Increasing progesterone (P4) during conceptus development is crucial for establishment of pregnancy in dairy cattle. We developed a robust technique to estimate d of conceptus attachment (CA) based on daily within-cow changes in concentrations of pregnancy-specific protein B (PSPB; BioPRYN). Highest sensitivity and specificity were obtained with a 12.5% increase in addition to 2 consecutive d of continuous increases when predicting CA. We hypothesized that increasing serum P4 post-ovulation would favor embryonic development and reduce time to CA in lactating dairy cows. The first objective was to determine the effects of treatment with hCG on d2, d5, and d2 and 5 post-ovulation on luteal function and serum P4. Treatment with hCG on d 2 & 5 or on d 5 increased P4 by inducing accessory CL (aCL) and increasing the volume of existing CL. On d 2, hCG increased P4 on d 5 post-ovulation. The second objective was to investigate the effects of treatment with hCG on time to CA. Treatment on d 5 reduced the percentage of cows with CA and increased time to CA. Primiparous cows benefited from ipsilateral aCL against pregnancy loss before d 100 post-AI/ovulation. Delayed time to CA was associated with pregnancy losses before d 35. The highest quartile of P4 on d 5, but not on d 19 & 20, was associated with reduced time to CA. Our hypothesis was not confirmed. Yet, early serum P4, but not late, was associated with time to conceptus attachment in multiparous dairy cows. Our findings indicated that treatment with hCG post-ovulation was not financially advantageous as treatment on d 5 or aCL induced by treatment reduced annual herd profits compared to standard reproductive programs such as Double-Ovsynch and Ovsynch.
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