PS6-13. Time to Luteolysis and Conceptus Attachment Vary with Parity in Holstein Heifers and Cows Diagnosed Non-pregnant by Ultrasound 32 Days After Timed AI.

Abstract

This observational study aimed to evaluate the association between time to luteolysis and parity in inseminated heifers and lactating Holstein cows diagnosed as non-pregnant by ultrasound at 32 d after timed artificial insemination (TAI). Ovulatory response between TAI and 1 d later was assessed in heifers and lactating cows from the same herd submitted to TAI for first or ≥ 2 services. Only animals that ovulated were included (n = 509). Blood samples were collected on d 14, daily from d 18 to 24, and on d 27 post-AI. Lactating cows received GnRH on d 25 as part of a resynchronization program. Pregnancy status was determined on d 32 post-AI, and serum progesterone (P4) and pregnancy-associated glycoproteins (PAG) were measured in non-pregnant animals (n = 234). Parity was categorized as heifers (n = 38), first lactation (Lact1; n = 55), second lactation (Lact2; n = 56), and ≥third lactation (Lact3+; n = 85). Conceptus attachment (CA) was defined as a ≥ 20% daily increase in PAG over 2 consecutive days, reaching ≥1.0 ng/mL. Time of luteolysis was defined as the interval between the last day with P4 ≥ 1 ng/mL and the first day with P4 < 1 ng/mL. Data were analyzed using Fisher's exact test. Among non-pregnant animals, 16.7% exhibited evidence of CA. The proportion of animals with CA differed by parity (P < 0.01; heifers: 0.0%; Lact1: 12.7%; Lact2: 21.4%; Lact3+: 23.5%). Time to luteolysis was affected by CA (P < 0.01) and by the interaction between CA and parity (P < 0.01). Among animals with CA, 84.6% did not undergo luteolysis by d 27, with no effect of parity. In contrast, among animals without CA, time to luteolysis differed by parity (P < 0.01). Most heifers (52.6%) underwent luteolysis between d 14 and 18, whereas a greater proportion of Lact1 and Lact3+ cows either underwent luteolysis between d 24 and 27 (22.9% and 20.0%, respectively) or did not undergo luteolysis by d 27 (20.8% and 21.5%, respectively). In Lact2 cows, luteolysis most frequently occurred between d 19 and 20 (22.7%). Progesterone concentration on d 14 and its interaction with parity was not associated with time of luteolysis in animals without CA having luteolysis after d 14. In conclusion, pregnancy loss after conceptus attachment increases with parity and is associated with prolonged luteal function following TAI. Additionally, time to luteolysis differs among parity groups in animals without evidence of conceptus attachment. These findings suggest that both early pregnancy loss and parity-related physiological factors may contribute to extended estrous cycles in lactating cows compared with heifers.