The introduction by recombinant DNA technology of new genes into major crops used by animals has raised important questions; about the safety of novel feeds. The recently amended European Council Directive 2001/18/EC requires an assessment of risks for human, animals and the environment before viable seeds can be imported or the plant itself can be cultivated in Europe. In addition, the Novel Food and the Novel Food Ingredient Regulation (EC Council Directive, 1997) covers the use of non-viable products of any genetically modified (GM) plant intended for food purposes. The conceptual basis for the analysis of risks remains that of substantial equivalence. In practice, compositional analysis of key nutrients and key toxicants used to compare a GM plant with its conventional counterpart is the major source of data used to establish substantial equivalence. Although laboratory animals are used in toxicological studies of the products of introduced genes, animals are rarely fed the entire transformed plants or their by-products. Despite not being required or recommended by the existing legislation, many new products have been intensively tested with farm animals to measure effects on performance and animal health, digestibility of key nutrients, wholesomeness and feeding value. In this paper, we consider the value of such studies for establishing the safety of GM feedstuffs for the target animals and whether animal feeding studies are always necessary to establish the safety for the consumer of products of animals fed GM material. Compositional analysis has always shown the genetically modified plants to fall within the range of established values. The equivalence in digestible energy and crude protein between isogenic and transformed plants expressing a wide range of modifications (insect resistance, herbicide tolerance, or the barnase/barstar system of sterility/fertility restoration genes) also has been clearly demonstrated in different species. In none of these experiments was animal performance, whether measured as growth rate, feed efficiency and carcass merit in beef cattle, egg mass in laying hens, milk production, composition and quality in dairy cows or digestibility in rabbits, affected by feeding transformed plants compared to animals fed control or isogenic plants. Detection of chloroplasts-specific gene fragments by polymerase chain reaction showed the presence of plant DNA fragments (199 base pairs) in lymphocytes and duodenal juice of the dairy cow, and in muscle, liver, kidney and spleen of broilers, However, tDNA expressing Bt, usually found as a single copy gene, could not be detected in milk, tissues sample or eggs of livestock fed Bt maize and is probably not cause for concern.