Effect of interval to resynchronization of ovulation on fertility of lactating Holstein cows when using transrectal ultrasonography or a pregnancy-associated glycoprotein enzyme-linked immunosorbent assay to diagnose pregnancy status

Protocol for blood sample collection and ultrasound examination for determination of pregnancy status in each Resynch treatment. Cows were enrolled in the study at the second PGF_2α injection of the Presynch + Ovsynch protocol (d −22). Nonpregnant cows after the first timed AI (TAI) were randomized to receive 1 of 2 resynchronization protocols beginning 25 (D25 Resynch) or 32 (D32 Resynch) d after TAI. Cows in the D25 Resynch treatment received the first GnRH injection of the Resynch protocol 25 d after TAI before assessment of pregnancy diagnosis. At 27 d after TAI, pregnancy diagnoses were determined by pregnancy-associated glycoprotein (PAG) ELISA assay, and nonpregnant cows continued the Resynch treatment with a PGF_2α injection at 32 d and a second GnRH injection 54h after the PGF_2α injection. Cows in the D32 Resynch treatment received the first GnRH injection 32 d after TAI 7 d before pregnancy diagnosis, and pregnancy status was established 7 d later by transrectal ultrasonography examination at 39 d. Cows diagnosed as not pregnant continued the Resynch treatment with a PGF_2α injection immediately after transrectal ultrasonography examination and a GnRH injection 54h after the PGF_2α injection. Cows in both Resynch treatments received TAI approximately 16h after the second GnRH injection. If cows failed to conceive after the first Resynch treatment, they remained in the same treatment for a second Resynch and TAI. A second pregnancy diagnosis by transrectal ultrasonography occurred 62 d after TAI to reevaluate pregnant cows from the D25 and D32 Resynch treatments after the transrectal ultrasonography evaluation at 39 d. Blood samples were collected at the initiation of each Resynch protocol to determine serum progesterone concentration. P=25mg of PGF_2α; G=100μg of GnRH; BS=blood sample for determination of progesterone concentration; PAG=blood sample collection for determination of PAG concentration; TU=transrectal ultrasonography; D=day; T=Tuesday; R=Thursday; F=Friday.