This study examines the possible effects of progesterone (P4) supplementation during the
time of pregnancy recognition, from Days 15 to 17 post-artificial insemination (AI), on
reproductive performance in high-producing dairy cows. Cows in their 15th day post-AI
were alternately assigned to a control, no-treatment group (C: n ¼ 257) or treatment
group (P4: n ¼ 287) on a weekly rotational basis according to the chronologic order of their
gynecologic visit. On the basis of the odds ratio, the interaction treatment previous
placenta retention had a significant effect (P ¼ 0.02) on conception rate. Thus, cows in P4
that had not suffered a retained placenta were 1.6 times more likely to conceive 28 to
34 days post-AI than the remaining cows. In nonpregnant cows, treatment had no effect on
subsequent return to estrus or AI interval and neither were any effects of treatment
observed on twin pregnancy and early fetal loss rates. The results of this study demonstrate
the efficacy of P4 supplementations during the time of pregnancy recognition after
AI in cows without a clinical history of placenta retention.