These are prescribed by grams (g) of active or generic substance per tonne of feed. For example, 500g per tonne (1000kg) of oxytetracycline (OTC) means that there are 500g of active medicine mixed in a tonne of feed. However, the manufacturer's product is normally available as a supplement - a mixture of the generic substance and usually a cereal base. Terramycin (OTC) 10% feed supplement means that OTC is present at a 10% level. Thus 1kg would contain 10% or 100g of oxytetracycline. To mix 500g of active ingredient per tonne in this case would therefore, require 5kg of the feed supplement.

Examples of in-feed medication are shown in (Fig.4-1). Feed can be analysed for the levels of antibiotic added. For technical and analytical reasons, do not expect 100% recovery rate. It can often be as little as 40-60%.