Abstract

In the American tropics, the majority of cattle populations are based on crossbred animals resulting from the mating of zebu (Bos indicus) with dairy breeds (Bos taurus). In Venezuela, as in other tropical regions, crossbred herds have been developed rapidly due to superior characteristics over other genetic groups, such as adequate heat and insect tolerance, milk production based mainly on pastures, and good quality meat to satisfy the animal protein demand of the human population (Soto et al. 2002 Theriogenology 57, 1503–1510). In order to compare the effectiveness of three ovulation synchronization protocols, a study on follicular characterization was carried out at a commercial farm in Zulia State, Venezuela. Seventy-seven crossbred heifers were randomly selected and assigned to the following treatments: OVSYNCH (T1, n = 27), Day 0: 20 mg of gonadotropinreleasing hormone (GnRH) IM; Day 7: 25 mg of prostaglandin F2α (PGF_2α) IM; Day 9: 20 mg of GnRH IM; and Day 10: timed AI (TAI). Modified OVSYNCH (T2, n = 25), similar to T1, but without GnRH on Day 9 and adding 1 mg of estradiol-17β on Day 8. Progestagen + E + PGF_2α (T3, n = 25), Day 0: 50 mg of progesterone and 5 mg of estradiol-17β IM; Day 7: 25 mg of PGF_2α; Day 8: 1 mg of estradiol-17β IM; Day 10: TAI. The results were analyzed by the Proc Freq and chi-square tests of the Statistical Analysis System (SAS; SAS Institute, Inc., Cary, NC, USA). Cows treated with T1 and T2 showed higher pregnancy rates than those treated with T3 (P < 0.05); 40%, 32%, and 12% for T1, T2, and T3, respectively. There was no difference between T1 and T2. Results of the parameters considered during follicular characterization examinations through ultrasound were: TALINIFID (initial size of the dominant follicle: 5.5 mm), DIAULFD (last day size of the dominant follicle: 10.2 mm), and TALFINFO (final size of the ovulatory follicle: 10.9 mm). No differences between treatments were found for follicular sizes during the evaluated times. In conclusion, Modified Ovsynch showed an effectiveness similar to that of the Ovsynch protocol. However, the combination of progestagen, estradiol, and PGF_2α produced a decreased pregnancy rate in dual-purpose crossbred heifers.