The effectiveness of anabolic agents in increasing rate of growth in farm animals; report on experiments in cattle.

In many countries anabolic agents are successfully used to increase the rate of growth of cattle. In the past widespread use was made of cheap synthetic estrogens such as diethylstilbestrol and hexestrol. However, new legislation in certain countries has restricted the use of synthetic estrogens. This has resulted in an intensive search by industry for alternative agents. On the one hand natural steroid hormones like estradiol, testosterone and progesterone were studied, while on the other hand products like an active compound of the resorcyclic acid lactones and the synthetic anabolic steroid trenbolone acetate were developed. Administration of anabolic agents to cattle is done in three ways. (1) The agent may be fed orally by incorporation into the concentrate feed or as a simple additive top dressing. If the agent is metabolised in the rumen, oral administration may still be possible by using coated materials which avoid rumen metabolism with subsequent absorption of the steroid from the small intestine. (2) The agent may be administered as a slow release implant, e.g. trenbolone acetate or various combined preparations of an androgen and estrogen. (3) Administration by repeated injection. This latter method is often impractical. Anabolic agents are normally administered to beef cattle or culled dairy cows during the last few months of the finishing period. Maximum weight gain performance in different types of cattle requires selection of the correct anabolic agent. Increased performance in female cattle is better when an androgenic steroid is administered. However, in intact males (bulls) best performance is only obtained when an estrogen, alone or in combination with an androgen, is administered. Castrate animals (steers) do best if a smaller amount of estrogen is combined with an androgen and administered, however there is evidence that androgen alone is as effective. In summary therefore, an additional response in growth of cattle may require the presence of an estrogen, as endogenous estrogen in the female and the exogenous form in the male. Only when this condition is met will administration of androgenic anabolic steroids result in maximum benefit, often improving the effect of estrogen given on its own. Anabolic agents have some beneficial effect on appetite. They may not affect the digestive processes of the alimentary system. They have a positive effect on nitrogen retention. The liver almost certainly has a central role in the regulation of nitrogen retention. Recent experiments suggest that urea entry rates in ruminants may be lowered, thus making available more nitrogen for protein synthesis. Muscle protein synthesis is altered with changes in carcass conformation. There are some changes in fat redistribution. How all these processes are integrated is not yet known. The possibility that anabolic agents establish a new hormonal status which is favourable to growth will be discussed...