Ovarian Follicular Populations Prior to and during Superovulation in Cattle: Relationship with Superovulatory Response

The present study examined the follicular populations prior to and during superovulation and investigated their relationship with superovulatory response in crossbred cattle. Eleven animals were administered i.m. 8 doses of Folltropin of 2.5 ml each (1.75 mg/ml) spread over 4 days beginning on Day 10 of oestrous cycle, and 30 and 20 mg Lutalyse was given alongwith the 5th and 6th injections of Folltropin, respectively, to induce luteolysis. The animals were artificially inseminated 48, 60 and 72 h after the first Lutalyse injection. The number of corpora lutea (CL) was recorded by palpation per rectum and embryos were recovered non-surgically on Day 6 (Day 0 day of superoestrus). The ovarian follicular population was examined by transrectal Ultrasonography 15 h prior to and 52 h after the first FSH injection, and then on the day of superoestrus and the day of flushing. The follicles were classfied on the basis of diameter as small (3-5 mm), medium (6-9 mm) and large (${\geq}10mm$). The total number of follicles increased significantly (p < 0.01) from $2.45{\pm}0.35$, 15 h prior to the first FSH injection to $8.09{\pm}1.12$, 52 h after the first FSH injection and then further to $13.27{\pm}1.89 $ on the day of superoestrus. A positive correlation was observed between the number of small follicles 15 h prior to the first FSH injection (r = 0.60, p < 0.05), the number of large follicles 52 h after the first FSH injection (r=0.59, p < 0.05) and the number of CL. The follicular population prior to and during superovulation was, however, not significantly different between high (> 6 CL) and low responders (${\leq}6CL$). The present study suggests that the follicular populations undergo dynamic changes during superovulation and that follicular populations prior to superovulation have a limited application as an indicator of the superovulatory response.