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Five experiments were conducted on commercial farms in Brazil designed to develop the basis for an estrus synchronization protocol using melengestrol acetate (MGA) in Bos indicus cattle. These studies resulted in the development of the following protocol: 0.5 mg·d-1 of MGA between d -14 and -1; 2.0 mg i.m. injection of estradiol cypionate on d -9; 48 h temporary weaning between d 0 and 2; and natural service beginning on d 0. The basis of this protocol was to induce estrous cyclicity before postpartum loss of body condition, prevent premature luteolysis, eliminate the need for labor required to detect estrus, and consequently increase the likelihood of pregnancy early during the postpartum period. This treatment effectively induced estrous cyclicity among anestrous cows, synchronized estrus activity, and prevented premature luteolysis with no negative effect on pregnancy. [ABSTRACT FROM AUTHOR]

This trial was to evaluate the effect of progesterone (P4) and/or 17β-estradiol (E2) on incidence of short cycles (SC) after induction of ovulation with calf removal (CR) and GnRH injection in anestrous Nelore cows. Cows (n=142; 43±12 DPP) were evaluated by 2 ultrasound (US) exams 8 d apart (day -16 and day -8) for cyclicity. Anestrous cows (n=114) were randomly assigned to receive a 6 d treatment with an intravaginal P4 device (CIDR® Pfizer Animal Health, Brazil) prior CR and/or 1 mg i.m. injection of 17β-estradiol 48h after beginning of CR (4 treatments; 2x2 factorial design). On day -2 all cows suffered CR for 54h and received a GnRH injection at the end of CR (day 0). Only cows that ovulated within 48 h after GnRH injection (evaluated by US on days 0 and 2) were used (Control, n=23; E2, n=25; P4, n=19; P4+E2, n=18). Blood samples were collected on days 0, 5, 7, 9, 12, 15 and 19 for corpus luteum lifespan evaluation, by serum P4 analysis. Cows were considered having SC if P4 decreased more than 50% between days 5 and 7. Percentage of cows exhibiting SC was analyzed by logistic regression and P4 were analyzed by PROC MIXED of SAS. P4 were higher on day 5 than on day 0 (P<.01) on all treatments. Serum P4 increased (P<.05) between days 5 and 7 on P4 (from 2.3±0.5 to 3.7±0.5 ng/mL) and P4+E2 (from 2.4±0.5 to 3.3±0.5 ng/mL) groups and decreased (P<.01) on Control (from 2.9±0.4 to 1.9±0.4 ng/mL) and E2 (from 1.9±0.4 to 0.8±0.4 ng/mL) groups. Cows treated with P4 (P4 and P4+E2) had higher serum P4 than cows not treated (Control and E2) on days 7, 9, 12 and 15 (P<.01). Incidence of SC was lower in cows treated with P4 than in cows not treated (23.5 vs. 79.2%; P<.01), while treatment only with 17β-estradiol did not differ (P>.10) from control group (76 vs 82.6%). Cows not treated with P4 that showed heat had the same incidence of SC than cows not detected in estrus (75 vs. 80%; P>.1). In conclusion, 6 d treatment with P4 prior ovulation on anestrous cows warranted a normal luteal lifespan, while administration of 1mg of 17β-estradiol did not prevent occurrence of SC. [ABSTRACT FROM AUTHOR]