Pattern of gonadotropin secretion and ultrasonographic evaluation of developmental changes in the testis of early and late maturing bull calves

This was a study that retrospectively analyzed serum gonadotropin secretion and the ultrasonographic appearance of the testis during development in pr epubertal bull calves to determine whether there were differences between e arly and late maturing bulls. Blood samples were taken every other week fro m 2 wk of age until puberty. Samples were also taken at 12 minute intervals for 12 hours at 4, 10, 20, 25, 30, 35, 40 and 45 wk of age. The GnRH treat ment was administered 10 hours after the start of each period of frequent b lood sampling. Bull calves fell into two distinctive groups, with one group maturing between 36.6 and 44.2 wk (n = 12) and the other between 46.4 and 48.9 wk of age (n = 8). In samples taken every other week mean serum LH con centrations were greater in early maturing bulls than in late maturing bull s at 12, 14 and 16 wk of age (P < 0.05). In blood samples taken every 12 mi nutes for 10 hours early maturing bull calves had higher mean serum LH conc entrations at 4 and 10 wk of age (P < 0.05) and higher LH pulse frequency a t 10 and 20 wk of age (P < 0.05). Mean serum LH concentrations at 4, 10 and 40 wk of age and LH pulse frequency at 10 and 20 wk of age were negatively correlated with age at puberty in bull calves. Mean pixel units of the rig ht and left testis were higher from 34 to 40 wk of age in early maturing th an in late maturing animals (P < 0.05). It seems possible that hormone meas urements and ultrasonographic characteristics of the testes could be develo ped into powerful tools for studies on the regulation of reproductive devel opment and may aid in the prediction of reproductive potential. (C) 2000 by Elsevier Science Inc.