Serum growth hormone concentrations after growth hormone or thyrotropin releasing hormone in cows

A single subcutaneous injection of 10, 50, or 100 mg bovine growth hormone into lactating Holstein cows increased concentrations of growth hormone, insulin, and glucose in serum above preinjection baselines for at least 16 h. Growth hormone concentrations in serum after injection of growth hormone or thyrotropin-releasing hormone were greater in cows during early (2 to 4 mo) as compared with cows during late (8 to 12 mo) lactation. Stage of lactation did not affect rate of metabolic clearance of growth hormone in eight cows, but larger cows cleared growth hormone more rapidly than smaller cows. Data of these experiments may help to explain how injections of growth hormone every 3 days stimulate milk yields.