The objective of this study was to investigate the effect of the presence or absence of Corpus luteum (CL) on the follicular
population during superstimulation in dairy cows (Holstein-Friesian cattle). Animals were divided into two groups as follows: (1)
Growing CL group (G1): Cows (n = 7) received a total dose of 28 Armour units (AU) follicle-stimulating hormone (FSH) through
the first 4 d (twice daily) after spontaneous ovulation (Day 0). (2) CL Absence group (G2): Cows (n = 10) received prostaglandin
F2a (PGF2a) at 9 or 10 d after ovulation. After 36 h, all the follicles (larger than 5 mm) were aspirated (Day 0). The FSH treatment
started 24 h after aspiration and continued for 4 d. The number of small (3 to <5 mm), medium (5 to <8 mm), and large (8 mm)
follicles was examined on Days 1, 3, and 5 in all groups. Blood samples were collected daily for 5 d, and progesterone (P4), estradiol
(E2), insulin-like growth factor-1 (IGF-1), and growth hormone (GH) in plasma were measured by enzyme immunoassays. The
results showed that in G1, the P4 level increased gradually from 0.5 ng/mL at Day 1 to 2 ng/mL at Day 5, whereas in G2, the P4 level
was completely below 0.5 ng/mL. All cows of the G2 group showed an increase of E2 at Day 3 or Day 4 followed by an increase of
IGF-1 within 24 h, while GH increased concomitantly with the E2 increase in 8 of 10 trials. On the other hand, cows of the G1 group
showed neither E2 nor IGF-1 increase. Moreover, at the end of the treatment, the number of follicles in the G2 group was
significantly increased compared with that of the G1 group (22.8  2.0 vs. 11.6  2.0). In conclusion, low P4 level during FSH
treatment enhanced multiple follicular growth and E2 secretion, which was followed by increase of IGF-1 and GH. Therefore, the
absence of the CL may play a critical role in the superovulation response by controlling the number of growing follicles