Follicle StimulatingHormone,LuteinizingHormone and Estradiol-1713Responsein GnRH Treated Prepuberal Holstein Heifers'

Theeffect ofi.v. injection of200 @tg gonadotropin releasing hormone(GnRU)onplasmaLU, FSH andestradiol-1 713(E2-17i3) wasinvestigated in18prepuberal Holstein heifers ateither 3,6or 9 months of age. GnRH caused increased levels of both LU and FSU within 20 min postinjection. Both LU and FSU plasma concentrations reached peak values by 20 mm following treatment in 3- and 9-month-old heifers, but in 6-month-old animals neither hormone reached maximum con centration until 2 h postinjection. Levels of both gonadotropins were not different with age. The profiles of both gonadotropins were similar following GnRU treatment (r = 0.86), but the relative magnitude of LU response (30-fold) was greater than FSU response (8-fold). FSU response was less variable in9-month-old heifers thaninyoungeranimals. Concentration of plasma E2-1713was not altered by GnRH treatment by 6 h postinjection. It appears thattheprepuberal heifer iscapable ofreleasing large quantities ofbothLU andFSH and that this response is not different at 3, 6 or 9 months of age. However, it does not appear that the GnRU-induced rise in plasma gonadotropins is sufficient stimulus to promote increased E2-1713 production within 6 h after GnRU administration in the prepuberal heifer.