The ability of different hormones administered on day five of the estrous cycle to induce the ovulation of the first wave dominant follicle (FWDF) and to form an accessory corpus luteum (CLa), and the respective effects on plasma progesterone concentrations were studied in recipient heifers. Fifty-two crossbred Holstein-Zebu heifers were randomly assigned to four treatments: T1-control, T2-subcutaneous administration of 500mg of rbST, T3-intramuscular administration of 100mug of GnRH and T4-administration of 3000IU of hCG (1000IU endovenous and 2000IU intramuscular). Transrectal palpation was performed on days 5, 13 and 60 to check original corpus luteum, CLa and pregnancy, respectively. CLa formation was as follow: T1-0/12 (0.0%), T2-0/13 (0.0%), T3-5/12 (41.7%) and T4-10/15 (66.7%), being T1 and T2 different from T3 and T4 (P<0.05). Plasma progesterone concentration (ng/ml) was determined by radioimmunoassay from blood samples collected from jugular vein on days 5, 13, 17 and 21. Thirty-two embryos, T1=8, T2=8, T3=7 and T4=9, were transferred to recipients at day 7. Only heifers not receiving embryos were collected on days 17 and 21. Differences on plasma progesterone concentration were observed only on day 13. The values were T1=5.01± 1.04, T2=4.80± 1.26, T3=6.42± 1.47and T4=11.16± 2.79, being T3 different from T1, T2 and T4, and T4 different from T1, T2 and T3 (P<0.05). No differences between pregnancy rates, T1=37.5% (3/8), T2=62.5% (5/8), T3=28.6% (2/7) e T4=33.3% (3/9), were found. These results showed that the FWDF on day 5 is capable to respond to GnRH-analog and hCG, to ovulate and forme an accessory corpus luteum, which allows for an endogenous increase in plasma progesterone concentration in heifers on day 13 of the estrous cycle, the critical period for establishment of pregnancy in bovine.
Cattle; Nelore; embryo; progesterone; dominant follicle; accessory corpus luteum