Fertility decline in aging roosters is related to increased testicular andplasma levels of estradiol

The relationships between testicular and plasma hormone levels and the decl ine in fertility in aging roosters were examined. Body mass, testicular mas s, and fertility were measured in roosters from 20 to 72 weeks of age. Plas ma was assayed for LH and testosterone, and estradiol and testicular extrac ts were assayed for testosterone and estradiol contents. Fertility increase d rapidly in young roosters to a peak of 96.2 +/- 3.9% at 37 weeks of age. Thereafter, fertility declined and by 72 weeks of age was significantly lon er than at 37 Reeks. Plasma LH reached 16.8 +/- 2.5 ng/ml at 27 weeks and remained high until 60 weeks of age, when it decreased significantly. Plasm a and testicular testosterone levels increased from low levels in young bir ds to a peak that coincided with highest fertility and declined thereafter. Plasma and testicular estradiol showed a striking inverse relationship wit h testosterone. Plasma estradiol was 29.4 +/- 4.0 pg/ml in 20-week-old bird s, decreased rapidly as testosterone increased, and increased again in olde r birds as testosterone decreased. Thus, the decline in fertility in aging roosters was associated with a decrease in plasma LH and testosterone and a n increase in plasma and testicular estradiol. It is suggested that plasma levels of LH and testosterone in roosters are regulated by a negative feedb ack mechanism involving estradiol that is produced not only by the aromatiz ation of testosterone in the brain but also by peripheral estradiol origina ting in the testes and that estradiol has a major role in the decline in fe rtility in aging roosters. (C) 1999 Academic Press.