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Fertilization and early embryonic development in androstenedione-immunized Merino ewes

Summary. Ewes were immunized against androstenedione (Fecundin) and assigned to be mated 14 days (179 ewes Group C) or 25 days (174 ewes Group B) after a booster immunization with Fecundin. The anti-androstenedione titres at these times were 6790 and 3240 respectively (P < 0·01). The remaining 169 ewes were untreated controls (Group A). Ewes were mated to entire rams (12 rams to 180 ewes) at their second oestrus after synchronization of oestrus. Immunization against androstenedione caused a shortening of the time from sponge removal to mating (Day 0) and a decrease in the percentage of ewes mated by the rams. Also, ovulation rate was increased after immunization (P < 0·01),being 1·42, 2·16 and 1·93 for Groups A, C and B respectively.

Egg recovery rates on Day 2 were lower in immunized ewes and there was some indication that fertilization rates were lowered. On Day 13 after mating a higher proportion of blastocysts was recovered from ewes in Group A than from those in Groups B and C. Immunization resulted in lower fertilization rates and smaller blastocysts with lower mitotic indexes (P < 0·01). At Days 24–32 of pregnancy fetal weight was lower in the immunized ewes. At all sampling stages, the proportion of ewes pregnant (fertility) was lowered in immunized ewes.

The results of the present study show that significant reproductive wastage occurs in androstenedione-immunized Merino ewes, with lower rates of embryo recovery and delayed embryonic development being found in comparison to controls. Despite increases in ovulation rate of 36–52% in Fecundin-treated ewes, the number of fetuses obtained per ewe mated decreased from 1·00 (control, Group A) to 0·89 (Group B) and to 0·69 (Group C). Higher titres of androstenedione antibody were associated with higher reproductive wastage.