Effect of Acute Challenge with an Extreme Dose of Somatotropin in a Prolonged-Release Formulation on Milk Production and Health of Dairy Cattle

Eight pregnant Holstein cows were given weekly injections of 15g of recombinant bST over a 2-wk period for a total dose of 30g to determine signs of acute toxicity. Cows were monitored intensively throughout the study, and samples were taken for analyses of hormones, metabolites, chemistries, hematology, and urine analytes. Animal health throughout the study was generally excellent. Mean rectal temperatures were significantly higher in treated cows (38.7 vs. 39.2°C). Least squares means for 3.5% FCM production were 15.9 and 23.0 kg/d, and net energy intakes were 29.4 and 26.9 Mcal/d for control and treated cows, respectively. Somatotropin concentrations reached more than 250 ng/d on d 10 and remained above 200ng/ml. Insulin and glucose concentrations were also increased but returned toward baseline values. Free fatty acid concentrations were higher in treated cows, but β-hydroxybutyrate was not affected. Most hematological measurements were unaffected except for a reduction in erythrocyte number in treated cows and decreases in hematocrit and hemoglobin, but values were within clinically normal ranges. Although cows received in 2 wk a dose that was equivalent to the amount administered during more than 2 yr of continuous use, no signs of acute toxicity to bST were observed.