Effect of time of prostaglandin administration on follicular response in ewes during oestrous cycle

The purpose of this study was to evaluate ovarian response, using transrectal ultrasonography, to two injections of Prostaglandin F-2 alpha (PGF(2 alpha)) given at different intervals. Fourteen Bergamasca ewes were injected with an i.m. dose of PGF(2 alpha) during the breeding season. Ewes were randomly alloted in two groups (n = 7) which received PGF(2 alpha) 7 d (PG7) or 9 d (PG9) after ovulation (when the dominant follicle of wave 2 was expected to be in the growth and plateau phase, respectively). Ovaries were examined daily with transrectal ultrasonography Aloka 500 with 7.5 MHz linear array transducer. Ultrasonography started 1 day prior to the second PG injection and ended 10 d after ovulation for both groups. Jugular vein blood samples were collected daily for progesterone assay. Plasma P-4 concentration was lower in PG7 than in PG9, immediately before PGF(2 alpha) treatment (4.10 + 0.17 vs 4.91 + 0.29 ng/mL, respectively). Mean maximum diameter size attained by the dominant follicle of wave 2 was larger in the group treated in PG7 when compared with PG9 (5.5 +/- 0.19 vs 4.29 +/- 0.26 mm). There was effect of day for total follicles. Number of small follicles (2-2.5 mm) was greater (4.57 +/- 0.78 vs 8.42 +/- 1.36) at day 6, for PG7 and PG9, respectively, whereas no changes were observed in medium follicles (3-3.5 mm) between groups treated. The proportion of large follicles (4 mm) increased after luteolysis. Values were lower for ewes treated on PG7, whereas values mantained stable over time for ewes treated on PG9. Ewes with rapid and complete luteal regression, ovulation occur from the dominant follicle of wave 2 in females treated on days 7 and 9.