Impact of reducing progesterone concentrations during follicle wave development on follicular dynamics, steroidogenesis, and fertility in beef heifers

The purpose of estrous synchronization in cattle is to effectively induce the ovulation of a follicle that produces elevated pre-ovulatory estradiol concentrations, contains a viable oocyte, and subsequently develops into a corpus luteum (CL) that is capable of producing increased progesterone concentrations. We hypothesize that reducing circulating concentrations of progesterone during follicle wave development will increase gonadotropin support of the ovulatory follicle, and subsequently increase dominant follicle growth and diameter and steroid production by the follicle and subsequent CL. These parameters, as well as estrual characteristics and fertility following AI were evaluated in a series of three experiments. The first experiment (Chapter 2) was conducted to evaluate the effect of reduced progesterone concentrations during follicle wave development on ovulatory follicle growth and diameter at ovulation, pre-ovulatory estradiol concentrations, and subsequent progesterone concentrations. Reducing progesterone concentrations during dominant follicle development (LoP4) resulted in an increase (P \u3c 0.05) in dominant follicle diameter from experimental d 0 (CIDR insert removal) to d 3 compared to control heifers with greater progesterone concentrations. Although pre-ovulatory estradiol concentrations and ovulatory follicle diameter did not differ between treatments, circulating concentrations of progesterone in the subsequent luteal phase tended (P = 0.097) to be increased in the LoP4 treatment. Two experiments (Chapter 3) were conducted to compare reproductive performance of heifers synchronized with three different estrous synchronization protocols. The objective of the first experiment was to compare estrual characteristics and AI pregnancy rates between three Select Synch + CIDR and timed-AI protocols and the objective of the second experiment was to compare timed-AI pregnancy rates between three CO-Synch + CIDR protocols. In both experiments, a modified protocol (Mod and Mod60 treatments) in which progesterone concentrations were reduced at initiation of synchronization via PGF2α administration followed by GnRH two d later was utilized. In the first experiment, heifers were synchronized with either the 5 d Select Synch + CIDR (5dSS), 7 d Select Synch + CIDR (7dSS), or the modified 7 d Select Synch + CIDR (Mod) protocol. Heifers were AI 12 h after observed estrus or heifers not detected in estrus by 60 h following CIDR insert removal were administered GnRH and timed-AI 72 h following CIDR insert removal. Estrous response was greater ( P \u3c 0.05) in heifers exposed to reduced progesterone concentrations during the synchronization period (Mod) compared to the 5dSS treatment, with estrous response in the 7dSS treatment similar to Mod. The interval to estrus was decreased (P \u3c 0.05) in the 7dSS and Mod treatments than the 5dSS treatment. Conception rate of heifers in estrus was greater (P \u3c 0.05) in the Mod and 5dSS than the 7dSS treatment, while timed-AI conception rate did not differ between treatments. Overall AI pregnancy rates were greater in the Mod and 5dSS than the 7dSS treatment. In the second experiment, however, reducing progesterone concentrations during a modified 7 d CO-Synch + CIDR program (Mod60; timed-AI at 60 h after PGF2α) did not increase timed-AI pregnancy rates when compared to the 5 d CO-Synch + CIDR protocol in which timed-AI was performed at either 60 (5dCO60) or 72 h (5dCO72) after PGF2α. Reducing progesterone concentrations during follicular wave development increases dominant follicle diameter in heifers following progestin removal and results in greater progesterone concentrations in the subsequent estrous cycle, potentially indicating that fertility is enhanced with this technique. In addition, the presence of this sub-luteal environment during estrous synchronization yields conception rates of heifers in estrus and AI pregnancy rates that are comparable to the 5dSS treatment, but does not enhance conception rates to estrous detection and AI or timed-AI pregnancy rates in beef heifers