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ABSTRACT: The study evaluated strategies for induction of ovulation at the end of timed AI (TAI) protocols initiated after a novel presynchronization strategy. A total of 909 lactating dairy cows from 6 dairy herds initiated a presynchronization protocol on d −15 with an intravaginal progesterone (P4) implant and 7 d later (d −8) were treated with 1.0 mg of estradiol cypionate (EC) and 0.530 mg of cloprostenol sodium (a synthetic analog of prostaglandin F2α; PGF), simultaneously with the P4 implant removal. On d 0, the protocol for synchronization of ovulation was initiated with 16.8 µg of buserelin acetate (a synthetic analog of GnRH) and insertion of a 2.0 g P4 implant, followed by a treatment with 0.530 mg of PGF on d 6, and a second PGF on d 7, concomitant with the P4 implant withdrawal. In Group EC, cows received 1.0 mg of EC on d 7 as an ovulation inducer. In G group, cows received 8.4 µg of GnRH 56 h after the first PGF (16 h before TAI). In Group EC/G, cows received both EC and GnRH. The TAI was performed on d 9 (48 h after P4 withdrawal) in all experimental treatments. There were no differences among treatments on pregnancy per AI (P/AI) on d 31 (40.4%; 367/909). There was a tendency for greater pregnancy loss in G group (19.8%) than in EC (12.2%) and EC/G (10.1%) groups. When the 2 groups that received EC were combined, the pregnancy loss was lower than when cows received only GnRH (11.2% [21/188] vs. 19.8% [17/86]; P = 0.05). There was an interaction between treatment and expression of estrus on P/AI, in which estrus positively affected P/AI in EC (53.3% vs. 20.0%) and EC/G (53.1% vs. 36.7%), but not in G group (41.5% vs. 38.7%). Overall, 81% of the cows had a corpus luteum (CL) on d 0 and 91% at PGF treatment of the breeding protocols. Fertility was greater in cows with CL at the time of the first PGF treatment than in cows without CL (45.9% vs. 17.7%). In summary, the strategies to induce final ovulation resulted in similar fertility despite the different physiological responses, such as expression of estrus, but further research is needed to definitively confirm the relationship between lack of EC and increased pregnancy loss.

ABSTRACT: This study aimed to determine the effect of circulating progesterone (P4) concentrations produced by a corpus luteum (CL) or released by an intravaginal P4 implant (IPI) on GnRH-induced LH release, ovulatory response, and subsequent CL development, after treatment with 100 μg of gonadorelin acetate (GnRH challenge). Nonlactating multiparous Holstein cows were synchronized and GnRH was used to induce ovulation (d −7). Over 4 replicates, cows that ovulated (n = 87) were randomly assigned to a 2 × 2 factorial arrangement (presence or absence of CL and insertion or not of an IPI at GnRH challenge), creating 4 groups: CL_IPI, CL_NoIPI, NoCL_IPI, and NoCL_NoIPI. On d −1.5, NoCL_IPI and NoCL_NoIPI received 2 doses of 0.53 mg of cloprostenol sodium (PGF2α), 24 h apart to regress CL. On d 0, cows were treated with 100 μg of GnRH and, simultaneously, cows from IPI groups received a 2-g IPI maintained for the next 14 d. Diameter of dominant follicle, ovulatory response, and subsequent CL volume were assessed by ultrasonography on d −1.5, 0, 2, 7, and 14. Blood samples were collected on d −1.5, 0, 1, 2, 3, 5, 7, and 14 for analysis of circulating P4 and at 0, 1, 2, 4, and 6 h after GnRH challenge for analysis of circulating LH. In a subset of cows (n = 34), the development of the new CL was evaluated daily, from d 5 to 14. The presence of CL at the time of GnRH challenge affected the LH peak and ovulatory response (CL: 5.3 ng/mL and 58.1%; NoCL: 13.2 ng/mL and 95.5%, respectively). However, despite producing a rapid increase in circulating P4, IPI insertion did not affect LH concentration or ovulation. Regardless of group, ovulatory response was positively correlated with LH peak and negatively correlated with circulating P4 on d 0. Moreover, new CL development and function were negatively affected by the presence of CL and by the IPI insertion. In summary, circulating P4 produced by a CL exerted a suppressive effect on GnRH-induced LH release and subsequent ovulation of a 7-d-old dominant follicle, whereas the IPI insertion at the time of GnRH had no effect on LH concentration or ovulation. Finally, elevated circulating P4, either from CL or exogenously released by the IPI, compromised the development and function of the new CL, inducing short cycles in cows without CL at the time of GnRH treatment.

Because progesterone (P4) is essential for pregnancy establishment and maintenance, we investigated the effect of increased concentrations of P4 on embryonic attachment and concentrations of pregnancy-associated glycoproteins (PAG). Additionally, we investigated the relationships among luteal regression, pregnancy loss, and PAG concentrations in cows undergoing pregnancy loss by d 33 of pregnancy. Lactating dairy cows were allocated into control (n = 40) and human chorionic gonadotropin (hCG; 3,300 IU on d 7 and 13 to promote greater circulating P4; GnRH = d 0; n = 46) groups. Progesterone was measured daily from d 7 to 33, and PAG was measured daily from d 17 to 33; both hormones were also measured on d 47 and 61. An increase in PAG >10% compared with d 17 was considered a marker for pregnancy. The gold standard for pregnancy diagnosis was ultrasound evaluation of embryonic heartbeat on d 33. Statistical analyses were done with PROC MIXED from SAS Institute Inc. Concentrations of P4 were greater from d 8 onward in the hCG group. Concentrations of PAG did not differ between groups from d 17 to 33, suggesting no effect of increased P4 on hastening embryonic attachment and placental development. Nevertheless, PAG was greater in the hCG group on d 47 and 61, suggesting greater placental area or PAG secretory capacity. Pregnancy loss between d 20 and 33 occurred in 24.6% of cows. About 50% of pregnancy loss was due to luteal regression and about 50% was due to conceptus failure; that is, a decrease in PAG in the absence of luteal regression. In conclusion, increased P4 does not hasten embryonic attachment or early placental development but it leads to increased PAG in the second half of the second month of gestation. Additionally, pregnancy loss seems to be initiated by either corpus luteum regression or conceptus failure.