Responses of High-Producing Dairy Cows to Long-Term Treatment with Pituitary Somatotropin and Recombinant Somatotropin

Abstract

Holstein cows (30) capable of high milk production were utilized to examine the effects of long-term administration of bovine somatotropin on lactational performance. Treatments were 0 (control), 13.5, 27 and 40.5 mg/day of recombinantly-derived bovine somatotropin and 27 mg/day of pituitary-derived bovine somatotropin. Hormone was injected i.m. once daily beginning at 84 .+-. 10 days postpartum and continuing for 188 days. Cows were fed ad lib a total mixed diet throughout the lactation. Recombinant somatotropin treatments increased average fat-corrected milk yield in a dose-dependent fashion from 23-41% over the control production (27.9 kg/day). Pituitary somatotropin (27 mg/day) increased milk production by 16%. Milk lactose, protein and fat composition was similar for all treatments. Control cows were in positive energy balance throughout the treatment period (4.7 Mcal net energy/day). Initially, the large increase in milk yield with somatotropin treatment caused cows to decrease in energy balance. Voluntary intake gradually increased, and by wk 10 of treatment, all somatotropin treatment groups were in positive energy balance. The gain in body weight over the treatment period was similar for all groups, ranging from 17-22%. Gross lactational efficiency (milk/U net energy intake) was improved by exogenous somatotropin whether calculated as observed, corrected for body weight changes or using the theoretical energy requirements for maintenance and milk production. Results were consistent with bovine somatotropin as a homeorhetic control that coordinated an array of physiological processes so that nutrients were partitioned for milk synthesis.