Abstract

Two experiments were designed to compare the effects of eCG treatment and temporary weaning on ovulation and pregnancy rates in postpartum cows. In Experiment 1, 39 lactating multiparous crossbred Bos indicus cows, 60 to 80 d postpartum with a body condition score (BCS) between 2.0 to 2.5 out of 5 were randomly allocated to one of four treatment groups in a 2 × 2 factorial design. On Day 0, all cows received a progesterone (P4) intravaginal device (DIB; Syntex, Argentina) and 2 mg estradiol benzoate (EB) i.m. (Syntex). On Day 8, DIB devices were removed and all cows received 150 µg D(+)cloprostenol i.m. (Ciclase; Syntex) and were randomly divided to receive 400 IU eCG i.m. (Novormon 5000; Syntex) at the same time or no treatment. In addition, half of the cows in each group had their calves weaned temporarily (TW) for 56 h at the time of DIB removal; the other half remained with their calves. All cows received 1 mg EB i.m. on Day 9 and were examined every 8 h by ultrasonography from the time of DIB removal until ovulation. Data were analyzed by ANOVA and chi-square test. The diameter of the dominant follicle on Day 8 (eCG: 7.8 ± 0.5 mm vs. no eCG: 8.2 ± 0.4 mm, and TW: 7.6 ± 0.4 mm vs. no TW 8.4 ± 0.5 mm), the proportion of cows that ovulated (eCG: 12/20, 60% vs. no eCG: 9/19, 47%; and TW: 13/20, 65% vs. no TW: 8/19, 42%, and the interval to ovulation (eCG: 72.0 ± 1.4 h vs. no eCG: 75.6 ± 2.0 h, and TW: 73.8 ± 1.6 h vs. no TW: 73.0 ± 1.8 h) did not differ among groups (P > 0.05). Although there was no effect of eCG treatment or eCG by TW interaction (P > 0.3) on the size of the preovulatory follicle (eCG: 11.1 ± 0.4 mm vs. no eCG: 10.1 ± 0.6 mm), it was smaller in cows TW (9.9 ± 04 mm), compared to those not TW (11.8 ± 0.3 mm; P < 0.05). Nevertheless, the growth rate of the ovulatory follicle was greater (P < 0.02) in cows treated with eCG (1.1 ± 0.1 mm/day) than in those not treated with eCG (0.6 ± 0.1 mm/day). In Experiment 2 769 lactating multiparous crossbred Bos indicus cows received the same treatments as those in Experiment 1 and were fixed-time inseminated (FTAI) 52 to 56 h after DIB removal. Cows were examined by ultrasonography on Day 0 (22.5% with corpora lutea (CL), 30.0% with follicles >8 mm, and 47.5% with follicles <8 mm) and 42 d after FTAI to determine pregnancy. Data were analyzed by logistic regression. Pregnancy rates were not affected (P > 0.7) by TW (TW: 141/397, 37.2% vs. no TW: 141/390, 36.1%) or the TW by eCG interaction (P > 0.7). However, pregnancy rates were higher (P < 0.05) in cows treated with eCG (154/377, 40.8%) than in those not treated with eCG (128/392, 32.6%). The use of eCG but not TW improved pregnancy rates following FTAI in postpartum Bos indicus cows. Results also suggest that the eCG-related increase in pregnancy rates may be due to the final growth rate of the ovulatory follicle.