Altered progesterone concentrations by hormonal manipulations before a fixed-time artificial insemination CO-Synch + CIDR program in suckled beef cows

We hypothesized that pregnancy outcomes may be improved by inducing luteal regression, ovulation, or both (i.e., altering progesterone status) before initiating a timed–artificial insemination (TAI) program in suckled beef cows. This hypothesis was tested in two experiments in which cows were treated with either PGF_2α (PG) or PG + GnRH before initiating a TAI program to increase the proportion of cows starting the program in a theoretical marginal (<1 ng/mL; experiment 1) or elevated (≥1 ng/mL; experiment 2) progesterone environment, respectively. The control was a standard CO-Synch + controlled internal drug release (CIDR) program employed in suckled beef cows (100 μg GnRH intramuscularly [IM] [GnRH-1] and insertion of a progesterone-impregnated intravaginal CIDR insert on study Day −10, 25 mg PG and CIDR insert removal on study Day −3, and 100 μg GnRH IM [GnRH-2] and TAI on study Day 0). In both experiments, blood was collected before each injection for later progesterone analyses. In experiment 1, cows at nine locations (n = 1537) were assigned to either: (1) control or (2) PrePG (same as control with a PG injection on study Day −13). The PrePG cows had larger (P < 0.05) follicles on study Day −10 and more (P < 0.05) ovulated after GnRH-1 compared with control cows (60.6% vs. 36.5%), but pregnancy per TAI was not altered (55.5% vs. 52.2%, respectively). In experiment 2, cows (n = 803) at four locations were assigned to: (1) control or (2) PrePGG (same as control with PG injection on study Day −20 and GnRH injection on study Day −17). Although pregnancy per TAI did not differ between control and PrePGG cows (44.0% vs. 44.4%, respectively), cows with body condition score greater than 5.0 or 77 or more days postpartum at TAI were more (P < 0.05) likely to become pregnant than thinner cows or those with fewer days postpartum. Presynchronized cows in both experiments were more (P < 0.05) likely than controls to have luteolysis after initial PG injections and reduced (P < 0.05) serum progesterone; moreover, treatments altered the proportion of cows and pregnancy per TAI of cows in various progesterone categories before the onset of the TAI protocol. In combined data from both experiments, cows classified as anestrous before the study but with elevated progesterone on Day −10 had increased (P < 0.05) pregnancy outcomes compared with anestrous cows with low progesterone concentrations. Progesterone concentration had no effect on pregnancy outcome of cycling cows. In summary, luteal regression and ovulation were enhanced and progesterone concentrations were altered by presynchronization treatments before the 7-day CO-Synch + CIDR program, but pregnancy per TAI was not improved.