Trophoblast differentiation and embryonic development are delayed in lactating cows compared to nulliparous heifers.

The objective of this study was to evaluate the effects of parity on early embryonic development and placentation in Holstein cattle. Pregnant cows were classified as nulliparous (n = 16), primiparous (n = 7), or multiparous (n = 8) after estrous synchronization for timed artificial insemination (d 0 = day of timed AI). Pregnancy was confirmed on d 28 after AI via transrectal ultrasonography. Cervical epithelial cells were collected daily from d 14 to 20 to assess ISG15 and MX2 mRNA abundance. Blood samples were collected on d 0, 4.5, 7, 10, and 12 and daily from d 14 to 30; additional samples were collected every 12 h from d 18 to d 24 to assess circulating pregnancy-associated glycoproteins (PAG) concentration. Data were analyzed using mixed-effects models for repeated measures, one-way ANOVA, and Pearson correlation, with Tukey-adjusted comparisons and significance set at P < 0.05. There was only an effect of time for MX2 abundance peaking on d17 and decreasing thereafter. For ISG15, mRNA abundance was greater in primiparous and multiparous compared with nulliparous on d 17, and greater in primiparous than nulliparous on d 18. Concentrations of PAG started to increase earlier in nulliparous heifers (d 18.9 ± 0.1; ±SEM) compared with primiparous (d 20.0 ± 0.3) and multiparous cows (d 20.6 ± 0.3). Overall, PAG was greater in nulliparous than primiparous from d 20 to 21.5 and in nulliparous than multiparous from d 19.5 to 22. Concentrations of progesterone (P4) were greater in nulliparous heifers from d 4.5 to 12, and greater P4 concentration was associated with earlier increase in PAG. In conclusion, this study demonstrates that parity influences early embryonic development, with earlier trophoblast differentiation and acquisition of PAG secretory capacity in nulliparous compared with primiparous and multiparous lactating cows.