Effects of long-term, near-term, and real-time energy balance, and blood progesterone concentrations, on the pregnancy rate of contemporary dairy cows.

This study aimed to contribute to understanding the interface between reproductive and nutritional energetic physiology in contemporary dairy cattle. Multiparous Holstein cows (n = 32) between 70 and 180 days in milk were used in a study starting 10 d prior to the artificial insemination (AI) date and were estrous synchronized using a hormonal regimen. Fourteen cows were determined pregnant on day 39 post-AI. Coccygeal blood samples of all cows were collected on d -10 and -3 prior to AI to determine estrous cyclicity, as well as at AI and at 6, 13 and 20 d post-AI. Milk progesterone was measured 20 d post-AI, and body condition was scored (BCS; 1-5 scale) on days -10, 0, 13 and 27 relative to AI. Blood non-esterified fatty acid concentrations, measured on the same days as BCS, and changes of BCS from d -10 to AI were not predictive of pregnancy outcome. The BCS of cows on the day of AI was greater (P = 0.02) for pregnant cows with an approximate minimum BCS for a high probability of conception being 2.50. Serum progesterone concentrations of pregnant cows were greater (P < 0.05) on days 6, 13 and 20 post-AI, as was milk progesterone at day 20 post-AI (P < 0.01). Pregnant cows had greater (P = 0.02) net energy output (NEL ), which is inconsistent with a common belief that low pregnancy rates in contemporary dairy cows are due to excessive milk production, but is consistent with published studies in this study area. The present research indicates that current low pregnancy rates in commercial high-producing multiparous dairy cattle may be partly due to breeding cows that have insufficient BCS to support pregnancy.