ANIMAL AND TEMPORAL EFFECTS ON OVARIAN FOLLICULAR DYNAMICS IN BRAHMANHEIFERS

The aims of the current study were to determine if the pattern of ovar ian follicular growth and development in Bos indicus heifers is differ ent to that reported in Bos taurus breeds, and to examine the factors that determine which dominant follicle will ovulate. In addition, the extent to which variation in follicular dynamics is attributable to va riation between animals and over time was evaluated. The ovaries of 17 Brahman heifers were examined daily by transrectal ultrasonography us ing a 7.5 MHz transducer for a total of 117 interovulatory intervals o ver a period of 10 months. Size and position of individual follicles g reater than or equal to 5 mm in diameter, and size of corpora lutea (C L) were recorded. Circulating progesterone concentrations were determi ned from plasma samples obtained twice weekly. Although size of domina nt follicles and CL within the ovaries of Bos indicus heifers were sma ller than reported for Bos taurus breeds, the overall patterns of domi nant follicle growth were similar. There were significant correlations between number of dominant follicles occurring prior to ovulation and time of appearance of the second dominant follicle, duration of detec tion of CL and size of the ovulatory follicle in the preceding oestrou s cycle (P<0.05). There were significant animal effects on a number of ovarian characteristics including number of dominant follicles per oe strous cycle (P<0.001), with one heifer having four dominant follicles in more than a third of oestrous cycles observed. In addition, change s in daylength over the 10 month period were related to changes in dur ation of the interovulatory interval, persistence and maximum diameter of CL and size of ovulatory follicles. Liveweight change over the sam e period was related to changes in maximum diameter of the first domin ant follicle.