Growth and regression of ovarian follicles during the follicular phase of the oestrous cycle in heifers undergoing spontaneous and PGF-2 -induced luteolysis

Abstract

Ultrasonography was used to monitor the growth, ovulation and regression of individual ovarian follicles .gtoreq. 5 mm during the late luteal and follicular phases of the oestrous cycle in heifers treated with injections of PGF-2.alpha. to induce luteolysis and in heifers undergoing spontaneous luteolysis. Six heifers were given a single injection of PGF-2.alpha. between Day 12 and 15 of the oestrous cycle and their ovaries were examined daily by transrectal ultrasonography until ovulation occurred. Another group of 5 heifers was examined daily by ultrasound from Day 14 or 15 of the cycle through spontaneous luteolysis and ovulation. Blood samples were taken twice daily from this group and analyzed for progesterone to determine when luteolysis occurred. All heifers were checked for oestrous behavior twice daily. Mean diameters of ovulatory follicles on each of the 3 days before oestrus were not different between PGF-2.alpha.-treated and untreated heifers. In both groups there was large variation among heifers in the sizes and growth rates of the ovulatory follicles. At 3 days before oestrus the diameters of ovulatory follicles were between 7.5 and 11 mm in PGF-2.alpha.-treated heifers and between 6 and 11.5 mm in untreated heifers. Non-ovulatory follicles decreased in size during the 3 days before oestrus and the number of non-ovulatory follicles within the size ranges of ovulatory follicles decreased. The ovulatory follicles was not consistently the largest follicles on the ovaries until the day of oestrus but was always one of the 2 largest follicles during the 3 days before oestrus. These results show that (1) there was variability among heifers in the size of the ovulatory follicle at various times before oestrus, (2) size alone was not a reliable predictor of the ovulatory follicle until the day of oestrus and (3) follicle dynamics during the 3 days before oestrus were similar in heifers undergoing spontaneous and PGF-2.alpha.-induced luteolysis.