Title

The ontogeny of the somatotropic axis in male and female Hereford calves from birth to one year of age and its response to exogenous somatotropin

Date of Completion

January 2003

Keywords

Biology, Animal Physiology|Agriculture, Animal Culture and Nutrition

Degree

Ph.D.

Abstract

The somatotropic axis is important in the regulation of growth and development. In beef cattle, manipulation of the somatotropic axis through the use of bovine (b) somatotropin (ST) has been reported to increase growth rate, and improve feed efficiency and carcass quality. We have reported an age-related response to bST treatment in growing calves, which may be associated with changes in insulin-like growth factor (IGF) binding proteins (BP)-2 and -3. Changes in components of the somatotropic axis, ST, IGF-I, IGFBP-2, and IGFBP-3, have been examined in cattle, however samples analyzed for IGFBP have been collected at infrequent intervals and in response to different physiological states. Therefore, the first objective of this dissertation was to examine the ontogeny of the somatotropic axis every week, from birth to one year of age, in growing Hereford calves. The second objective was to examine the response of the somatotropic axis to bST, every 50 d, from birth to one year of age. In experiment one, males began to grow faster than females at about 110 d of age. At this same time point, concentrations of ST began to decline in the males and concentrations of IGF-I began to plateau in males and females. Average concentrations of IGFBP-3 were greater in males, and concentrations of IGFBP-2 were greater in females. In experiment two, all calves were responsive to bST treatment beginning at birth, with an increase in ST, IGF-I, and IGFBP-3. Similar to experiment one, age-related changes were observed, however increased growth rate and greater concentrations of IGFBP-3 in males were not observed. Similar to our previous work, in which growth rate was increased in bST treated calves older than 250 d of age, concentrations of IGFBP-3 increased following bST treatment at 250 d of age. This indicates the importance of IGFBP-3 in the growth response to bST treatment in calves. Therefore, we conclude that calves are more responsive to bST at about 250 d of age due to the greater magnitude of response in concentrations of IGFBP-3. ^