Physiology and Endocrinology: Hormonal Control of Estrus in Beef Cattle

The objective of the experiment was to compare follicular dynamics, ovulatory response to GnRH, steroid hormone concentration patterns, and synchrony of estrus and ovulation among estrous-cycling and anestrous postpartum beef cows following treatment with long-term progestin-based protocols. Beef cows (n = 40) were assigned to treatments based on age, days postpartum, BCS and estrous cyclicity status. Blood samples were taken 10 and 1 d before treatment to determine estrous cyclicity status (progesterone ≥0.5 ng/mL estrous cycling). CIDR Select (T1, n = 20) treated cows received a controlled internal drug-release insert (CIDR;1.38 g progesterone) from d 0 to 14 followed by GnRH (100 μg, i.m.) on d 23, and PG (25 mg, i.m.) on d 30. Cows assigned to the 14-d CIDR-PG (Show-Me-Synch T2, n = 20) treatment received a CIDR insert from d 0 to 14 and PG on d 30. Ultrasound was used to determine response to GnRH for T1 treated cows or follicle turnover for T2 treated cows coincident with timing of GnRH for T1; follicle size at AI; and pregnancy diagnoses. T1 treated cows had a higher ovulatory response to GnRH than cows in T2 (P < 0.001; T1 = 17/20, 85% ovulatory response; T2 = 1/20, 5% follicle turnover); however, progesterone at PG did not differ between treatments (P = 0.17). Mean diameter of the dominant follicle at GnRH, PG and AI did not differ between treatments (P > 0.05). Estrous response, determined from Heatwatch, during the 2 synchronized periods, following CIDR removal and after PG, did not differ (P = 1.0) between treatments. Variances for interval to estrus after CIDR removal and PG were similar for both treatment groups (P > 0.05). Synchronized AI conception rates did not differ between T1 and T2 treated cows (P > 0.05; 72% vs. 58%, respectively); and there was no difference in synchronized AI pregnancy rates between treatments (P > 0.05; T1 = 68%; T2 = 55%). In summary, CIDR Select and Show-Me-Synch protocols were equally effective at synchronizing estrus in postpartum beef cows. This project was supported by National Research Initiative Competitive Grant no. 2005-55203-15750 from the USDA Cooperative State Research, Education, and Extension Service.