Progestagen synchronisation in the absence of a corpus luteum results in the ovulation of a persistent follicle in cyclic ewe lambs

Progestagens are widely used to synchronise oestrous in sheep but the effec ts on follicular dynamics are not clear. We tested the hypothesis that when luteolysis occurs early during progestagen synchronisation prolonged growt h of the ovulatory follicle will occur. Cyclic ewe lambs (40.0 +/- 0.3 kg) were divided into three groups: eight ewes (Long group) received a progesta gen sponge (60 mg medroxyprogesterone acetate) from Days 5 to 19 after oest rous and eight ewes (Short group) received a progestagen sponge on Day 5 wh ich was replaced on Day 10 and again on Day 15, and removed on Day 19 after oestrous. On Days 6 and 7, ewes in both groups received prostaglandin. A t hird group (n = 5, Control) did not receive any treatment. The growth and d evelopment of follicles greater than or equal to 2 mm in diameter were char acterised using daily transrectal ultrasonography. On Day 18, blood samples were collected every 12 min for 8 h from five ewes in the Long and Short g roups. Data were analysed by ANOVA. The maximum diameter and age (emergence to ovulation) of the ovulatory follicle was greater (P < 0.01) in ewes in the Long group (7.4 +/- 0.3 mm and 12.1 +/- 0.6 days) than in ewes in the S hort group (6.3 +/- 0.2 mm and 5.1 +/- 0.5 days) and Control group (6.3 +/- 0.4 mm and 6.8 +/- 0.5 days). On Day 18 of the cycle, MI pulse frequency a nd oestradiol concentrations were greater (P < 0.05) in ewes in the Long gr oup (3.2 +/- 1.1 pulse per 8 h and 1.15 +/- 0.09 pg ml(-1)) than the Short group (0.8 +/- 0.4 pulses per 8 h and 0.54 +/- 0.08 pg ml(-1)). We suggest that the negative feedback efficacy of a long-term progestagen s ponge decreased with time and led to an increase in LII pulse frequency and prolonged growth of the ovulatory follicle. We conclude that, in the absen ce of luteal progesterone, synchronisation with a single progestagen sponge for 14 days resulted in higher LII pulse frequency and ovulation of a pers istent follicle with a larger maximum diameter, compared with controls. (C) 2000 Elsevier Science B.V. All rights reserved.