ASSOCIATION BETWEEN THE DURATION OF DOMINANCE OF THE OVULATORY FOLLICLE AND PREGNANCY RATE IN BEEF HEIFERS

After luteolysis, subluteal concentrations of progesterone or treatmen t with a synthetic progestagen result in an extended period of dominan ce (persistence) of the dominant follicle in cattle. Two experiments s tudied (1) the relationship between the duration of dominance of the o vulatory follicle and pregnancy rate and (2) the ability of a persiste nt dominant follicle to ovulate and farm a normal functioning corpus l uteum. In Expt 1, beef heifers were either untreated (n = 30) or given a synthetic progestagen implant (3 mg norgestomet) for 12 days starti ng on day 16 of their cycle (n = 32). The mean duration of dominance o f the ovulatory follicle differed (P < 0.05) between treated and contr ol heifers (10.8 +/- 1.2 and 3.3 +/- 0.8 days, respectively) and 20 of 26 control and 7 of 30 treated heifers were diagnosed pregnant 28 day s after artificial insemination (P < 0.01). In Expt 2, on the first da y of dominance of the second dominant follicle, heifers received eithe r a PGF(2 alpha) analogue alone (controls; n = 18), or prostaglandin a nd a norgestomet implant for 6 (T6; n = 19) or 10 days (T10; n = 20). Increases in the duration of dominance of the second dominant follicle (controls, 4.1 +/- 0.2 days; T6, 8.6 +/- 0.2 days; T10, 12.1 +/- 0.2 days; P < 0.05) resulted in a decrease in pregnancy rate (controls, 14 of 16; T6, 11 of 19; T10, 0 of 13; P less than or equal to 0.05). Pro gesterone concentrations on days 7 and 12 and the area of luteal tissu e on day 12 after artificial insemination were not different (P > 0.05 ) between treatments. It is concluded that (1) treatment with a synthe tic progestagen towards the end of the luteal phase causes a variable extension of the period of dominance of the ovulatory follicle with a significant reduction in pregnancy rate, (2) the persistent dominant f ollicle can ovulate and form a functional corpus luteum, and (3) the p regnancy rate is sequentially decreased as the duration of dominance i ncreases from 4 to 8 days, and is further significantly reduced if the duration of dominance exceeds 10 days.