Control of estrus in Bos indicus cattle under tropical conditions. Effect of utilization of Norgestomet combined with estrogens

The effect of norgestomet implant in combination with estrogens for estrus control in zebu cattle was evaluated. The research was conducted on seven cattle ranches of the state of Veracruz. An ear implant containing 6 mg of norgestomet was applied, during 9 days, to 387 Bos indicus females. Additionally, on the day of the application of the implant, an intramuscular injection of estradiol valerate (5 mg) and norgestomet (3 mg) was also applied. Estrus detection was accomplished by continuous observation during 96 hours after removal of the implant. All cows were inseminated between 8 to 1 2 hours after heat detection. Pregnancy diagnosis was performed 45 days after insemination. Within the 96 hours following implant removal, 85.8% of the cows showed signs of estrus. The average time interval between implant removal and estrus was 43.9 ± 13.2 hours and 64 % of cows in estrus were detected between 1 900 and 0700 hours. Conception rate at first insemination was 25.7% and 22% of all treated cows became pregnant during the first four days following implant removal. The proportion of cows showing estrus was significantly modified by the physiological status of the cows prior to the treatment and 90.2 % of the cows that were dry and cycling showed a synchronized estrus after the treatment but only 75.9% of cows in anestrus and suckling showed estrus after treatment. Both conception and gestation rates were lower in anestrus and suckling cows than in cycling cows, regardless if these latter were suckling or not, but these differences were not significant. It is concluded that the physiological status of the cows when the treatment is applied limits its efficacy and, even though most of the cows respond by showing an estrus after the implant is removed, the chances of good fertility are only moderate, specially in those cows that are in anestrus and suckling when the treatment is applied.