EFFECT OF PATTERN OF ADMINISTRATION OF BOVINE GROWTH-HORMONE ON LACTATIONAL PERFORMANCE OF DAIRY-COWS

Bovine growth hormone was administered to Holstein cows (late lactation) in a 4 .times. 4 Latin square design. Treatments were (1) control, subcutaneous injection of placebo, (2) subcutaneous injection of 25 IU growth hormone every 2nd d, (3) daily subcutaneous injection of 25 IU growth hormone, (4) continuous subcutaneous infusion of 25 IU/d growth hormone. Daily profiles of growth hormone in plasma consisted of: (1) a relatively constant concentration averaging 4.5 ng/ml for control, (2) a single peak of 21 to 25 ng/ml followed by a steady decline to baseline for treatments involving daily or alternate day injection (in the latter case concentrations maintained at baseline during the 2nd d after injection), and (3) a relatively constant elevation to 10.6 ng/ml for the continuous subcutaneous infusion. Total growth hormone in plasma increased in proportion to average daily dose of exogenous growth hormone. Milk yield responses in the daily injected and infused groups did not differ and averaged 28% over controls. Gross efficiency (milk energy/consumed energy) was increased 16% by daily administration of 25 IU growth hormone. Responses to the alternate day injection were approximately half those predicted for an average dose of 12.5 IU/d. No particular pattern in plasma growth hormone concentration need be attained to achieve lactational response to exogenous growth hormone. However, extending the interval between injections beyong 24 h may diminish the response per unit of growth hormone injected.