|Institution:||The Ohio State University|
|Department:||Veterinary Clinical Sciences|
|Keywords:||Animal Sciences; Endocrinology; Physiology; Progesterone, Used CIDR, Fertility, Beef Cows, LH, Oocyte Developmental Capacity|
|Full text PDF:||http://rave.ohiolink.edu/etdc/view?acc_num=osu1420620191|
Synchronization programs aim to effectively create an ideal hormonal environment to successfully induce the development of a dominant follicle containing a healthy oocyte that is capable of being fertilized and developing to term. It has been repeatedly suggested that greater gonadotropin stimulation for a finite period during follicular development may enhance fertility in beef cattle. It was hypothesized that decreased progesterone (P4) concentrations during early follicular development would result in increased luteinizing hormone (LH) pulse frequency, leading to oocytes of greater developmental capacity, ultimately increasing pregnancy rate to AI in beef cattle. In the first experiment, decreased P4 concentration during early follicular development and its effect on pregnancy rate to timed-AI and estrus-AI were investigated in multiparous beef cows in Ohio (Experiment 1.1) and Montana (Experiment 1.2). In both experiments, P4 concentrations at the end of P4 treatment were, as designed, greater in the high P4 (HiP4) than the low P4 (LoP4) treatment. The administration of prostaglandin F2a (PGF) to induce luteolysis prior to emergence of the new follicular wave, in conjunction with either insertion (Experiment 1.1) or maintenance (Experiment 1.2) of a previously used CIDR, resulted in decreased P4 concentrations during emergence and early development of the ovulatory follicle in the LoP4 treatment. It was demonstrated that decreased P4 concentrations during follicular development resulted in greater estradiol (E2) concentrations and increased follicle diameter at termination of P4 treatment (Experiment 1.1), but did not influence pregnancy rates attained at the subsequent synchronized ovulation/estrus in beef cows (Experiments 1.1 and 1.2).In the second experiment, the effect of P4 concentrations during early follicular development on LH secretion and characteristics of oocytes that reflect developmental competence were investigated in primiparous beef cattle. The animal model used was similar to Experiment 1.1. Concentrations of P4 were lesser and E2 were greater in the LoP4 than HiP4 treatment throughout follicular development. LH pulse frequency was greater in the LoP4 than HiP4 treatment, however, LH mean concentration and pulse amplitude did not differ between treatments. Among treatments, follicles aspirated per cow, total oocytes recovered per cow, recovery rate, percentage of oocytes graded 1 to 3, oocyte diameter, percentage of oocyte stained for BCB (BCB+), and the relative expression of oocyte mRNA for FST did not differ. In conclusion, decreased P4 concentrations during early follicular development increased LH pulse frequency and enhanced E2 concentrations in both early antral and dominant follicles. However, the parameters used to investigate developmental competence in oocytes in the current experiment did not differ between HiP4 and LoP4 treatments. In addition, differences in P4 concentrations do not impact pregnancy rates in beef cows, when adequate intervals of proestrus are afforded.