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Responses of High-Producing Dairy Cows to Long-Term Treatment with Pituitary Somatotropin and Recombinant Somatotropin^{1, 2,}

Department of Animal Science, Cornell University, Ithaca, NY 14853-0281

Melvin J. DeGeeter and Gregory M. Lanza

Monsanto Company, St. Louis, MO 63167

ABSTRACT

Thirty Holstein cows 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 intramuscularly, once daily beginning at 84 ± 10 days postpartum and continuing for 188 days. Cows were fed ad libitum a total mixed diet throughout the lactation. Recombinant somatotropin treatments increased average fat-corrected milk yield in a dose-dependent fashion from 23 to 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 Meal net energy/day. Initially, the large increase in milk yield with somatotropin treatment caused cows to decrease in energy balance. However, voluntary intake gradually increased, and by week 10 of treatment all somatotropin treatment groups were in positive energy balance. Thus, the gain in body weight over the treatment period was similar for all groups, ranging from 17 to 22%. Gross lactational efficiency (milk per unit of 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 are consistent with bovine somatotropin as a homeorhetic control that coordinates an array of physiological processes so that nutrients are partitioned for milk synthesis.

¹ Supported by Cornell University Agricultural Experiment Station and Monsanto Co.

² The authors gratefully acknowledge the assistance of staff at Cornell University (M. Partridge, H. N. Erb, G. Birdsall, R. D. Smith, D. M. Galton, W. R. Butler, B. Palhof, B. M. Perkins, T. Kimmich, and the workers at the Cornell University Dairy Teaching and Research Unit) and staff at Monsanto Co. (G. L. Bachman, L. A. Bentle, J. A. Duque, K. A. Dwyer, R. W. Hammond, R. L. Hintz, R. G. Hoffman, K. Jacobson, V. K. Meserole, J. W. Mitchell, F. W. Morgan, J. P. O'Donovan, G. J. Rogan, R. L. Ryan, R. H. Taylor, and D. H. Will).

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