Bioactivity of milk from bST-treated cows

Hypophysectomized male Sprague-Dawley rats were fed milk from cows treated with recombinant bovine somatotropin (bST) to determine whether immunoreactive bST in milk has a growth-promoting effect following oral ingestion. The first three treatment groups were fed either 1) milk from control cows, 2) milk from control cows with 500 ng bST/mL added directly to the milk or 3) milk from bST-treated cows. Treatments 4, 5 and 6 were the same as 1, 2 and 3, respectively, except that the milk was heat-treated before being fed. Although injection of bST into hypophysectomized rats (0, 5, 20 or 80 micrograms bST/d) resulted in a dose-dependent response in average daily weight gain, width of the tibial epiphyseal growth plate and fractional weights of selected organs, ingestion of milk containing added bST (resulting in intakes of 1.3 +/- 0.03 mg bST/d) had no effect on these growth indices. Levels of immunoreactive bST in milk from bST-treated cows were not significantly different from those of untreated cows (4.2 +/- 1.9 ng/mL versus 3.3 +/- 1.7 ng/mL, respectively, p greater than 0.05). Heat treatment reduced the immunoreactive quantities of bST in milk by 85 to 90% (p less than 0.001). Feeding heat-treated milk to hypophysectomized rats resulted in reduced liver weights (p less than 0.001) compared to those of similar rats fed milk that was not treated by heat. The effects of milk from bST-treated cows were not different from those of milk from untreated cows following oral ingestion by hypophysectomized rats.