Effect of increasing energy levels of diet and heat synchronization on steroid hormones and reproductive indices in Holstein heifers

The aim of the present study was to determine the effects of increasing energy levels of diet and heat synchronization on steroid hormones and fertility in Holstein heifers. Eighty heifers with average bodyweight of 330±20 kg were randomly allocated in equal numbers into four groups including control (A), high energy diet (B) (10% higher than NRC recommendation), heat synchronization (C) (CIDR-Ovsynch) and high energy diet heat synchronization (D). Heifers in groups B and D were fed initially one year and 30-day pre-synchronization periods, and they were inseminated after observation of heat. Blood samples were collected after first heat signs (groups A & B) and at the beginning of synchronization in each phase of estrus cycle for the determination of plasma progesterone, estrogen and BHBA concentrations. Also, reproduction indices such as service per conception rate and intervals between first heat and AI leading to pregnancy were evaluated. The results showed that estrogen level was significantly increased (p