Prolific strains of Barbarine sheep are characterized by increased ovulation rate due to extended period of ovulatory follicle recruitment and co-dominance effects

Preovulatory follicle dynamics and plasma oestradiol around the time of onset of oestrus as well as ovulation rate were assessed, during non-breeding season, in prolific (W; n = 20) and non prolific (AB; n = 20) strains of the Barbarine sheep. Oestrus cycles were synchronized with progestagen sponges for 14 days followed by introduction of rams. Transrectal ultrasonographic monitoring of the ovaries was carried out from the day of pessaries removal up to oestrous exhibition. Blood samples were collected every 4 h and were analyzed for plasma concentrations of oestradiol by radioimmunoassay. Laparoscopy was performed 11 days after introduction of rams to assess number of corpora lutea (CL). Results indicate that along the first 5 days after introduction of rams, 20 and 14 ewes of respectively the AB and W ewes exhibited oestrus (P < 0.01). Ewes of the W strain had more medium (3.5-5.4 mm) follicles than AB ones during the follicular phase; the difference being significant (P < 0.01) 24 h before oestrus. The number of medium follicles prior to oestrus changed little in AB ewes but declined rapidly from 5.8 ± 0.7 to 4.0 ± 0.6 in W females (P < 0.05). Differences in medium follicles between strains were not reflected in the number of large (≥5.5 mm) or the growing large follicles during the follicular phase. Around oestrus, plasma oestradiol concentrations were higher for W in comparison to AB ewes (22.3 ± 12.6 vs. 11.8 ± 6.2 pg/ml; P < 0.01) and mean ovulation rates were 1.8 ± 0.8 and 1.1 ± 0.3 for respectively W and AB ewes (P < 0.01). In conclusion, the increased ovulation rate in the W strain is related to an extended period of follicle recruitment with appearance of co-dominance effects.