was to determine the effects of GnRH injection on day 12 post mating, on ovarian function and their subsequent effects on conceptus growth in ewes. Ewes in the experiment were divided into two similar treatment groups (10 ewes each) according to age, body weight and physiological condition as follows: The first group (Control): Ewes in the first group represented the control ewes which were allowed for natural mating without hormonal treatment. The second group: In this group, ewes were intramuscularly injected (Day 12 post mating) with 1 ml GnRH analogue. Experiment 3:In this experiment, 40 ewes were divided into three treatment groups: the 1st group represented the control ewes which were allowed for natural mating. The control ewes were exposed to fertile ram from the contemporary to that of treatment ewes start time up to the end of the breeding season (end of January) or for a period covering 2 cycles. During January breeding season, ram of proven fertility was introduced to ewes for 35 days and was rested for 7 days and again introduced for another period of 35 days and so on until the end of the experiment. The 2nd, 3rd and 4th treatment groups intramuscularly injected (Day 0) with 1 ml GnRH analogue followed 5, 6 or 7 days later by intramuscular injection with 0.7 ml PGF2α. A second dose of 1 ml GnRH analogue was given on day 7, 8 or 9, and artificial insemination of treated does was carried out 24 h later. The results of the present work could be summarized as follows: Results show that GnRH treatment of ewes on day 12 postpartum markedly increased estrous rate to 100% and decreased postpartum 1st estrus interval to 22 days as compared to 90% and 30 days in the control group, respectively. Results show that GnRH treatment of ewes on day 12 post-mating increased lambing rate to 80% and litter size to 1.62/litter as compared to 70% and 1.14/litter in the control group, respectively. The differences were significant (P<0.05) only in litter size. Results show that one out of 10 ewes (10%) exhibited estrous activity after PGF2α injection in G2 versus 30% (3 out of 10 ewes) in both G2 and G3. This means higher response to PGF2α injection (estrus rate) for ewes that were injected with PGF2α on day 6 or 7 than those injected on day 5 (30 vs. 10%,). According the results, the recommendation of this study, the administration of exogenous GnRH on day 12 postpartum improved uterine involution in term of decreasing 1st postpartum estrus interval, and increased fertility and fecundity of born lambs. Based on the results obtained from the second experiment, the administration of ewes with exogenous GnRH on day 12 post-mating resulted in greater lambing rate and fecundity of born lambs (2nd Exp.). Based on the foregoing results of the 3rd experiment, using GnRH-PGF2-GnRH protocol during breeding season with different days of PGF2 injections failed to affect fertility of ewes, but may be used for synchronization of estrus and ovulation to reduce service and lambing interval for flocks of greater number of ewes (G1 treatment group).