The efficacy and mechanism of action of anabolic agents as growth promoters in farm animals

Abstract

Anabolic agents are substances with physiological functions similar to those of the sex steroids which increase nitrogen retention and protein deposition in farm animals. When used correctly they significantly increased the daily live-weight gain and food conversion efficiency (FCE) in cattle and sheep but were less effective in pigs and poultry. A hypothesis to explain the role of sex steroids in the growth rate of cattle suggested that both androgens and oestrogens were necessary to realise the maximum growth rate. Thus in practice the best responses in growth rate and also FCE were observed when androgens were used in females and androgens combined with oestrogens were administered to steers. The mechanism of action of anabolic agents is not fully understood. The actions of androgens and oestrogens may be different. There is the possibility that androgens may have a direct action on the muscle cell. Oestrogens, but not androgens, may act indirectly through the regulation of the concentration of growth hormone and insulin in the plasma. Evidence is also presented to show that both androgens and oestrogens depress the concentrations of thyroid hormones in the plasma of cattle and sheep and this effect may be associated with their mechanism of action.