Effects of Postnatal Implanting With A Commercial Growth Promotant on Bovine Uterine Development

Abstract

Clinicians investigating heifer infertility have associated implanting hormonal growth promotants in neonatal calves with subsequent reproductive failure. Specifically, controlled studies have shown that heifers implanted with zeranol (Ralgro®) at birth have significantly decreased pregnancy rates as yearlings. It has long been established that neonatal animals of several species, exposed to exogenous steroid hormones and zenobiotics, undergo various alterations in the development of their urogenital tracts. For example, human female fetuses exposed to diethylstilbestrol (DES) in utero have an increased risk of uterine and cervical neoplasia as women. Could such developmental alterations occur in implanted heifers?

This study was designed to investigate the effects of 100 mg progesterone and 10 mg estradiol benzoate (Synovex^® C) implanted in neonatal heifers on the reproductive tracts of the same animals as adult, 15-month-old heifers. Four groups of five heifers per group were implanted either at birth, 21 days of age, 45 days of age (the earliest on-label dose), or not implanted (controls). The heifers were allowed to develop until 15 months of age, when they were slaughtered while in the luteal phase of a synchronized estrus cycle, and their uterine tissues were examined. The results will be discussed as well as potential implications for beef heifers in this presentation.