350 EFFECT OF DIFFERENT SUB-DOSES OF EQUINE CHORIONIC GONADOTROPIN ON PREGNANCY AND TWINNING RATES OF CYCLIC ZEBU COWS SUBMITTED TO FIXED-TIME ARTIFICIAL INSEMINATION

Abstract

The injection of a low dose of eCG has the potential to induce multiple ovulation and pregnancies in cattle. The present study aimed to evaluate the ovarian response, conception rate and incidence of twin pregnancies of cyclic cows receiving 1 of 2 low doses of eCG. Multiparous Nellore (Bos t. indicus) cows with plasma progesterone levels >1 ng∙mL–1 on at least one of 2 blood samples collected at 10-day intervals (Day –10 and Day 0) received an intramuscular (IM) injection of 2 mg of oestradiol benzoate (EB; Estrogin®, AUSA, São Paulo, Brazil) and a vaginal device (DIP) containing 1 g of progesterone (Primer®, Tecnopec, São Paulo, SP, Brazil) on Day 0. On Day 8, the DIP was removed and cows received an IM injection of 150 μg of cloprostenol (Veteglan®, Hertape Calier, Juatuba, MG, Brazil). At this time, the animals were randomly distributed into 3 groups. Group 1 (n = 30) received an IM injection of 2 mL of saline, whereas groups 2 (n = 41) and 3 (n = 23) received 600 IU and 900 IU of eCG (Novormon® MSD Saude Animal, São Paulo, Brazil), respectively. Twenty-four hours later (Day 9), all groups received 1 mg of EB and were submitted to fixed-time artificial insemination (FTAI) 30 h later (i.e. 54 h after DIP removal). Oestrus observation was performed daily from the time of the withdrawal of the DIP until the day of FTAI. Ovaries were examined ultrasonically at the time of FTAI, the following day and 7 days after FTAI. Pregnancy diagnosis was done by ultrasonography 30 days after FTAI and the incidence of twin or single calves was recorded at birth. Data were analysed by chi-square test. The rate of expression of oestrus was 70.0% (group 1), 82.9% (group 2), and 78.2% (group 3; P = 0.25). Cows that had 2 or more large follicles at the time of FTAI was 0% (group 1), 14.6% (group 2), and 34.8% (group 3; P < 0.05). The ovulation rate of cows in group 1 (80.0%) was higher than cows in groups 2 (48.8%) and 3 (52.2%; P < 0.05). The conception rates for groups 1, 2, and 3 were 50.0, 26.8, and 39.1%, respectively (P < 0.05). Two animals in group 3, one in group 2, and none of group 1 had twin pregnancies on Day 30 after FTAI. Only one of these cows (group 3) had a twin calving. It was concluded that the injection of 600 or 900 IU eCG, in an oestradiol/progestogen FTAI protocol does not result in an increase in the rate of twin calvings, but may negatively affect pregnancy rates of cyclic Nellore cows.Financial support was provided by FAPESP (proc. 2011/13096–0).