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

Abstract 

The objective of this study was to compare 2 strategies for resynchronization of ovulation based on nonpregnant diagnoses using transrectal ultrasonography or a pregnancy-associated glycoprotein (PAG) ELISA. Lactating Holstein cows (n=1,038) were submitted for first postpartum timed artificial insemination (TAI) using a Presynch+Ovsynch protocol. After the initial breeding, cows were randomly assigned to initiate resynchronization 25 d (D25) or 32 d (D32) later. Pregnancy status of cows initiating Resynch 25 d after TAI was determined 27 d after TAI by using a PAG ELISA, whereas pregnancy status of cows initiating Resynch 32 d after TAI was determined 39 d after TAI using transrectal ultrasonography. Cows diagnosed as not pregnant continued the Resynch protocol by receiving an injection of PGF_2α 7 d after the initial GnRH injection and a second GnRH injection 54h after the PGF_2α injection. Cows in both treatments were inseminated approximately 16h after the second GnRH injection. Blood samples for analysis of progesterone (P₄) were collected at the first GnRH injection of each Resynch protocol. Pregnancies per AI (P/AI) of nonpregnant cows initiating Resynch 25 vs. 32 d after first postpartum TAI did not differ 39 d after TAI and were 28.3 vs. 30.9% for D25 vs. D32 cows, respectively. Mean P₄ at the first GnRH injection of Resynch was greater for D32 than for D25 cows (3.67±0.22 vs. 2.83±0.22 ng/mL), indicating that the Resynch treatments were initiated at different stages of the estrous cycle. After blocking P₄ concentration into low (<1.0ng/mL) or high (≥1.0ng/mL) classes, P₄ class was not found to affect P/AI 39 d after TAI. Early resynchronization was not found to affect P/AI 39 d after TAI; however, early resynchronization did decrease days between inseminations and the interval from the initial nonpregnant diagnosis to conception. Earlier detection of nonpregnant cows using the PAG ELISA in conjunction with a TAI resynchronization program may improve the rate at which cows become pregnant in a dairy herd compared with transrectal ultrasonography conducted at a later stage after TAI.

Key words: resynchronization, transrectal ultrasonography, pregnancy-associated glycoprotein