16 STRATEGIES OF FOLLICULAR WAVE SYNCHRONIZATION WITH ESTRADIOL BENZOATE IN GYR (BOS TAURUS INDICUS) CATTLE

Abstract

Physiological and behavioral differences between Bos taurus and Bos indicus can influence the response to a fixed AI protocol. The objective of this study was to examine the reduction of the usual dose of 2 mg of estradiol benzoate (EB) to 1 mg at the beginning of a fixed-time AI protocol, aiming at follicular regression. In a second step, we evaluated the effect of EB on follicle development during follicular pre-deviation and dominance. The experiment was performed at Monte Verde Farm (Uberaba, MG, Brazil). Twenty-two cows (n = 10 heifers and n = 12 nonlactating cows) underwent an estrous synchronization protocol with a progesterone-releasing intravaginal device (P4; Sincrogest^®, Ouro Fino Animal Health, São Paulo, Brazil) and received 1 (G_1mg, n = 11) or 2 mg (G_2mg, n = 11) of EB (Sincrodiol^®, Ouro Fino Animal Health), on a random day (designated Day 0). Follicular dynamics was monitored once per day by ultrasonography from Day 0 to 4 with blood sample collections. In a second step, females received 2 mg of EB on Day 3 (G_D3, pre-deviation, n = 4) or Day 5 (G_D5, dominance, n = 4) of the estrous cycle (Day 0 was the ovulation). Following these treatments, follicular development was monitored daily for 6 days with blood sample collections. The statistical analysis was conducted using the SAS System for Windows 2 (2003; SAS Institute Inc., Cary, NC, USA). The explanatory variables included in the statistical model were the dose of EB, animal category (cows and heifers), and their interaction. The mean test was used to compare intervals from EB treatment to follicular atresia and follicular wave emergence using ANOVA. Progesterone concentrations between groups were compared using the Wilcoxon test. Independently of animal category or stage of the estrous cycle, both EB doses (1 or 2 mg) induced follicular atresia in 2.2 ± 0.9 and 2.1 ± 1.2 days (P > 0.05), respectively. Emergence of a new follicular wave was observed, from Day 0 to 4, in 64% (7/11) of females from G_1mg and in 45% (5/11) from G_2mg, and the interval between treatment and follicular emergence was 3.4 ± 0.8 and 3.0 ± 1.0 days (P > 0.05), respectively. Plasma progesterone concentrations of the 22 animals increased from 2.1 ± 2.0 ng mL^–1 to 7.6 ± 3.0 ng mL^–1 by 24 h after the device insertion (P < 0.05), reaching peak concentration (8.0 ± 3.0 ng mL^–1) by 48 h after treatment beginning, decreasing to 6.4 ± 2.5 ng mL^–1 by 72 h, and remaining constant up to 96 h. Estradiol benzoate injection at follicle pre-deviation (G_D3) caused follicular atresia (2.0 ± 1.4 days) and emergence of a new follicular wave in 3.7 ± 0.1 days in all animals (4/4). However, EB injection during follicle dominance (G_D5) did not synchronize a new follicular wave and follicles persisted during the time of monitoring. Furthermore, EB applied at dominance hastened luteolysis in 50% (2/4) of the treated animals. In conclusion, a reduced dose of EB (1 mg) at the beginning of the protocol with P4 effectively induces follicular atresia. To synchronize a wave emergence at any stage of the estrous cycle, EB must be associated with an exogenous source of progesterone.