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A study was designed to compare the effect of a prostaglandin-based synchronization protocol on ovarian follicular dynamics in sheep with the FecB (Booroola) mutation. Forty dry Romney sheep (57.6 ± 7.3 kg; 6.1 ± 1.1 years) were randomly selected from both Invermay Booroola (BB; n= 20) and commercial (non-FecB carriers, ++; n= 20) flocks. All ewes had their estrous cycles synchronized with 2 i.m. injections of PGF (150 μg of cloprostenol, Estrumate, Schering-Plough Coopers Animal Health Ltd., New Zealand) administered 7 days apart. Ewes were monitored by transrectal ultrasonography (Aloka 900-SSD and a 7.5-MHz linear-array transducer, Aloka, Tokyo, Japan) daily from Day -2 to the day of ovulation. Data were analyzed by Student''s t-test or Wilcoxon Rank Sum test. Variances were compared with Barlett''s test. Paired t-test compared the number of preovulatory follicles in each genotype after PGF treatments and intervals from PGF to ovulation after PGF. Data are presented as mean (± SEM). The number of corpora lutea (CL) and total CL area at the time of the first and second PGF treatment were 4.4 ± 0.6; 5.7 ± 1.4 and 672.1 ± 133.5 mm2; 999.0 ± 145.9 mm2in the BB and 2.1 ± 0.2; 2.1 ± 0.5 and 342.3 ± 60.7 mm2; 401.3 ± 68.6 mm2in ++ ewes, respectively. These 2 variables were higher (P 0.61) between BB (3.4 ± 0.2; 3.0 ± 0.3d) and ++ (3.5 ± 0.2d; 3.0 ± 0.1d) ewes. However, interval from the second PGF to ovulation was more variable (P= 0.002) in the BB than in the ++ ewes. Data of both groups were combined and a mean significant difference of 0.6 ± 0.2d (P< 0.003) was found between the first and second PGF-to-ovulation intervals. The interval from the first PGF to emergence of the next follicular wave was shorter (P< 0.02) and more variable (P< 0.03) in the BB (2.7 ± 0.4d) than in the ++ (3.5 ± 0.2 d) group. Preovulatory follicles were smaller in Booroola, but higher in number, than in ++ ewes, whereas the luteal area was similar. Within the BB ewes, the higher number of follicles that ovulated after the second PGF than after the first injection may be due to a higher follicular response to an elevated rebound in circulating FSH after the first PGF. A high number of growing follicles of the first follicular wave may also have contributed to this event. These findings warrant further research aimed at the study of the interaction between FSH and follicle dynamics in estrus synchronized sheep carrying the FecB mutation. [ABSTRACT FROM AUTHOR]