Diameter of the Larger Follicle at the Moment of the Estradiol Application and the Gestation Rate in Cows Submitted to Artificial Insemination at Fixed Time

Objective: This study was conducted to evaluate the effect of the follicular diameter upon gestation rate of cows submitted to Artificial Insemination at Fixed Time (IATF). Methods: Forty-seven crossbred cows Bos taurus x indicus presenting cyclical luteal ovarian activity and under nursing were used. The cows were given 150μg of PGF_2α via Intra-Muscular (IM). The diameter of the present largest follicle was measured through Ultra-Sound Examination (USE). After 24 hours, the cows were given 2μg of estradiol via IM and they were again evaluated through USE for certification of the follicular dominance. Later, they were divided into two treatments: treatment 1 (cows with dominant follicle <13mm) and treatment 2 (cows with dominant follicle ≥13mm). After 48 hours from the application of E2, the cows were Artificially Inseminated (AI). However, those cows presenting estrus in advance were inseminated at 12 hours after manifestation of the same one. Ten days after AI the concentration of progesterone (P4) was quantified by radioimmunoassay using a commercial kit. The gestation diagnosis was accomplished at 30 and 100 days after IA through USE. Findings: Differences (p<0.05) were observed in the gestation rates, that is 76% and 45.5% for animals of the treatments 1 and 2, respectively. The serum concentrations of P4 were 1.34±0.49 and 2.16±1.17 for the treatments 1 and 2 respectively (p<0.01). A low correlation (r=0.45) was observed between P4 levels and the presentation of pregnancy. Application: Cows with dominant follicles <13 mm in diameter at the time of application E2 have a higher gestation rate even with lower production of P4.