Effect of experimental manipulation of circulatory cortisol levels in lambs on their growth rate and carcass quality

Abstract

Relationships between the activity of the adrenal cortex and the productivity of lambs were investigated by administering cortisol acetate or melengestrol acetate. Initial experiments indicated that cortisol acetate administered every second day effectively maintained elevated levels of serum cortisol and that the administration of melengestrol acetate brought about some suppression in the levels of serum cortisol. However, when administered to growing lambs over an 8-week period, melengestrol acetate at a rate of 0.6 mg every second day had no measurable effect on serum cortisol. Cortisol acetate at a dosage of 50 mg every second day increased serum cortisol concentrations for the first 4 weeks only, after which they declined to the levels in the control lambs. In the group treated with cortisol acetate, serum and adrenal levels of cortisol at slaughter were significantly lower than the control group, but there were no differences in meat tenderness. Elevated levels of cortisol at slaughter were obtained by acute administration of cortisol acetate at the rate of 150 mg/day for 3 days. Meat from these animals was appreciably more tender than that from the controls. It is concluded that the negative relationships reported to exist between endogenous cortisol levels and growth rate or meat tenderness of cattle, either do not exist in lambs or they are not the simple cause and effect type.