Investigation of mechanisms associated with establishment and maintenance of pregnancy in beef cattle
Preovulatory estradiol concentrations and expression of estrus have been shown to be associated with greater pregnancy rates in beef cattle. Experiment 1 was designed to determine if supplemental estradiol (E2) at GnRH-induced ovulation would decrease pregnancy loss in postpartum beef cows. The 7-day CO-Synch + CIDR(R) protocol was used to synchronize estrus/ovulation in 326 cows. The protocol included administration of GnRH (GnRH1) on day -9, treatment with exogenous progesterone via a CIDR(R) (controlled internal drug release) from day -9 to day -2, administration of prostaglandin F2[alpha] (PG) on day -2, and an additional administration of GnRH (GnRH2) 48 h after PG (day 0) in cows that did not express estrus by that time. At 48 h post PG, cows were either estrual (Adequate endogenous E2 for estrus expression; n = 105) and assigned to the Positive Control group or nonestrual and randomly assigned to either no treatment (Low endogenous E2; Negative Control; n = 112) or administration of a 2 mL IM dose of 0.1 mg estradiol 17-[beta] (Low endogenous E2 + adequate E2 supplementation; Estradiol; n = 109). To examine the timing of embryonic loss in the preceding treatments genetically identical pools of embryos, of similar developmental stage and quality grade, were transferred on day 7. To determine if there was an effect of E2 supplementation on the timing of pregnancy failure, interferon-stimulated gene expression in peripheral white blood cells and circulating progesterone, circulating pregnancy associated glycoprotein concentrations, and transrectal ultrasonography were used to detect evidence of a conceptus/fetus on days 19, 30, and 55/58, respectively. In regards to presence of a conceptus/fetus on specific days, there was no treatment by day interaction (P = 0.9400); however, there was a main effect of day (P < 0.0001), and there was a tendency for an effect of treatment (P = 0.0800). Regardless of treatment, the proportion of cows that had evidence of a conceptus/fetus on days 7, 19, 30, and 55/58 was 100 [plus-minus] 0.02a, 40 [plus-minus] 0.02b, 32 [plus-minus] 0.02c,d, and 26 [plus-minus] 0.02D, respectively (a,b,c,dP < 0.05; D,dP = 0.0856). When the tendency for treatment as a main effect was evaluated, the Positive Control and Estradiol groups had similar (P = 0.7095) overall proportions of cows considered pregnant. The Negative Control group had lower (P = 0.0388) and tended to have lower (P = 0.0786) overall proportions of cows considered pregnant than the Positive Control and Estradiol groups, respectively. There was no treatment by Period of Loss interaction (P = 0.9690), nor a main effect of treatment (P = 1.000) when analyzing loss. There was a main effect of Period of Loss (P [less than] 0.0001) such that greater (P [less than] 0.0001) loss occurred in Period A (day 7-19) than both Period B (day 19-30) and C (day 30-55/58). The amount of loss that occurred in Periods B and C were similar (P = 0.1441). In summary, estradiol supplementation, at 48 h post PG, to nonestrual cows may potentially improve pregnancy rates; however, a second replicate is needed to solidify this conclusion. Recently, a timed AI protocol (i.e. Split-time AI) was developed in which heifers that express estrus are inseminated at a predetermined time (e.g. 66 h post PG) and heifers that are nonestrual at that time are inseminated 24 h later (e.g. 90 h). Experiment 2 was designed to test the hypothesis that delaying the first timepoint from 66 h to 72 h would increase estrous response and pregnancy rate when using sex-sorted semen. Estrus was synchronized in 794 heifers across 4 locations with the 14-d CIDR(R)-PG protocol. Heifers were administered a progesterone intravaginal insert (CIDR(R) on day 0, which was removed on day 14. On day 30, PG was administered, intramuscularly, and estrus detection aids were applied. Split-Time AI was performed based on estrous status. Within location, heifers were blocked based on breed composition, source, sire, RTS, and BW and assigned within block to one of two experimental approaches: Approach 66: heifers that were estrual prior to 66 h after PG administration were inseminated at 66 h, and remaining heifers were inseminated 24 h later (90 h), with GnRH administered to heifers that were nonestrual by that time; or Approach 72: heifers that were estrual prior to 72 h were inseminated at 72 h, and remaining heifers were inseminated 24 h later (96 h), with GnRH administered to heifers that were nonestrual by that time. Within approach, heifers were preassigned to receive either SexedULTRA 4M(TM) sex-sorted or conventional semen. Overall estrous response did not differ between approaches. However, the proportion of heifers that were estrual by the first timepoint (66 h or 72 h following PG administration) was greater (P < 0.0001) with Approach 72 (76 %; 302/395) compared to Approach 66 (61 [percent]; 242/399). Pregnancy rates to STAI differed (P = 0.0005) between semen type but were not affected by an approach x semen type interaction or by approach. Conventional semen (59 [percent]; 240/404) pregnancy rates to STAI were greater than sex-sorted semen (48 %; 187/390). Among heifers that were estrual by Timepoint 1, pregnancy rates tended (P = 0.08) to differ between semen type (Conventional: 62% [174/280]; Sex-sorted: 55% [146/264]). Pregnancy rates of nonestrual heifers were reduced (P < 0.01) with sex-sorted semen (25%; 15/61) compared to conventional semen (41%; 23/56). In summary, when using sex-sorted or conventional semen for STAI in heifers following the 14-d CIDR(R)-PG protocol, pregnancy rates did not differ when timing of STAI was delayed by 6 hours. However, the proportion of estrual heifers prior to the first timepoint for STAI was greater when using the later timepoints.