Influence of the length of proestrus on fertility and endocrine function in female cattle

Previous research from our laboratory in beef cattle suggests that ovarian follicle maturity and subsequent fertility is influenced by length of proestrus across a range of follicle sizes. To test this hypothesis an animal model was used in which ovulation from similar sized follicles was induced following either a long (LPE; ∼2.25 days) or short (SPE; ∼1.25 days) proestrus (interval from PGF2α administration to a GnRH-induced LH surge). Specific objectives were to compare pregnancy rates and luteal phase concentrations of progesterone (Experiment 1) and to characterize preovulatory concentrations of estradiol, the GnRH-induced LH surge, and concentrations of progesterone in the subsequent estrous cycle (Experiment 2) between the LPE and SPE treatments. In Experiment 1, ovulation from follicles that were previously synchronized using follicular aspiration was induced with GnRH (Day 0) after either 2.25 days (LPE; n = 40) or 1.25 days (SPE; n = 38) of proestrus. Lactating and non-lactating cows were inseminated 12 h following GnRH administration. Ovulatory follicle diameter was similar between treatments. Pregnancy rates to AI were greater (P < 0.01) in the LPE (50.0%) compared to the SPE (2.6%) treatment. The proportion of cows having a short luteal phase in the subsequent estrous cycle was greater (P < 0.01) in the SPE than LPE treatment. In cows with a luteal phase of normal length, timed-AI pregnancy rates and concentrations of progesterone in the subsequent luteal phase were greater (P < 0.05) in the LPE than SPE treatment. In Experiment 2, a similar experimental approach was taken with non-lactating beef cows and ovulation was induced following either 2.2 days (LPE; n = 8) or 1.2 days (SPE; n = 8) of proestrus. Ovulatory follicle diameter was similar between treatments. Concentrations of estradiol during the proestrus period were greater (P < 0.05) in the LPE than SPE treatment from Days −1.9 to Day 0 (GnRH administration). Concentration of LH during the GnRH-induced LH surge and concentrations of progesterone in the subsequent estrous cycle did not differ between treatments although there was a tendency (P = 0.10) for increased incidence of short luteal phases in the SPE treatment. In conclusion, decreasing the length of proestrus before induction of ovulation of a large follicle resulted in lesser pregnancy rates and an increased incidence of short luteal phases. The impact of a shortened proestrus on concentrations of progesterone in cows with luteal phases of normal length varied among experiments. Decreased circulating concentrations of estradiol during the preovulatory period and/or reduced concentrations of progesterone during the subsequent estrous cycle in cows that do not experience a shortened luteal phase may represent the mechanism responsible for reduced fertility with the SPE treatment.