During the evolution of our major species of domestic animals, a significant number of metabolic pathways have been lost by irreparable damage to the genetic information that encodes them. The end products of these pathways must be supplied as essential nutrients in the diet, and there are instances where this source is insufficient for maximum productivity. Genetic engineering now provides the ability to transfer genes encoding these pathways from organisms where the pathways are functional, thus restoring functional biosynthetic capacity to the target species. In order for this to be useful, however, it is necessary to identify substrate or nutrient limitations to important production traits. This is not a simple task, because many production traits are influenced by a variety of different factors depending on the actual husbandry situation, and the identification of these factors requires a detailed knowledge of the physiology of the production trait. Such knowledge is often not available, because the detailed physiology of the particular production trait is only partly understood. There are, however, two current areas of research where this approach may succeed.
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