Characterization of luteal dynamics in lactating dairy cows for 32 days after synchronization of ovulation and timed artificial insemination

Abstract Text: Approximately 20% of cows diagnosed not pregnant 32 d after TAI do not have a CL, and cows that begin a resynchronization protocol in the absence of a CL have about 10% fewer pregnancies per AI compared to cows with a CL. An understanding of luteal dynamics after synchronization of ovulation and timed AI (TAI) may help to refine strategies for resynchronizing cows failing to conceive. Lactating Holstein cows (n=141) were synchronized for first TAI (80 to 86 DIM) using a Double Ovsynch protocol. Thrice weekly (MWF) from 4 to 32 d after TAI, luteal diameter was measured using ultrasonography and blood samples were collected for evaluation of progesterone (P4) concentrations. Pregnancy status was determined using ultrasound 32 d after TAI. Cows (n=13) were removed if they had twins (n = 2), if they did not synchronize (n=4), or if they had pregnancy loss (n=7). For cows diagnosed pregnant (n=48), luteal volume increased from 4 to 13 d after TAI then remained constant until 32 d, whereas P4 increased from 4 to 15 d after TAI then remained constant until 32 d. For cows diagnosed not pregnant 32 d after TAI (n = 80), P4 profiles were evaluated using statistical cluster analysis (PROC CLUSTER of SAS) based on the day after TAI that P4 decreased to < 1 ng/mL. Cows diagnosed not pregnant were segregated into 5 clusters: 1) luteal regression 15 d after TAI (1.3%, 1/80); 2) luteal regression 18 to 22 d after TAI (55.0%, 44/80); 3) luteal regression 25 to 27 d after TAI (18.8%, 15/80); 4) luteal regression 29 to 32 d after TAI (3.8%, 3/80); and 5) original CL present 32 d after TAI (21.3%, 17/80). Pregnancy-associated glycoproteins (PAG) were measured in serum samples collected 25 and 32 d after TAI using a commercial assay (IDEXX Laboratories, Inc., Westbrook, ME). Relative serum PAG levels (mean ± SEM S-N values) differed among clusters at both 25 (P = 0.03) and 32 (P < 0.01) d after TAI and were similar for cows in clusters 2, 3, and 4 but were greater for cows in cluster 5. We conclude that cows maintaining their original CL for 32 d after TAI were initially pregnant but underwent pregnancy loss based on residual serum PAG levels at 24 and 32 d after TAI. Supported by Hatch project WIS01171

Keywords: Progesterone; Luteal Dynamics; Pregnancy loss