Effects of estradiol benzoate or human chorionic gonadotropin for ovulation resynchronization protocols on pregnancy rate per TAI.

Background: Programs of resynchronization of ovulation have the objective of allowing reinsemination of females that did not get pregnant after fixed time artificial insemination (TAI), in the lowest time interval possible. These protocols are usually composed of administration of estradiol benzoate (EB) concomitant to the insert of progesterone-based (P4) intravaginal implants 22 to 24 days after the first TAI. However, little information is available on the impact of hormonal manipulation in a pre-established pregnancy. The objective of this study was to evaluate if resynchronization of ovulation, started 24 days after the first TAI, would interfere in the function of the corpus luteum (CL), production of P4, and pre-established pregnancy. Materials, Methods u0026 Results: For this study, 526 Brangus heifers with ages between 24 and 26 months, were subjected to TAI program in the beginning of the breeding season. Synchronization protocol for the first TAI started with insertion of an intravaginal implant containing 750 mg of P4 and administration of 2 mg of estradiol benzoate (EB) intramuscular (im) on day -9 (D-9). After seven days (D-2), P4 implants were removed, and 150 μg of D-cloprostenol (PGF), i.m., and 1 mg of estradiol cypionate (EC), i.m., were administered. The TAI was performed between 48 and 54 h after removal of the P4 implant (D0). Twenty-four days after the first TAI (D24), heifers were randomly divided into the following experimental groups: Control (n = 167, no treatment), EB (n = 208, 1 mg of EB, i.m.), and hCG (n = 151, 1000 IU of hCG, i.m.). Heifers of EB and hCG groups received a new intravaginal device containing 750 mg of P4 on D24. On day 31 (D31), P4 implants were removed and pregnancy diagnosis were performed by ultrasound. Pregnancy rates from the first TAI on D31 were 58.7% (98/167), 53.4% (111/208) and 52.9% (80/151) respectively for groups Control, EB and hCG. Heifers diagnosed as non-pregnant received 150 μg of PGF, i.m., and 1 mg of EC, im, and the second TAI was performed 48 to 54 h after removal of the implant (D33). On D31, subgroups of pregnant heifers of each experimental group were randomly selected to determine of area of CL and serum concentration of P4: Control (n = 13), EB (n = 26), and hCG (n = 24). The area of CL was significantly greater (P u003c 0.05) in group hCG (3.42 ± 0.76 cm) as compared to groups EB (2.44 ± 0.57 cm) and Control (2.61 ± 0.61 cm). Similarly, serum concentration of P4 was significantly higher (P u003c 0.05) in group hCG (12.43 ± 3.48 ng/mL) as compared to EB (6.92 ± 3.04 ng/ml) and Control (7.29 ± 2.45 ng/mL) groups. Discussion: The use of EB and hCG in programs of resynchronization of ovulation 24 days after TAI did not interfere with the result of pre-established pregnancy. It is likely that the mechanism of action of EB does not affect luteal activity, production of P4, and consequently, has no negative effect in maintenance of pregnancy in protocols of resynchronization of ovulation. Treatment with hCG increased the area of CL and production of P4; however, this effect did not favor pregnancy rates on the first TAI.