The effect of progestin and luteinizing hormone (LH) pulse frequency on dynamics of dominant follicle growth during the first follicular wave after oestrus was examined in non-lactating Holstein cows by ultrasonography. On day 8 of the cycle, cows (n = 8) received a luteolytic dose of prostaglandin F2α (PGF2α; 25 mg) and an ear implant of Norgestomet (6 mg). On day 18, cows were assigned to a crossover design in which the implants were retained (T1) or replaced with a new implant (T2). All implants were removed on day 23. After oestrus, cows underwent a normal intervening oestrous cycle. On day 8 of the third cycle, T1 and T2 were reversed among cows. Ultrasonography and blood sampling were performed on alternate days throughout the experiment. On days 10 and 19 of the third cycle, blood was sampled every 15 min for 8 h in concert with an additional control group (n = 3) sampled on day 10 of the cycle. Progesterone concentration on day 8 before PGF2α was 6.5 ± 0.5 ng ml−1. Dominance of the first wave dominant follicle was extended beyond day 18 in 15 of 16 cows for T1 and T2 periods. The original dominant follicle ovulated in five of eight T1 and none of eight T2 periods (P < 0.01). New dominant follicles were detected on day 24 ± 1 in T1 (n = 3) and on day 20.6 ± 1 in T2 (n = 8; P < 0.01) cows. Growth rate of the dominant follicle from day 8 (15.8 ± 0.4 mm) to day 18 (22.9 ± 0.6 mm) was 0.7 mm day−1 (n = 15). Preovulatory sizes for the initial and new dominant follicles were 26.8 ± 1 mm (n = 5) and 15.3 ± 0.8 mm (n = 11), respectively (P < 0.01). Intervening oestrous cycles comprised three (n = 5) and two (n = 3) follicular waves. An interaction of treatment by day (P < 0.01) for LH pulses in 8 h was detected (pulses on days 10 and 19 for T1 (5.2 ± 0.5 and 7 ± 0.7) versus T2 (5.7 ±0.5 and 3 ±0.4)). A mean of 0.7 ± 0.3 LH pulses in 8 h was detected on day 10 for control cows (n = 3). Increased LH support appears to maintain a dominant follicle, whereas high progesterone concentrations decrease LH pulse frequency leading to turnover of the dominant follicle in cattle.
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