Time of ovulation in the South Australian Merino ewe following synchronization of estrus. 2. Efficacy of GnRH treatment and its relevance to insemination programs utilizing frozen-thawed semen.
In a study of the time of ovulation following synchronization of estrus in the ewe, the effect of time of treatment with GnRH (24 vs 36 h after pessary removal) and dosage (6.25 to 100 ug per ewe) were examined. All treatments synchronized the time of ovulation irrespective of when untreated ewes commenced to ovulate. As part of an evaluation of GnRH treatment in artificial insemination programs, an assessment was made of the quality of eggs obtained from control ewes and ewes treated with GnRH at either 24 or 36 h after pessary removal. Treatment at 24 h increased the number of retarded embryos (P < 0.01) and unfertilized ova (P < 0.01) collected per ewe, reduced the number of embryos collected per ewe (P < 0.01), and reduced (P < 0.05) the percentage of pregnant ewes compared with other groups. However, there were no differences between control ewes and ewes treated with GnRH at 36 h. GnRH treatment at 36 h was consequently examined as a means of improving conception rates following the intrauterine insemination of frozen-thawed semen. Insemination of GnRH-treated ewes 8 to 12 h before the median time of ovulation resulted in a nonsignificant increase (range 5.7 to 7.3%) in the percentage of ewes of mature age which became pregnant. Insemination 0 to 4 h before the median time of ovulation resulted in a nonsignificant decrease in the percentage of pregnant ewes. GnRH treatment did not influence the number of fetuses per ewe. Reasons for the failure of this treatment to significantly improve ewe fertility are discussed.
Walker
S K
SK
Department of Obstetrics and Gynecology, University of Adelaide Box 498 GPO, Adelaide, South Australia 5001 Australia.
Smith
D H
DH
Ancell
P
P
Seamark
R F
RF