The effects of varying the interval from follicular wave emergence to progestin withdrawal on follicular dynamics and the synchrony of estrus in beef cattle.

The objective of this experiment was to examine the effects of varying the interval from follicular wave emergence to progestin (controlled internal drug-releasing insert, CIDR) withdrawal on follicular dynamics and the synchrony of estrus. A secondary objective was to assess the effects of causing the dominant follicle (DF) to develop in the presence or absence of a corpus luteum (CL) on follicular dynamics and the synchrony of estrus and ovulation. The experiment was designed as a 2 x 2 x 2 factorial arrangement of treatments with injection of GnRH or estradiol-17 beta and progesterone (E2 + P4) at treatment initiation, duration of CIDR treatment, and injection of PG (prostaglandin F2 alpha) or saline at the time of CIDR insertion as main effects. Estrous cycles (n = 49) in Angus cows were synchronized, and treatments commenced on d 6 to 8 of the estrous cycle. Cows were randomly assigned to receive a CIDR containing 1.9 g of P4 for 7 or 9 d. Approximately half the cows from each CIDR group received either GnRH (100 micrograms) or E2 + P4 (1 mg of E2 + 100 mg of P4) at CIDR insertion. Cows in GnRH or E2 + P4 groups were divided into those that received PG (37.5 mg) or saline at CIDR insertion. All cows received PG (25 mg) 1 d before CIDR removal. Daily ovarian events were monitored via ultrasound. The intervals from GnRH or E2 + P4 treatment to follicular wave emergence were 1.4 and 3.3 d, respectively (P < 0.05). The interval from follicular wave emergence to CIDR removal was longer (P < 0.05) for cows treated with GnRH (6.6 d) than those treated with E2 + P4 (4.7 d) and longer (P < 0.05) for those fitted with a CIDR for 9 d (6.5 d) than those with a CIDR in place for 7 d (4.8 d). Cows treated with PG or GnRH at CIDR insertion had a larger (P < 0.05) DF at CIDR removal than those treated with saline or E2 + P4. Treatment with a CIDR for 9 d also resulted in a larger (P < 0.07) DF at CIDR removal compared with cows fitted with a CIDR for 7 d. The interval from CIDR removal to estrus was shorter (P < 0.05) in cows treated with PG than those treated with saline. The synchrony of estrus and ovulation was not affected by any of the treatments (P > 0.05). Altering the interval from follicular wave emergence to progestin removal or creating different luteal environments in which the DF developed caused differences in the size of the DF at CIDR removal and the timing of the onset of estrus, but it did not affect the synchrony of estrus or ovulation.