The factors that affect output in animal production systems are growth rate, feed conversion efficiency (FCE), carcass conformation, muscle and fat content, and kill out percentage. These factors are affected by genetic potential, provision of optimal nutrition and environmental conditions to disease free animals. In addition, certain exogenous chemicals are known to affect animal performance, and these can be classified as follows:

• i)use of chemical messengers which bind to specific receptors in target tissue

• ii)use of anti-microbial compounds which alter the microflora in the gastro-intestinal tract and thereby improve performance.

This paper will deal specifically with the use of beta adrenergic agonists and growth hormone (GH) which are currently being researched.