Effect of exogenous bovine somatotropin on pubertal mammary development in heifers.

Nine pairs of identical twins were used to examine the effect of exogenous bovine somatotropin on mammary growth and development in dairy heifers. One twin received a daily subcutaneous injection of somatotropin (20 IU) and the other received excipient. Treatments commenced at 8.0 mo of age (179 kg live weight) and continued for 15.6 wk, at which time heifers were slaughtered and mammary development evaluated. Treatment with somatotropin resulted in an increase in mammary parenchyma and a decrease in extraparenchymal tissue and weight of mammary glands. Increases in parenchyma were 46% as determined by computer assisted x-ray tomography and 18% as determined by dissection. Differences between the two techniques related to the ability of computer assisted x-ray tomography technique to exclude fat deposits and connective tissue from parenchymal estimates. Chemical composition (39% water, 7% protein, and 54% fat) as well as histological and cytological appearance of the mammary parenchyma were not affected by the treatment. Therefore, treatment with exogenous somatotropin around puberty enhances the growth rate of mammary parenchymal tissue. Additional studies are required to examine whether this enhanced mammary development will increase milk yield during subsequent lactations.