Effect of somatotropin on kinetics of nonesterified fatty acids and partition of energy, carbon, and nitrogen in lactating dairy cows.

The utilization of energy, carbon, and nitrogen, and irreversible loss rates of nonesterified fatty acids were examined in lactating cows during treatment with bovine somatotropin. Six multiparous Holstein cows at approximately 62 d postpartum received daily intramuscular injections of somatotropin (29.4 mg/d) or excipient during two 21-d treatment periods in a crossover experimental design. Milk yield was increased 11% (37.1 versus 41.3 kg/d), whereas intake and digestibility of DM, energy, nitrogen, and carbon were not affected. Cows remained near zero net energy balance during both treatments (1.31 versus -1.52 Mcal/d for control and somatotropin, respectively). The major effect of somatotropin was to partition net energy into milk while tissue energy accretion was reduced. Postabsorptive use of nitrogen and carbon was altered by somatotropin as reflected by greater secretion in milk. Urinary losses of nitrogen and tissue use of carbon were reduced. Plasma concentrations and irreversible loss rates of nonesterified fatty acids did not differ between treatments. Somatotropin treatment of lactating cows results in partitioning of nutrients away from tissue deposition toward milk synthesis. The extent to which different metabolic processes are altered depends upon the energy balance of the somatotropin-treated animal.