The effect of different levels of progesterone (P4) concentrations on follicle growth and ovulatory capacity was evaluated in 40 crossbred Bos indicus x Bos taurus cyclic heifers submitted to distinct PGF2α + progesterone-based protocols. Heifers in CIDR_PGF8 group (n = 10) received 2.0 mg i.m. estradiol benzoate (EB) and a new controlled internal drug release containing 1.9 g of progesterone (CIDR) on day 0 of study. At the time of CIDR withdrawal (day 8), heifers received an i.m. injection of PGF2α and 24 h later a second EB i.m. injection (0.5 mg). The three other groups received EB injections and CIDR insertion/withdrawal as aforementioned, except that an i.m. injection of PGF2α was administered on day 5. In addition, heifers in the CIDR_PGF5 group (n = 10) received a new CIDR, while heifers in the CIDR1x_PGF5 (n = 9) and CIDR2x_PGF5 (n = 11) groups received a previously used CIDR for 8 and 14 days, respectively. Heifers that received a PGF2α injection on day 5 showed lower circulating P4 than heifers treated on day 8 (CIDR_PGF5 = 1.98 ± 0.21 ng/ml; CIDR1x_PGF5 = 1.69 ± 0.17 ng/ml and CIDR2x_PGF5 = 1.33 ± 0.08 ng/ml versus CIDR_PGF8 = 3.31 ± 0.45 ng/ml). The dominant follicle (DF) growth rate was slower in those heifers receiving PGF2α injection on day 8 (CIDR_PGF8 = 0.72 ± 0.13 mm/day) than groups treated on day 5 (CIDR_PGF5 = 0.96 ± 0.12 mm/day; CIDR1x_PGF5 = 1.06 ± 0.15 mm/day and CIDR2x_PGF5 = 1.01 ± 0.06 mm/day). In consequence, preovulatory follicle diameter on day 10 was smaller in those animals injected on day 8 (CIDR_PGF8 = 8.81 ± 6.7 mm) than in those treated on day 5 (CIDR_PGF5 = 10.00 ± 0.58 mm CIDR1x_PGF5 = 10.5 ± 0.69 mm and CIDR2x_PGF5 = 10.5 ± 0.35 mm). For heifers receiving PGF2α injection on day 5, no significant differences on plasma P4 concentrations, follicular growth rate and DF diameters were observed among heifers that received new or previously used CIDR inserts. These results suggest that the presence of corpus luteum during synchronization protocols is the main factor responsible for the increase in the plasma P4 concentrations and inhibition of DF growth.
