27 FOLLICULAR RESPONSE TO DIFFERENT eCG DOSES IN AN OVSYNCH + PROGESTERONE PROTOCOL IN BEEF HEIFERS

Abstract

An experiment was designed to evaluate the effect of different doses of eCG on ovarian follicular dynamics in heifers treated with a Ovsynch plus progesterone protocol. Twenty-five cyclic yearling Black Angus heifers (373.0 ± 35.7 kg), in 2 replicates, received an injection of 100 μg of GnRH (Ovurelin, Bomac Laboratories Ltd., Auckland, New Zealand) i.m. and an intravaginal progesterone device (1.38 g of progesterone; Eazi-Breed CIDR, Pfizer Animal Health, New Zealand) on Day 0 (beginning of the experiment), followed by 500 μg of cloprostenol (PGF, Estrumate, Intervet/Schering-Plough Animal Health, Auckland, New Zealand) i.m. on Day 7, and a second 100 μg of GnRH injection given i.m. on Day 9 (56 h after PGF). At the time of PGF treatment, heifers were randomly assigned to 5 treatment groups to receive 0, 300, 500, 700, or 1000 IU of eCG (Folligon, Intervet/Schering-Plough Animal Health) i.m. Heifers were monitored by ultrasonography (Aloka 900-SSD equipped with a 7.5-MHz linear-array transducer; Aloka, Tokyo, Japan) daily from Day 0 to 9 (GnRH), and then every 12 h until ovulation. Data were analyzed by one-way ANOVA or Kruskall-Wallis test, and means or ranks were compared with LSD or Wilcoxon rank sum tests, respectively. Because a replicate effect was observed (P < 0.05) in the size of the dominant follicle at the second GnRH and prior to ovulation, replicate effect was included in the analysis. The luteal area at PGF treatment was significantly greater (P < 0.01) in heifers that ovulated (750.0 ± 97 mm²) in response to the first GnRH treatment than in those that did not (301.6 ± 42.7 mm²). The diameter of the dominant follicle at the time of PGF treatment was also greater (P < 0.05) in ovulating (11.2 ± 0.4 mm) than in nonovulating (9.7 ± 0.5 mm) heifers. The interval from the first GnRH treatment to the emergence of the next follicular wave was longer (P = 0.50) and more variable in heifers that did not ovulate (2.9 ± 0.4 d; n = 27) than in those that ovulated (1.9 ± 0.2 d; n = 23). There was no effect (P < 0.37) of eCG on the interval from PGF to ovulation (86 ± 1.9 h). The number of ovulations after the second GnRH was higher (P = 0.01) in the group of heifers treated with 1000 IU of eCG (1.8 ± 0.4) than in the other groups (1.0 ± 0.0; 1.1 ± 0.1; 1.2 ± 0.1; 1.0 ± 0.1). There was an effect of day of follicular wave emergence on the number of ovulations (P < 0.01). Heifers with a wave emerging 1 to 3 days after the first GnRH (n = 37), had one ovulation (1.0 ± 0.0), whereas heifers with a wave emerging on Day 4 (3 out of 4 heifers) and Days 5 to 7 (n = 9), ovulated 2 or more follicles. In summary, the multiple ovulation effect occurred when eCG was given to heifers with a follicular wave emerging on or after Day 4, and was potentiated when heifers received 1000 IU of eCG. Although the dose of eCG given at the time of PGF treatment in an Ovsynch program has a significant effect on follicular development, the time of emergence of the dominant follicle appeared to be more important in the ovulation of preovulatory follicle/s after the eCG and the second GnRH treatment.