Changes in concentrations of plasma immunoreactive follicle-stimulating hormone, luteinizing hormone, estradiol-17 beta, testosterone, progesterone, and inhibin in heifers from birth to puberty

This study was designed to clarify the characteristics of changes in plasma concentrations of reproductive hormones in heifers from birth to puberty. Weekly or daily hormonal changes were observed in 39 heifers. Daily changes in the concentration of follicle-stimulating hormone (FSH) demonstrated a consistent cycle of hormone changes over a 7- to 8-day period in heifers fr om approximately 10 days to 9 months old. Weekly changes in reproductive ho rmones showed that there were three brief periods in heifers between birth and puberty in which dramatic changes occur. The first period was the first week after birth, during which a reciprocal relationship between steroid h ormones and gonadotropins was observed. At birth, the concentrations of ste roid hormones were higher than those at any other age. These hormone levels rapidly decreased within the first week after birth. Gonadotropin levels, however, increased from birth to 1 week of age. The second period of major change was at approximately 4 weeks of age when there was an increase in th e concentrations of luteinizing hormone (LH), estradiol-17 beta, testostero ne, and immunoreactive inhibin. The third period was the last 5 weeks befor e the first ovulation, when there was an increase in the concentrations of estradiol-17 beta followed by an increase in (LH). These results suggest th at regular hormone changes start from 10 days after birth and that the peri ods from birth to 4 weeks of age and the last 5 weeks before the first ovul ation in heifers are important to the development of reproductive functions before puberty. (C) 2000 Elsevier Science inc. All rights reserved.