Timed artificial insemination in beef cattle using GnRH agonist, PGF2alphaand estradiol benzoate (EB)

The present work evaluated low-cost protocols for timed artificial insemina tion (TAI) in beef cattle. In Experiment 1, cycling nonlactating Nelore cow s (Bos indicus, n=98) were assigned to the following groups: GnRH-PGF (GP) and GnRH-PGF-GnRH (GPG), whereas cycling (n=328, Experiment 2) or anestrus (n = 225, Experiment 3) lactating (L) cows were divided into 3 groups: GP-L , GPG-L and GnRH-PCF-Estradiol benzoate (GPE-L). In Experiment 4, lactating cows (n=201) were separated into 3 groups: GP-L, GPE-L and G 1/2PE-L. Anim als from Experiment 1, 3 and 4 were treated (Day 0), at random stages of th e estrous cycle, with 8 mug of buserelin acetate (GnRH agonist) intramuscul arly (im), whereas in Experiment 2 half of the cows received 8 and the othe r half 12 mug of GnRH (im). Seven days later (D 7) all animals were treated with 25 mg of dinoprost trometamine (PGF2 alpha, im) except those cows fro m the G 1/2PE-L group which received only 1/2 dose of PGF2 alpha (12.5 mg) via intravulvo-submucosa (ivsm). Alter PGF2 alpha injection the animals fro m the control groups (GP and GP-L) were observed twice daily to detect estr us and Al was performed 12 h afterwards. The cows from the other groups rec eived a second GnRH injection (D 8 in GPG-L and d9 in GPG groups) or one in jection of estradiol benzoate (EB, 1.0 mg, D 8 in GPE-L group). All cows fr om GPG and GPG-L or GPE-L groups were AI 20 to 24 or 30 to 34 h, respective ly, after the last hormonal injection. Pregnancy was determined by ultrason ography or rectal palpation 30 to 50 days after AI. In the control groups ( GP and GP-L) percentage of animals detected in heat (44.5 to 70.3%) and pre gnancy rate (20 to 42%) varied according to the number of animals with corp us luteum (CL) at the beginning of treatment. The administration of a secon d dose of GnRH either 24 (Experiment 2) or 48 h (Experiment 1) after PGF2 a lpha resulted in 47.7 and 44.9% pregnancy rates, respectively, after TAI in cycling animals. However, in anestrus cows the GPG treatment induced a muc h lower pregnancy rate (14.9%) after TAI. The replacement of the second dos e of GnRH by EB (GPE-L) resulted in a pregnancy rate (43.3%) comparable to that obtained after GnRH treatment (GPG-L, 47.7%, Experiment 2). Furthermor e, the use of 1/2 dose of PGF2 alpha (12.5 mg ivms, Experiment 4) resulted in pregnancy rate (43.5%) similar to that observed with the full dose (im). Both protocols GPG and GPE were effective in synchronizing ovulation in cy cling Nelore cows and allowed approximately a 45% pregnancy rate after TAI. Additionally, the GPE treatment is a promising alternative to the use of G PG in timed Al of beef cattle, due to the low cost of EB when compared to G nRH agonists. (C) 2001 by Elsevier Science Inc.