Profiles of serum IGF-I concentrations in calves from birth to eighteen months of age and in cows throughout the lactation Cycle

Blood samples were obtained from 16 female and 7 male holstein calves at birth and at 3-mo intervals until 15 to 18 mo of age. Monthly blood samples were also obtained from 34 holstein cows beginning 1 mo prepartum and continuing until the subsequent dry period. Serum concentrations of IGF-I were determined by RIA after serum-binding proteins had been removed by acid-gel chromatography. In calves there was an age-dependent increase in serum IGF-I concentrations, which reached a plateau after 9 mo of age. From 3 mo of age, these concentrations were greater in male animals. Analysis of data after retrospective grouping of female calves into fast, medium and slow growth rate groups indicated different (P < 0.05) IGF-I concentrations between groups; levels were greatest in the fastest-growing animals and lowest in the slow growth rate group. However, only weak relationships existed between serum IGF-I levels at 3 or 6 mo of age and body weight at 15 or 18 mo of age. In lactating animals, serum IGF-I concentrations were lowest at parturition. These levels slowly increased throughout lactation and returned to pre-partum levels within 15 d of drying off. Weak, negative relationships (P < 0.01) were found between serum IGF-I concentrations and milk production in 11 first lactation heifers (R = −0.34) and in 23 cows (R = −0.25). Serum IGF-I levels were not different between groups composed of the five best or five worst producing cows. The results of this study indicate that serum IGF-I concentrations are of little use in predicting lactational performance of dairy cows or growth performance of calves.