EFFECT OF ACUTE TREATMENT WITH PROGESTERONE ON THE TIMING AND SYNCHRONY OF OVULATION IN BOS-INDICUS HEIFERS TREATED WITH A NORGESTOMET IMPLANT FOR 17 DAYS

The aim of the present study was to develop a treatment protocol for t he precise synchronization of oestrus that would avoid the development of persistent dominant ovarian follicles. Bos indicus heifers, in whi ch oestrous cycles had been presynchronized, were allocated randomly, according to the day of their oestrous cycle, to one of five treatment groups. All heifers received a subcutaneous ear implant containing 3 mg of norgestomet for 17 days starting on day 0 and an injection of an analogue of prostaglandin F-2 alpha on days 0 and 4. Heifers in group 1 (control group; n = 7) received no other treatment, while heifers i n groups 2 (n = 8), 3 (n = 7), 4 (n = 7), and 5 (n = 7) received a sin gle progesterone-releasing controlled internal drug release device (CI DR) for 24 h on days 10, 12, 14 and 16, respectively. Treatment with a single CIDR delayed the mean time of ovulation and the day of emergen ce of the ovulatory follicle in heifers treated on days 14 and 16 comp ared with control heifers (P < 0.05). There was less variation in the interval to ovulation in heifers treated on day 10 compared with other treated heifers (P < 0.05). The variation among heifers in the day of emergence of the ovulatory follicle and the age of the ovulatory foll icle at ovulation was less for all groups treated with a CIDR than for the control group (P < 0.05). The duration of dominance and variation in the duration of dominance of the ovulatory follicle was less in he ifers treated with a CIDR device on days IO and 16 than for control he ifers (P < 0.05). Mean age (days from emergence to ovulation) of the o vulatory follicle did not differ among treatment groups (P > 0.05). Co ncentrations of LH and oestradiol decreased coincident with increased concentrations of progesterone on the days of CIDR treatment in treate d compared with control heifers (P < 0.02) but increased again after r emoval of the CIDR. A smaller proportion of follicles in the growing p hase of follicular development at the time of CIDR treatment become at retic compared with follicles that had reached a plateau phase of foll icular growth (14.3% (1/7) versus 90.5% (19/21), respectively; P < 0.0 01). It was concluded that acute treatment with progesterone can influ ence the growth pattern of ovarian follicular development. However, th e effect varies with the stage of ovarian follicular development. Shor t term treatment with progesterone 7 days before the end of a 17 day p eriod of norgestomet treatment resulted in precise synchrony of ovulat ion without the ovulation of a persistent dominant ovarian follicle.